Question

(i) If the lens is placed in water instead of air, how does its focal length change?


(ii) Which lens, thick or thin has greater focal length?​

Answer 1

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• Since the relative refractive index of glass with respect to water is less than that of glass with respect to air, the bending of light at the interface of glass water is lesser and hence a parallel beam of light falling on the lens is bent less, to meet at point on the principal axis and hence the lens has more focal

• Thick lens will have shorter and consequently thin lens will have greater focal length. Because, For a thick lens, the optical path length of the light is more, than for a thin lens, thus, the bending of light will be more in case of a thicker lens. Consequently, it has a shorter focal length

Answer 2

Answer:

i. Since the relative refractive index of glass with respect to water is less than that of glass with respect to air, the bending of light at the interface of glass water is lesser and hence a parallel beam of light falling on the lens is bent less, to meet at point on the principal axis and hence the lens has more focal.

ii. Thick lens will have shorter and consequently thin lens will have greater focal length. Because, For a thick lens, the optical path length of the light is more, than for a thin lens, thus, the bending of light will be more in case of a thicker lens. Consequently, it has a shorter focal length.