# OPTICS

## Unit12: OPTICS

Q.1. What do you understand by reflection of light? Draw a diagram to illustrate reflection at a plane surface:

Ans. REFLECTION OF LIGHT

When light travelling in a certain medium falls on the surface of another medium, a part of it turns back in the same medium. This is called reflection of light.

Reflection of Light at a Plane Surface

When a ray of light from air along the path OA falls on a plane mirror M, it is reflected along the path OB. The angle of incidence i.e.ZAON represented by i is equal to the angle of reflection i.e. ZNOB represented by r. This is called reflection of light.

Q.2. Define the following terms used in reflection of light:

(i) Normal

(ii) Angle of incidence

(iii) Angle of reflection

Ans. NORMAL

The perpendicular to a reflecting or refracting surface at the point of incidence of the ray concerned is called normal.

ANGLEOF INCIDENCE

The angle between the incident ray and the normal is called angle of incidence.

The angle between the normal and the reflected ray is called angle of iflon.

Q.3. State laws of reflection. Describe how they can be verified graphically? LAWS OF REFLECTION

There are two Jaws of reflection:

The incident ray, the normal, and the reflected ray at the point of incidence all lie in the same plane.

The angle of incidence is equal to the angle of reflection i.e. Z / = Z r.

Graphical Verification of the Laws of Reflection

Reflection of light is illustrated. From this figure, it is clear that; the incident ray OA and the reflected ray OB and the normal all lie in the same plane. From this figure, it is also clear that Zi = Zr.

Q.4. What do you mean regular and irregular reflection of light?

Ans. REGULAR REFLECTION

The reflection of light in only one direction is called regular reflection.

For example, a smooth surface reflects the rays of light in one direction only. For regular reflection of light, the angle of incidence is equal to the angle of reflection

i.e. Zi = Zr.

IRREGULAR REFLECTION

The reflection of light in many directions is called irregular reflection. For example, a rough surface reflects the light rays in many directions.

Q.5. Define spherical mirrors. What are its types?

Ans. SPHERICAL MIRROR

A mirror whose polished, reflecting surface is a part of a hollow sphere of glass or plastic is called a spherical mirror.

Types of Spherical Mirrors

Depending upon the nature of reflecting surface, there are two types of spherical mirrors:

1. Concave mirror
2. Convex mirror

Concave Mirror .

A spherical mirror whose inner curved surface is reflecting is called concave mirror.

In case of a concave mirror, the size of the image depends on the position of the object. Both virtual and real images can be formed by a concave mirror.

Convex Mirror

A spherical mirror whose outer curved surface is reflecting is called convex mirror.

In case of a convex mirror, the size of the image is always smaller than the object. Only virtual and erect image is formed by a convex mirror.

Q.6. Define the following terms:

(i) Pole

(ii) Centre of curvature

(iv) Principal axis

(v) Principal focus

(vi) Focal length

Ans. POLE

The midpoint of the curved surface of the spherical mirror is called pole of the mirror

CENTRE OF CURVATURE

A spherical mirror is a part of a sphere. The centre of this sphere is called centre of curvature.

The radius of the sphere of which spherical mirror is a part is called radius of curvature.

PRINCIPAL AXIS

The line joining the centre of curvature and pole of the spherical mirror is called principal axis.

PRINCIPAL FOCUS

In case of a concave mirror:

The point where the rays of light parallel to the principal axis after reflection converge is called principle focus.

Concave mirror is also called corverging mirror. Since rays of light actually pass through the principal focus, therefore, it is called a real focus.

In case of a convex mirror:

The point where the rays of light parallel to the principal axis after reflection appear to come from situated behind the mirror is called principal focus.

Convex mirror is also called diverging mirror. Since rays of light do not actually pass through the principal focus but appear to do so, therefore, it is called a virtual focus.

FOCAL LENGTH

The distance from the pole to the principal focus measured along the principal axis is called focal length.

Q.7 How light is reflected through spherical mirrors?

