Question

Read the passage given below and answer the questions that follow.

There are two types of lenses: Converging lenses and diverging lenses, depending on whether they converge or diverge an incident beam of light. They are also called convex or concave lenses. Lenses are usually made of glass. Convex lenses are more popular as they form a real image of an object. They are widely used in our daily life, for instance, in microscopes, telescopes, projectors, cameras, spectacles, etc. Microscopes are used to view small and nearby objects, whereas telescopes are used to see distant objects.

1. State any one factor on which the focal length of a lens depends.
2. Give an example where a convex lens behaves like a diverging lens.
3. What type of lens is used in a camera?
4. Write an expression for magnifying power of a compound microscope when its final image lies at the least distance of distinct vision (D).
5. State any one difference between a reflecting telescope and a refracting telescope.

Answer 1

(i) The refractive index is based on the two surfaces' curvatures and the glass from which it is constructed.

(ii) When placed in a transparent material with a higher refractive index than the lens, a convex lens acts similarly to a diverging lens. E.g., An air bubbles in water behaves like a diverging lens.

(iii) Convex lenses are used in photography cameras in order to capture true images on screen, as concave mirrors do not capture true images.

(iv) For a compound microscope, its magnifying power is given as 

m = `- v_0/u_0 (1 + "D"/f_e)`

Where r₀ = distance of the object from the objective lens.

f₀, f_e = focal lengths

D = least distance of distinct vision

(v) Differences between a reflecting telescope and a refracting telescope:

Reflecting telescope	Refracting telescope
It uses mirrors.	It uses lenses.
It requires a much purer material than a refracting telescope.	It requires a less purer material than a reflecting telescope.
It can be made much larger than refracting telescope.	It can be less than reflecting telescope.