# Homework Help: Spectrum change underwater

1. Apr 25, 2005

### stigelsm

Hey this may be a stupid question but here goes...
If you're underwater, say in a pool or something, and someone shines a light into the water will you see a color change? Why or why not?

2. Apr 25, 2005

### OlderDan

3. Apr 25, 2005

### Staff: Mentor

I don't think this question is stupid at all, but I believe the answer is simpler than you may think. The frequency of the light doesn't change as it moves from one media to another. And, in order for you to see the color, the light must enter your eye. Does that give you a hint at the answer?

4. Apr 26, 2005

### GCT

what are you saying, a flashlight? You'll have no significant refraction, thus I'm guessing...no; based on what I think you meant by the question.

5. Apr 26, 2005

### stigelsm

spectrum of light underwater continued...

Not really a flashlight, something like a laser light

6. Apr 26, 2005

### OlderDan

If the light hits the surface at a shallow angle, there will be significant refraction and there will be dispersion. If the light were a pencil beam the size of your eye, and you were at any appreciable depth, you would only see a narrow region of the spectrum. If it is not a pencil beam, you would see light of different colors coming from different direction, so what you see will be a bit blurred, but perhaps not so discolored.

Then depending on how deep you are, there will be a scattering effect as there is it the atmosphere. That is what causes the sky to be blue and sunsets red. I don't know how big that effect is in water, but it will be there.

As I said, it's somewhat complicated and depends on where the light is coming from, how deep you are, etc, but the simple answer is yes. How much is another matter.

7. Apr 26, 2005

### Staff: Mentor

OlderDan is correct, there will be a (probably small) amount of absorption or scattering of the light from the water, and thus the set of frequencies that make it to your eye may produce a different color response (compared to travel through air).

If the light source is monochromatic, then it will appear as the same color to your eye whether viewed under water or in air.

8. Apr 26, 2005

### DaveC426913

It depends on how much water you are talking about. (Nonpure) water preferentially absorbs strongly in the red and yellow end of the spectrum, which is why everything is blue below 10 feet or so - those colours are missing. A red laser light won't penetrate as far underwater before it is absorbed.

A monochromatic light will not change colour (though it will get dimmer), but a polychromatic light will change colour.

9. Apr 26, 2005

### stigelsm

Thanks so much to everyone for your help with this question, it is very much appreciated!!