Ans. One curved surface of spherical mirrors (may be inner or outer) is always polished surface. When light falls on this surface, it suffers reflection as polished surfaces are good reflectors of light. This surface of spherical mirrors is called reflecting surface.

Q.8 Why we take the principal focus of a concave mirror as real and a convex mirror as virtual?

Ans. The principal focus of a concave mirror is a real focus point because the reflected rays actually pass through this point. However, the principal focus of a convex mirror is a virtual focus point because the reflected rays do not actually pass through it but appear to do so, therefore, it is called virtual focus.

Q.9  Why a concave mirror is used for makeup or shaving?

Ans. People use a concave mirror for makeup or shaving because when a person stands between the principal focus and pole of the concave mirror, he/she sees an enlarged, erect and virtual image of his/her face.

Q.10 Which mirror is used in motorcycles and automobiles?

Ans. Convex mirrors are used in motorcycles and automobiles which enable the driver to see the automobiles coming behind him.

Q.11 Why convex mirrors are fixed on the blind turns on the roads of the mountains?

Ans. Convex mirrors give wide angle view of the other side of the blind turns. Convex mirrors of small focal lengths ae used in this case which give diminished, erect image of the automobiles coming from the other side of the blind turns.

Q.12. How are focal length and radius of curvature related to each other?

Ans. RELATION BETWEEN FOCAL LENGTH AND RADIUS OF CURVATURE

The focal length is related to the radius of curvature by a relation:

This means that as the radius of curvature is reduced, so too is the focal length of the reflecting surface.

Q.13. Describe the characteristics of focus of a concave and a convex mirror. Ans. CHARACTERISTIC OF FOCUS OF A CONCAVE AND A CONVEX MIRROR

Convex Mirror

Concave Mirror

• The focus lies behind the mirror.
• The focus is virtual as the rays of tight after reflection appear to come from the focus

The focus is in front of the mirror.

The focus is real as. the rays of light after reflection converge at the focus.

Q.14. Define mirror formula. Also write its equation. MIRROR FORMULA

The relationship between object distance, image distance from the mirror and ibca/ length of the mirror is called mirror formula.

If p is distance of the object from the mirror, q is distance of the image from His mirror and f is the focal length of the mirror, then mirror formula can be written as:

1′- 1 + i

f ” P q

0.15. What are the sign conventions for concave and convex mirror?

Ans. SIGN CONVENTIONS

Quantity When Positive (+)

When Negative (-)

Object distance p Image distance q Focal length f

Real object Real image Concave mirror

Virtual object Virtual image Convex mirror

Q.16. Define refraction of light Describe the passage of light through parallel sided transparent material.

Ans.  REFRACTION OF LIGHT

The process of bending of light as it passes from air into glass and vice versa is called refraction of light.

Refraction of light can be explained with the help of Fig. 12.3. A ray of light IO travelling from air falls on the surface of a parallel-sided glass block.

At the air-glass interface, the ray of light IO changes direction and bends towards the normal and travels along the path OR inside the glass block. When refracted ray leaves the glass, ft bends away from the normal and travels along the path MF. This is called refraction of light.

Q.17. Define the following terms used in refraction:

(i) Angle of incidence

(ii) Angle of refraction

Ans. ANGLE OF INCIDENCE

The angle made by the incident ray with the normal is called angle of incidence.

ANGLE OF REFRACTION

The angle made by the refracted ray with the normal is called angle of refraction.

Q.18.State the laws of refraction of light and show how they may be verified using rectangular glass slab and pins.

Ans. LAWS OF REFRACTION OF LIGHT

(i)   The incident ray, the refracted ray, and the normal at the-point of incidence all lie in the same plane.

 Substance Iidnof Refraction (a) Diamond 2.42 Cubic Zirconia 2.21 Glass (flint) 1.66 Glass(crown) 1.52 Ethyl Alcohol 1.36 Ice 1.31 Water 1.33 Air 1.00

(ii) The ratio of the sine of the angle of incidence i to the sin of the angle of refraction r is always equal to a constant.

i.e.

sin i

sin r

= constant

where the ratio sin i/sin r is known as the refractive index of the second medium with respect to the first medium. So, we have

sin i sin r

it is called Sneil s law.

