Q140 / 150 · MHT CET 2022 (08 Aug Shift 1) · Physics

A convex lens is dipped in a liquid whose refractive index is equal to refractive index of lens material. Then its focal length will

Correct answer
A
become infinite
B
remain same
C
decrease
D
become zero

Solution

Given that refractive of liquid is equal to refractive index of lens. $\Rightarrow n_{\text {lens }}=n_{\text {liquid }}$ Let the focal length be $f$. From lens makers formula, we get $\frac{1}{f}=\frac{\left(n_{\text {lens }}-n_{\text {liquid }}\right)}{n_{\text {liquid }}}\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$ Here, $n_{\{\text {lens }\}}-n_{\{\text {liquid }\}}=0$ Therefore, the focal length will be infinite.

Hence, the correct answer is option: 'A'.

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