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Answer the following with respect to a concave lens (L).

Answer the following with respect to a concave lens (L). ICSE 2025 Improvement Physics Solved Question Paper.

(a) Describe the path of the rays X and Y through the lens.

(b) Give one use of this lens.

(c) Calculate its power if the focal length of this lens is 20 cm.

Refraction Lens

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Answer

(a) Ray X : As the ray is parallel to the principal axis of the concave lens then after refraction, it appears to come from the second focus F, as shown in below diagram.

Ray Y : As the ray is incident at the optical centre of the lens then it passes undeviated through the lens because angle of incidence of the ray with the normal at optic centre is zero as shown in below diagram.

Answer the following with respect to a concave lens (L). ICSE 2025 Improvement Physics Solved Question Paper.

(b) A concave lens is used in spectacles for correcting short-sightedness (myopia).

(c) Given,

  • Focal length = -20 cm = -0.2 m

Then,

Power=1focal length (in metres)=1−0.2=−102=− 5⇒Power=− 5 m\text {Power} = \dfrac{1}{\text {focal length (in metres)}} \\[1em] = \dfrac{1}{-0.2} \\[1em] = - \dfrac {10}{2} = -\ 5 \\[1em] \Rightarrow \text {Power} = -\ 5\ \text m

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