What is meant by refractive index of a material? How would you line the refractive index of a rectangular glass stab.

REFRACTIVE INDEX

The refractive index of a medium is the ratio of the speed of light in a vacuum flte the speed of light in the medium. Speed of tight in vacuum Refractive .index = speed of light in medium

If refractive index is denoted by n, speed of light in a vacuum by c and speed of light in the medium by v, then

c n   =  v

(12.2)

Q.20. What is meant by the term total internal reflection?

Ans. TOTAL INTERNAL REFLECTION

When the angle of incidence becomes larger than the critical angle, no refraction of light occurs. The entire light is reflected back into the denser medium. This is known as total internal reflection of light.

Explanation: When a ray of light enters from a denser medium to a rare medium, it bends away form the normal (Fig.12.5-a). If the angle of incidence increases, the angle of refraction also increases.

For a particular value of the angle of incidence, the angel of refraction becomes 90° known as critical angle (Fig. 12.5-b). When the angle of incidence becomes greater than the critical angle, no refraction of light occurs. The entire light is reflected back into the denser medium (Fig,12.5-c). This is known as total internal reflection.

Q.21. State the conditions for total internal reflection.

Ans. There are two essential conditions for the total internal reflection:

1. The ray of light should travel from a denser medium to a rare medium:
2. The angle of incidence should be greater than the critical angle.

Q.22. What is critical angle? Derive a relationship between the critical angle and the refractive index of a substance.

Ans. CRITICAL ANGLE

The angle of incidence in the denser medium for which corresponding angel of refraction is 90 ° in the rare medium is called the critical angle.

When a ray of light passes from a denser medium (glass) to a rare medium (air), the refracted ray bends away from the normal and the angle of refraction is greater than the angle of incidence.

It is clear from Fig. 12.6 that as the angle of incidence i increases, the angle of refraction r also increases in such a way that r is always greater than i, till for a particular value of angle of incidence, the corresponding angle of refraction becomes 90° and the refracted ray grazes along the surface AB (Fig. .12.6).

If the refractive index of air with respect to glass is n, then in this case the ray OD is travelling from glass to air and is refracted in air therefore, by applying Snell’s

law, we will find the value of refractive index.

Now for ray OD

Angle of incidence i = C Angle of refraction  r = 90° According to Snell’s law

sigg _ sin C _ 1 sin^^in 90° ~ n 1 ‘ or sin C – —

Q.23. What do you mean by convex and concave lens. What are their different types?

Ans. CONVEX LENS

The tens which causes incident parallel rays to converge at a point is known as convex lens or converging lens.

This lens is thicker at the centre and thinner at the edges.

Types of Convex Lenses

There are three types of a convex lens: Double convex lens Plano-convex lens Concavo-convex lens

The lens which causes the incident parallel rays of light to diverge from a point is called concave lens or diverging lens.

This lens is thinner at the centre and thicker at the edges.

Types of Concave Lens

There are three types of concave lens:

(i) Double concave lens

(ii) Plano-concave lens

Q.24. A coin is placed at the focal point of a converging lens. Is an image formed? What is its nature?

Ans.   When a coin is placed at the focal point or the principal focus, no mage is formed because the refracted rays are parallel and never meet.

Q.25. What is prism? Describe the passage of light through a glass prism and measure the angle of deviation.

Ans.  PRISM

A prism is a transparent body haying three rectangular and two triangular surfaces.

Angle of Prism

The angle of the triangular surface opposite to its base is known as angle of prism.

In case of a triangular prism, the emergent ray is not parallel to the incident ray. It is deviated by the prism from its original path. The incident ray PE makes an angle of incidence i at point E and is refracted towards the normal N as EF. According to the law of refraction n = sin i/sin r.

