# A problem in optics

1. Apr 18, 2006

### Abhishek.S

A hiker stands on an isolated mountain peak near sunset and observes a rainbow formed by water droplets in the air 8 km. away. The valley is 2 km. below the mountain peak and entirely flat. What fraction of the complete circular arc of the rainbow is visible to the hiker?

I tried to solve the problem with the fact that the deviations of red light and violet light are 42 and 40 degrees resp.
I got the width of the rainbow he sees as 8tan42-8tan40 = 0.49
But what is the total width?

2. Apr 18, 2006

### andrevdh

I just want to mention first that stating that the rainbow forms at a distance of 8 km is a bit artificial, since it really forms at infinity (Well actually there is nothing out there, the rainbow forms on your retina, so each observer has his own personal rainbow inside of his eye! But if you were to stand next to me and I say "Do you see the rainbow there?" you would agree with me. Another observer flying over us in his airoplane would actually see the rainbow as a circle in another place. You can even photograph it and it will appear on the film, so we all say seeing is believing, but it still isn't actually out there! Some people might disagree with this statement of mine, this is my personal view on the subject. It makes you think does'nt it?), but that aside with the given info one can calculate the radius of the red circle of the rainbow at this distance, $r_r$. That enables you to evaluate the angle $\alpha$ that is cut off by the horizon from this circle and finally the arc that is below the horizon $s_{below}$.

There probably is a much more elegant way of calculating this, but for now it eludes me.

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Last edited: Apr 18, 2006