Geometrical optics. Photometry. Quantum nature of light > Refraction of light. The lens formula > Problem 13.2.3

Statement

$13.2.3.$ A snail sits on the far wall of a rectangular aquarium of width $l$. By what factor will the apparent angular size of the snail change if the water is drained from the aquarium? The observer is located at a distance $L$ from the aquarium.

Solution

In the previous problem we proved that the apparent width of the aquarium will be $n$ times smaller than the real one. The angular size at large distances is inversely proportional to the distance $\to$

$$\alpha' = \alpha \frac{L+\frac{l}{n}}{L+l}$$

Answer

$$\alpha' = \alpha \frac{L+\frac{l}{n}}{L+l}$$