The refracted ray EF makes an angle r inside the prism and travels to the other end of the prism. : Refraction through This ray emerges out from prism at point F making             a triangular glass prfcm an angle e. Hence the emerging ray FS is not parallel to the incident ray EF but is deviated by an angle D which is called angle of deviation.

Q.26. Define the following terms applied to a lens:

(i) Principal axis (ii) Optical centre (iii) Focal length

Ans. PRINCIPAL AXIS

The line passing through the centre of the lens is called principal focus.

OPTICAL CENTRE

A point on the principal axis at the centre length lens is called optical centre.

FOCAL LENGTH

The distance between the optical centre and the principal focus is known as focal length.

0.27. What is meant by the principal focus of a

(a) Convex lens

Ans. (a) In case of a convex lens:

The light rays travelling parallel to the principal axis of a convex lens after refraction meet at a point on the principal axis called principal focus or focal point.

Q.28. How does a convex tens or converging tens forms a virtual image of a real object?

Ans.  When the object is placed between the convex lens and F, the image will be virtual. The ray diagram is shown in the figure given below:

Q.29. How does a concave lens or diverging lens can form a real image of a real object?

Ans. A concave lens cannot form a real image of a real object in the air. If the concave lens is placed in a medium of refractive index greater than that of glass, then, it will form real image of a real object.

Q.30. What are the differences between real and virtual images?

Ans. DIFFERENCES BETWEEN REAL AND VIRTUAL IMAGES

A real image can be obtained on the screen; but a virtual image cannot be obtained on the screen.

Q.31. Describe how light is refracted through lenses?

Ans. REFRACTION THROUGH LENSES

1.In lenses image is formed through refraction of light. Image formation in convex lens can be explained with the help of three principal rays as shown in the figure given below

The ray parallel to the principal axis passes through the focal point after remains by the lens.

1. The ray passing through the optical centre passes through the lens and passes undeviated.
2. The ray passing through the focal point becomes parallel to the principal axis after refraction by the lens.

Q.32. Explain image formation with ray diagram when the object is placed beyond 2F from a convex lens?

Ans. When the object is placed beyond 2F, the image formation in a convex lens

The image is formed between F and 2F, it is real, inverted and smaller than the object.

Q.33 Explain image formation with ray diagram, when the object is placed at 2F from the convex lens.

Ans. When the object is placed at 2F, the image formation in a convex lens will be as follows:

The image is formed at 2F, it is real, inverted and of the same size as the object.

Q.34 Explain image formation with ray diagram, when the object is placed between F and 2F of a convex lens.

Ans.  When the object is placed between F and 2F, the image formation in a convex lens will be as follows:

Q.35   Explain the image formation with ray diagram, when the object is placed at F (focal point) of a convex lens.

Ans.  When the object is placed at F, the image formation in a convex lens will be as follows

No image is formed because the refracted rays are parallel and never meet.

Q.36 Explain the image formation with ray diagram, when the object is placed between the convex lens and F.

Ans.   When the object is placed between the convex lens and F, the image formation will be as follows:

The image is behind the object, it is virtual, erect and larger than the object.

Q.37 In which direction the light bends when it enters from one medium to another?

Arts. When light enters from rare to denser medium, it bends towards the normal at the point of incidence.

On the other hand, when light enter from denser to rare medium, it bends away from the normal.

Q.38. Define power of a lens and its unit

Ans. POWER OF A LENS

The reciprocal of the focal length of a lens is called its power.

Power of lens P = 1/focal length (in metres) Unit:   The SI unit of power of a lens is Dioptre, denoted by D. If f is expressed in

metres so that 1D = 1 m  .

Dioptre

The power of a lens is one Dioptre whose focal length is one metre.

Q.39. What is difference between the power of a convex lens and a concave lens?

Ans. Because the focal length of a convex lens is positive, therefore, its power is also positive. Whereas the focal length of a concave lens is negative, therefore, it has negative power.

Q.40. Define lens equation or formula. Also write its equation.

