First slide

Ray optics

Question

A fish looking up through the water sees the outside world, constant in a circular horizon. If the refractive index of water is $\frac{4}{3}$ and the fish is 12 cm below the water surface, the radius of this circle in cm is:

Difficult

A

$36 \sqrt{7}$
B

$36 / \sqrt{7}$
C

$36 \sqrt{5}$
D

$4 \sqrt{5}$

Solution

Radius of circle of illuminance,
$r = \frac{h}{\sqrt{μ^{2} - 1}} = \frac{12}{\sqrt{{(\frac{4}{3})}^{2} - 1}}$
$\Rightarrow r = \frac{36}{\sqrt{7}} cm$

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