# Question on optics

1. Apr 19, 2010

### semc

An opaque cylindrical tank with an open top has a diameter of 3m and is completely filled with water. When the afternoon sun reaches an angle of 28o above the horizon, sunlight cease to illuminate and part of the bottom of the tank. How deep is the tank?

I don't understand how to approach this question but more importantly I guess I should ask why doesn't the bottom part of the tank is not illuminated?

2. Apr 19, 2010

### Rajini

Hi,
To make things easier i suggest you to draw a schematic..
I think it is simple.Draw a cylinder..that is enough..Now draw a line..with 28 degree to the surface of the tank or to the bottom of the tank..now you can apply sin or cosine rule to get the height of cylinder? By this way i got 1.41 m height of cylinder.
good luck

3. Apr 19, 2010

### tiny-tim

Hi semc!

It's afternoon, so the sun is going down …

even if the tank was empty, when the sun is low enough, the sunlight won't light up all the bottom of the tank.