Ans.  LENS FORMULA

The relation between the object and image distance from the lens in terms of the focal length of the lens is called lens formula.

It can be written as:

1 -1 -1 T – £ + q

This formula is valid both for concave and convex lenses.

Q.41. What are the adopted rules about the sign conventions for the lenses?

Ans.  SIGN CONVENTIONS FOR LENSES Focal length

1. f is positive for a converging lens.
2. f is negative for a diverging lens.

Object Distance

1. p is positive, if the object is towards the left side of the lens, it is called a real object.
2. p is negative, if the object is on the night side of the lens. It is called virtual object.

Image Distance

1. q is positive for a real image made on the right side of the lens by real object.
2. q is negative for a virtual image made on the left side on the lens by real object.

Q.42. Describe some uses of lenses.

Ans. USES OF LENSES

Lenses of many different types are used in optical devices such as cameras, eyeglasses, microscopes, telescopes, and projectors.

Q.43. Write the construction and working of a camera.

Ans. CAMERA

A simple camera consists of a light-proof box with a converging lens in front and a light sensitive plate or film at the back. The lens focuses images to be photographed onto the film. In simple lens camera, the distance between lens and film is fixed which is equal to the focal length of the lens. In camera, object is placed beyond 2F. A real, inverted and diminished image is formed in this way as shown below:

Q.44. How does a slide projector or movie projector work? Explain.

Arts.   The working of a slide projector or a movie projector is shown in the figure given below:

The light source is placed at the centre of curvature of a converging or a concave mirror. The concave mirror is used to reflect light back in fairly parallel rays. The condenser is made up of 2 converging lenses that refract the light so all parts of the slide are illuminated with parallel rays. The projection or converging lens provides a real, large and inverted image. It must be real to be projected on a screen. The slide (object) must be placed between F and 2F of projection lens so as to produce a real, large, and inverted image. Because the image is inverted, the slide must be placed upside down and laterally inverted so we can see the image properly.

Q.45. What is photograph enlarger? How does it work?

Ans.  PHOTOGRAPH ENLARGER

In the case of photograph enlarger, object is placed at distance of more than F but less than 2F. In this way, we get a real, inverted and enlarged images as shown in the figure given below:

The working principle of photograph enlarge is basically the same as that of a slide projector It uses a convex lens to produce a real, magnified and inverted image of the film on a photographic paper.

Q.46. Describe the use of totally internal reflecting prism.

Ans. TOTALLY INTERNAL REFLECTING PRISM

Many optical instruments use right-angled prism to reflect a beam of light through 90° or 180° (by total internal reflection) such as cameras, binoculars, periscope and telescope. One of the angles of a right-angled prism is 90°. When a ray of light strikes a face of prism perpendicularly, it enters the prim without deviation and strikes the hypotenuse at an angle of 45° (Fig.12.22-a). Since the angle of incidence 45° is greater than critical angle of the glass which is 42°, the light is totally reflected by the prism through an angle of 90°. Two such prisms are used in periscope (Fig.12.22-b). In Fig.12.22-c, the light is totally reflected by the prism by an angle of 180°. Two such prisms are used in binoculars (Fig. 12.22-d).

Q.47. What is optical fibres? Describe how total internal reflection is used in light propagating through optical fibres.

Ans. OPTICAL FIBRE

Total internal reflection is used in fibre optics which has number of advantages in telecommunication field. Fibre optics consist of hair size threads of glass or plastic through which light can be travelled (Fig. 12.23). The inner part of the fibre optics is called core that carries the light and an outer concentric shell is called cladding. The core is made from glass or plastic of relatively high index of refraction. The cladding is made of glass or plastic, but of relatively low refractive index. Light entering from one end of the core strikes the core/cladding boundary at an angle of incidence greater than critical angle and is reflected back into the core. In this way, light travels many kilometres with small loss of energy.

In Pakistan, optical fibre is being used in telephone and advanced telecommunication systems. Now we can listen thousands of phone calls without any disturbance.

