# Light cone from water-air

1. Nov 19, 2004

### daveed

this is the entire question:
at the shallow end of a swimming pool, the water is 70 cm deep. The diameter of the cone emerging from the water into the air above, emitted by a light source 10.0 cm in diameter at the bottom of the pool and measured by an observer on the edge of the pool 2.5 meters away is:

a)1.6m
b)1.7m
c)1.75m
d)1.8m

it would seem to me that you would use snell's law to find an angle of refraction, however, how do you know at what angle the light goes in the water? if it goes straight it wouldn't refract at all... im confused...

2. Nov 21, 2004

### Staff: Mentor

total internal reflection

Consider the light emitted at all angles. What's the angle of the refracted light if the angle of incident light is $\theta$? As the angle of incidence gets larger, at some angle (the critical angle) you won't get any refracted light. You'll have so-called "total internal reflection". Figure out that angle, then use it to figure out the diameter of the "cone" of light that emerges from the water.