Question

Consider a converging lens with a focal point . Without changing the shape and size of the lens, what could you do to move the focus closer to the lens?a. Make the lens from a material with a larger index of refraction.b. There is nothing you can do: the shape alone determines the focal length. c. Make the lens from a material with a smaller index of refraction.

Answer 1

Answer:

the correct one is A

Explanation:

The focal length of a lens is given

          1 / f = (n-1) (1 /R₁ - 1 /R₂)

where n is the refractive index of the lens and R₁ and R₂ the radii of curvature of the two surfaces.

In our case they indicate that the shape of the lens has not changed, therefore the radii R₁ and R₂ remain constant.

Therefore, the only way to change the focal length is to change the refractive index and as we see the two quantities are inverse, that is, to decrease the focal length we must increase the refractive index

When examining the answers the correct one is A