Q.48 What is the principle of optical fibres?

Ans. The optical fibres work on the principle of total internal reflection.

If light rays enter at one end of an optical fibre at an angle of incidence greater than* the  critical angle, then these rays undergo total internal reflection repeatedly at the walls and come out at the other end without any-loss of intensity. Thus, light travels along the fibre no matter how it may be curved and comes out with the same intensity.

Q.49. What is a light pipe? Describe its uses.

Ans.  LIGHT PIPE

Light pipe is a bundle of thousands of optical fibres bounded together.

Uses: Light pipes are used to illuminate the inaccessible places by the doctors or engineers. For example, doctors view inside the human body They can also be used to transmit images from one place to another.

Q.50. Describe the use and working of endoscope.

Ans.  ENDOSCOPE

An endoscope is a medical instrument used for exploratory diagnostics, and surgical purposes.

An endoscope is used to explore the interior organs of the body. Due to its small size, it can be inserted through the mouth and thus eliminates the invasive surgery. The endoscopes used to diagnose the stomach, bladder and throat are called Gastroscope, Cystoscope and Bronchoscope respectively. An endoscope uses two fibre-optic tubes through a pype. A medical procedure using any type on endoscope is called endoscopy. The light shines on the organ of patient to be diagnosed while entering through one of the fibre tubes of the endoscope. Then light is transmitted back to the physician’s viewing lens through the other fibre tube by total internal1 reflection. Flexible endoscopes have a tiny camera attached to the end. Doctors can see the view recorded by the camera displayed on the computer screen.

Q.51.Define the terms resolving power and magnifying power or angular magnification.

Ans. RESOLVING POWER

The ability of an instrument to distinguish between two closely placed objects or point sources is called resolving power.

MAGNIFYING POWER

The angular magnification or magnifying power is the angular size of the final image produced by the magnifying glass divided by an angular size of the object seen without the magnifying glass.

Angular size of final image produced by magnifying glass  Let Angular size of object seen without magnifying glass

If 9′ is the angular size of the final image q^

produced by the magnifying glass and 6 is the angular j size of the object seen without the magnifying glass, – n

then magnifying power can be written as:

indicates that the lens produces virtual

image which is enlarged and upright with respect to he a. object. If do is the near distance of the object from eye

which is usually equal to 25 cm, then magnifying power becomes: Image formation.

Q.52. What is simple microscope? How is image formed in it?

Ans.  SIMPLE MICROSCOPE

A simple microscope is an magnifying glass (convex lens) which is used to produce magnified image of small objects. „.

Formation of Image in Simple Microscope

The object is placed nearer to the lens than the principal focus such that an upright, virtual and magnified image is seen clearly at 25 cm from the normal eye.

Q.53. What is a compound microscope? Describe its some features and uses.

Ans. COMPOUND MICROSCOPE

A compound microscope has two converging sets of lens, the objective and the eyepiece.

Features of Compound Microscope

Following are some features of a compound microscope:

1. It gives greater magnification than a single lens.
2. The objective lens has a short focal length, f0 < 1 cm.
3. The eyepiece has a focal length. F e of a few cm.

Uses of Compound Microscope

A compound microscope is used to study bacteria and other micro objects. It is used for research in several fields of sciences like, Microbiology, Botany, Geology, and Genetics.

Q.54. What is telescope? Describe the working of a refracting telescope.

Ans. TELESCOPE

An optical instrument which is used to observer distant objects using lenses or mirrors is called telescope.

Refracting Telescope

A telescope that uses two converging lenses is called refracting telescope.

Working of Refracting Telescope

The ray diagram of refracting telescope is shown in the figure. When parallel rays from a point on a distant object pass through objective, a real image I-i is formed at the focus F0 of the objective lens. This image acts as an object for the eyepiece. A large virtual image I2 of Ij is formed by the eyepiece at large distance from the objective lens. This virtual image makes an angle 6 at the eye.

