Level 1: A Concave Mirror Under Water: Does Its Focal Length Change?
A lens in a liquid of index one point two five.
A convex lens made of glass (n = 1.5) has a focal length of 20 cm in air. It is immersed in a liquid of refractive index 1.25. Its new focal length is:
By the lensmaker's formula, the power of a lens is proportional to its relative refractive index minus one, where the relative index is that of the glass divided by that of the surrounding medium.
The lens becomes 2.5 times weaker, so its focal length is 50 cm. Scaling by the liquid's index alone, which gives 25 cm, is the common mistake; what matters is how much the glass differs from its surroundings.
Every Level 2 starts from a quick question, Level 1, and changes one thing. Seeing what that change does is the skill JEE Main tests. Try it here, free and with no signup, then read the worked answer.
