Question

When a biconvex lens of glass having refractive index $1.47$ is dipped in a liquid, it acts as a plane sheet of glass. This implies that the liquid must have refractive index:
A. Equal to that of glass
B. Less than one
C. Greater than that of glass
D. Less than that of glass

Answer 1

Hint: To solve the question, use the approach of thinking that a lens’ job is to deviate the incident light rays and this happens because of the fact that the light goes from travelling in a medium with a certain refractive index into a medium with a lesser or higher refractive index, thus bending the light rays. If the light rays are not bending, it must mean that the lens is nothing but the medium itself.

Complete step by step answer:
We will solve the question by using the approach as explained in the hint section of the solution to the asked question. Let us first explain the working principle of lens:
The lens uses the refraction of light to deviate the incident light rays and either converge them or diverge them. This basically happens since the refractive index of the medium and the material of the lens is different and thus, when the light enters a denser medium or a rarer medium from a rarer or a denser medium respectively, it experiences refraction and thus bending of light rays. Hence, for a lens to work, the refractive index of the lens and the medium surrounding it must be different. But if the lens acts as a glass and no light rays are bent or converge or diverge, it means that the lens is nothing but the surrounding medium itself in terms of refractive index, i.e. the refractive index of the lens is exactly the same as the medium surrounding the lens. Hence, we can safely say that the refractive index of the liquid in which the biconvex lens is put in the question has exactly the same refractive index as the material of the lens itself, i.e. the liquid has a refractive index of $1.47$.
Hence, the correct option is nothing but option (A) as it states the option of refractive index of the liquid being equal to that of the glass or lens.

Note: Many students might tick the option (B) without actually going extrapolating the data and information given in the question believing that both the incidents, refractive index being less that one and lens not bending the light rays are rare to be seen. Refractive index can never practically be less than one as the speed of light in vacuum is the maximum possible speed of any object in any medium in the universe.