Q,55.  Mention the magnifying powers of the following optical instruments:

• Simple microscope
• Compound microscope
• (Hi) Refracting telescope

Ans. (i) Magnifying power of a simple microscope is given by

(ii) Magnifying power of a compound microscope is given by

(iii) Magnifying power of a refracting telescope is given by

Q.56. Draw the ray diagrams of

(i)   Simple microscope

(ii)  Compound microscope

Refracting telescope.

Ans. The ray diagram of a simple microscope is

The ray diagram of a compound microscope is

The ray diagram of a refracting telescope is

Q.57. With the help of a ray diagram, how you can show the use of thin converging lens as a magnifying glass?

When the object is placed between F and O, the convex lens forms magnified virtual image. Then, the convex lens is called magnifying glass.

Q.58. Describe how image is formed in human eye. Draw ray diagrams to show the formation of images in the normal human eye.

Ans. The imager formation in human eye is shown in

the figure. Human eye acts, like a camera. In place of the

film, the retina records the picture. The eye has a

refracting system containing a converging lens. The lens

forms an image on the retina which is a light sensitive

layer at the black of the eye. In the camera, the distance

of lens from film is adjusted for proper focus but in the

eye, the lens changes focal length. Light enters the eye

through a transparent membrane called the cornea. The Fig. 12.32

iris is the coloured portion of the eye and controls the amount of light reaching the

retina. It has an opening at its centre called the pupil. The iris controls the size of the

pupil while in dim light pupil is enlarged,

Q.59. Describe the mechanism of eye for focusing the image of an object onto the retina.

Ans. The eye has different adjusting mechanics for focusing the image of an object onto the retina. Its ciliary muscles control the curvature and thus the focal length of the lens, and allow objects at various distances to be seen.

s If an object is far away from the eye, the deviation of light through the lens must be less To do this, the ciliary muscles relax and decrease the curvature of the lens, thereby increasing the focal length. The rays are thus focused onto the retina producing a sharp image of the distant object.

objects at various to be seen

Flg.1 2.33 Human ay» accommodation If an object is close to the eye, the ciliary muscles increase curvature of the lens, thereby, shorter, nig the focal length. The divergent rays from the nearer object are thus bent more so as to come to a focus on the retina.

Q.60. Define the term accommodation. How defects in accommodation be corrected?

Ans. ACCOMMODATION

The variation of focal length of eye lens is called accommodation.

Defects in accommodation may be corrected by using different types of lenses and eyeglasses. –

Q.61. What do you mean by near point and far point of the eye?

Ans. NEAR POINT

The minimum distance of an object from the eye at which it produces a sharp image on the retina is called near point.

FAR POINT

The maximum distance of an distant object from the eye on which the fully relaxed eye can focus is called far point.

Q.62. Draw the ray diagram for image formation in human eye when object is placed at near point

Ans. When the object is placed at near point, the ray diagram for image formation in human eye will be as follows:

Q.63. What is meant by the terms nearsightedness and farsightedness. How these defects can be corrected?

Ans. NEARSIGHTEDNESS

The disability of the eye to form distinct images of distant objects on its retina is known as nearsightedness.

This is also known as shortsightedness.

Near sightedness may be due to the eyeball being too long. Light rays from a distant object are focused in front of the retina and a blurred image is produced

The nearsighted eye can be corrected with gla”ss or contact lenses that use diverging lenses. Light rays from the distant objects are now diverged by this lens before entering the eye. To the observer, these light rays appear to come from far point and are therefore focused on the retina, thus forming a sharp image.

FARSIGHTEDNESS

The disability of the eye to form distinct images of nearly objects on it retina is known as farsightedness.

it is also known as longsightedness.

When a farsighted eye tries to focus on a book held closer than the near point, it shortens its focal length as much as it can. However, even at its shortest, the focal length is longer than it should be. Therefore, the light rays from the book would form sharp image behind the retina. f

This defect can be corrected with the aid of a suitable converging lens (Fig.12.36-b). The lens refracts the light rays arid they converge to form an image on the retina. To an observer, these rays appear to come from near point to form a sharp virtual image on the retina

SOLVED INFORMATION

Q.1  In  large shopping centres, convex mirrors are used for security purposes. Do you know why?

