Question

The lens formula $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$ is valid when

• A. medium on both sides of the lens is air.
• B. lens is either thick or thin.
• C. medium on both sides of lens is different.
• D. lens is thin and medium on both sides of lens is same.

Answer 1

The correct option is D lens is thin and medium on both sides of lens is same.

Lens formula is not applicable for thick lens, its applicable for thin lens. This is because the separation between the two refracting surfaces will also be small.
The medium on either side of the lens needs to be the same.

Why this question? Caution: It must be very clear that medium should be same on both sides of lens to apply lens formula.