What is a magnifying lens or simple microscope? How can a convex lens behave as a magnifying glass? What is its magnifying power? Why must a convex lens of short focal length be used as a magnifying glass?

Magnifying Lens or Simple Microscope:

A magnifying lens or simple microscope is a single convex lens that is used to magnify objects. It works on the principle of forming a magnified image of a small object that is placed close to the lens. This type of microscope does not have multiple lenses like more complex optical systems, such as compound microscopes, and relies solely on the magnification ability of the single lens.

How Can a Convex Lens Behave as a Magnifying Glass?

A convex lens is a type of lens that is thicker in the middle than at the edges. When you hold an object close to the convex lens, the lens bends (refracts) light rays passing through it in such a way that they converge at a point beyond the object. However, the image formed by the lens is not real and is located on the same side as the object.

  • Magnifying Glass Effect: When you place the object within the focal length of the convex lens, a virtual image is formed. This virtual image is upright, magnified, and appears on the opposite side of the lens from the object, making it look larger and more detailed to the observer.
  • The closer the object is to the lens (within the focal length), the larger and clearer the magnified image will appear to the viewer.

Magnifying Power of a Convex Lens:

The magnifying power (M) of a convex lens is the ratio of the angular size of the image formed by the lens to the angular size of the object if viewed without the lens. The formula for magnifying power is given by:

M=Angle subtended by the image/Angle subtended by the object

For a convex lens used as a magnifying glass, the magnifying power (M) can be approximated as:

M=D/f

Where:

  • is the near point of the eye (usually taken as 25 cm, the closest distance at which the human eye can clearly see an object without strain).
  • is the focal length of the lens.

In practice, the lens should be positioned such that the object is slightly closer than the focal point to achieve maximum magnification.

Why Must a Convex Lens of Short Focal Length Be Used as a Magnifying Glass?

A convex lens with a short focal length is preferred as a magnifying glass because it allows for higher magnification. Here’s why:

  1. Higher Magnifying Power: According to the formula M=D/f, the magnifying power is inversely proportional to the focal length. A shorter focal length results in a higher magnifying power.
  2. Smaller Object Distance: A lens with a short focal length can form a larger virtual image at a closer object distance. This allows the object to be placed nearer to the lens, which is key to achieving high magnification.
  3. Increased Image Size: With a short focal length, the image formed by the lens is larger and more detailed, enhancing the magnifying effect. This is especially useful when examining small objects such as insects, text, or tiny details.

Related Questions:

  1. What is reflection of light? Define the following terms: Incident ray, reflected ray, normal, angle of incidence, and angle of reflection. Describe the laws of reflection.
  2. What are different spherical mirrors? Define different terms involved in spherical mirrors.
  3. Why is a concave mirror called a converging mirror? Why is a convex mirror called a diverging mirror?
  4. Define the principal focus of convex and concave mirrors. Define focal length. Which mirror has a positive focal length? Which has a negative focal length and why?
  5. What is the spherical mirror formula? Also, write down the sign conventions for the spherical mirror formula.
  6. Differentiate between real and virtual images. What are three basic rays by which we can draw a ray diagram to form an image by a spherical mirror?
  7. What is refraction of light? Show by ray diagram how light bends when it travels from rare to denser medium and from denser to rare medium. Define the refractive index of a medium. What are its different formulae?
  8. What are the laws of refraction? Explain with the help of ray diagrams
  9. Explain the phenomenon of total internal reflection. What are necessary conditions to perform total internal reflection? Derive the formula for the refractive index of a medium in terms of the critical angle.
  10. What is optical fibre? How does light travel through it? Explain.
  11. What is a lens? What are its types? Define the basic terminologies of lenses.
  12. Define and explain the terms: resolving power and magnifying power.
  13. When you look at the front side of a polished spoon, your image is inverted, and from the back of the spoon, your image is erect. Explain why?
  14. Which mirror is used by girls for makeup and why?
  15. Why are large convex mirrors fixed at blind turns of mountains?
  16. Which mirrors are used for the rear view of vehicles and why?
  17. If a person is walking in a pool, why do his legs appear shorter in water?
  18. Why do diamonds sparkle brightly?
  19. When white light passes through a prism, it disperses into its seven colors. Why does dispersion take place in a prism?
  20. A magnifying glass can burn the paper. How is it possible?