Ans.  In large shopping centres, convex mirrors are used to see hidden areas or places, it helps to avoid any stealing or theft and for better supervision.

Q.2  Why the position of fish inside the water seems to be at less depth than that of its actual position?

Ans.  Position of fish inside the water seems to be at less depth due to refraction of light at the surface of water.

Q.3  Will the bending of the light be more or less for a medium with high refractive index?

Ans.  Bending of light is more for a medium with high refractive index.

Q.4  Where a pen is placed in front of a convex lens if the image is equal to the size of the pen? What will be the power of the lens in dioptres?

Ans.   To obtain an image of the pin where its size is equal to the size of the pen, the pen must be placed at 2F from the convex lens.

Q.5 How the size of the pupil of our eye will change: (a) in dim light (b) in bright light

Ans. (a) In dim light, the size of the pupil of our eye increases,

(b) In bright light, the size of the pupil of our eye decreases.

SUMMARY

When light travelling in a certain medium falls on the surface of another medium, a part of it turns back in the same medium. This is called reflection of light. ‘

There are two laws of reflection:

(i) The incident ray, the reflected ray, and the normal all lie in the same plane.

(ii) The angle of incidence is equal to the angle of reflection (i.e. Zi = Zr)

In mirrors, image formation takes place through reflection of light while in lenses image is formed through refraction of light.

The equation relating the distance of the object, p from the mirror/lens, distance of the image q and the focal length f of the mirror/lens -is called mirror/lens formula.

Magnification of a spherical mirror or a thin lens is defined as “the ratio of the image height to the object height.”

Power of a lens is defined as “the reciprocal of its focal length in metres”. Thus power of a lens P = 1 /focal length (in metres). The SI unit of power of a lens is “Dioptre”, denoted by a symbol D. If f is expressed in metres so that 1D = 1 m~1. Thus, 1 Dioptre is the power of a lens whose focal length is 1 metre.

The refractive index of light n of a material is the ratio of the speed of light c in vacuum to the speed of light v in the material.

Speed of light in vacuum _ c Speed of light in medium ~ v

The bending of light from its straight path as it passes from one medium into another is called refraction.

Refraction of light takes place under two laws called laws of refraction. These are stated as:

(i)       The incident ray, the refracted ray, and the normal at the point of incidence all lie in the same plane.

(ii) ‘” The ratio of the sine of the angle of incidence i to the sine of the angle of refraction r is always equal to a constant.

sin i …       x.      *•   sin i .          ,.   ..       , i.e.      ^-pj-p –

constant/where the ratio ^-p is equal to the refractive

index n of the second medium with respect to the first medium

This is called Spell’s law.

The angle of incidence for which the angle of refraction becomes 90° is called critical angle.

When the angle of incidence becomes larger than the critical angle, no refraction occurs. The entire light is reflected back into the denser medium. This is known as total internal reflection of light.

A simple microscope, also known, as a magnifying glass, is a convex lens which is used to produce magnified images of small objects.

A compound microscope is used to investigate structure of small objects and has two converging lenses, the objective and the eyepiece.

Telescope is an optical instrument which is used to observe distant objects using lenses of mirrors.

A telescope that uses two converging lenses is called refracting telescope.

A telescope in which the objective lens is replaced by a concave mirror is called reflecting power telescope.

The magnification is defined as “the ratio of the angle subtended by the image as seen through the optical device to that subtended by the object at the unaided eye”.

The resolving power of an instrument is its ability to distinguish between two closely placed objects.

The ability of the eye to change the focal length of its lens so as to form a clear image of an object on its retina is called its power of accommodation.

The disability of the eye to form distinct images of distant objects on its retina is known as near sightedness-

The disability of the eye to form distinct images of nearby objects on its retina is known as farsightedness.