Compare the shadows cast by a pole in the river and a pole on the shore

[hidemi]

Homework Statement

A pole stands in a river, half in and half out of the water. Another pole of the same length stands vertically on the shore at a place where the ground is level. The shadow cast by the pole in the river on the river bottom is:

A. slightly longer than the shadow of the pole on land
B. much longer than the shadow of the pole on land
C. shorter than the shadow of the pole on land
D. shorter than the shadow of the pole on land if the Sun is high and longer if the sun is low
E. the same length as the shadow of the pole on land

The answer is C

Relevant Equations

v=f*λ

Is it because the speed of light is less fast in the water than on land as such leads to different in length of shadow?

 

[jbriggs444]

Is it because the speed of light is less fast in the water than on land as such leads to different in length of shadow?

Only indirectly.

If you traced a ray of light from the source to the tip of the pole and then straight past that until it strikes the river bottom, that would be the edge of the shadow, right?

What happens to that light ray when it strikes the surface of the river water?

 

[kuruman]

In a way yes. Consider the ray that just grazes the top of the pole. Draw a ray diagram for each of the two cases.

 

[hidemi]

What happens to that light ray when it strikes the surface of the river water?

The ray of light would deflect close to the normal line which makes the shadow less longer in the water.

Thanks both of you!