# Dispersion of light in a swimming pool

I've noticed that in this outdoor swimming pool, the near and far sides of the black racing stripe appear blue and orange, respectively.

I've drawn some ray diagrams trying to rationalize this, but I can't get a satisfactory answer for why the light that experiences a higher index of refraction (blue) should appear on the near side of the black stripe, and likewise for red. Shouldn't the bottom of the pool emit light equally in every direction? If so, how can different sections appear as different colors? Sorry if the phenomenon isn't clear in the photo; if it isn't, just take my word for it.

My best guess is that, for whatever reason, light is preferentially emitted upward. So, that light is separated with blue hitting the bottom of my cornea and red the top. This feels wrong to me. Anyone have a better explanation for this dispersion?

• Welcome to StackExchange! Take a look at the help center to learn how this site works. – Pritt Balagopal Jun 3 '17 at 16:11
• It is chromatic aberration but I do not think that the colour sequence is not consistent with dispersion occurring on the surface of the water. Is the photograph the right way up, ie was the "blue edge" closest to the camera? – Farcher Jun 4 '17 at 9:08
• @Farcher Google 'swimming pool lines' or something similar and you can see the same effect with always orange on top. – Quantum spaghettification Jun 4 '17 at 19:14

I'm no expert and I am not claiming that this is the actual answer but I think it is a possibility. I think it is a result of chromatic aberration. I won't go into mathematical detail but show you some pictures of the results:

(Image by George Shuklin from wikimedia released under CC BY-SA 1.0)

This webpage also gives a picture which shows the blue and orange edge due to chromatic aberration. I won't reproduce it here since I don't know the licensing on it.

You talk about the cornea in your post - indicating you see it with your eyes as well as with a camera. This post by BarsMonster explains how we do have chromatic aberration in the eyes, but our brains usually account for it. I suspect (although could be wrong) that in this case the aberration is exaggerated.

• Yes, chromatic aberration occurred to me, but I threw it out because I see the same effect with my naked eye. The post you linked changes this, though; if aberration is a possibility with naked eye observation, then that might be the answer. – valen Jun 3 '17 at 18:18
• @valen Just to exclude the obvious - do you wear glasses? – Quantum spaghettification Jun 3 '17 at 18:37
• No. No contacts either. Also, I see the same thing with naked eye as with polarized sunglasses. – valen Jun 3 '17 at 18:42
• @valen Chromatic aberration is what this is. It's nothing to do with your eye or the camera, however: it is to do with the varying refractive index of the water. – tfb Jun 3 '17 at 20:54

Why? That phenomenon is perfectly reasonable.

Recall the fact that refractive index of water is greater for violet than for red. Consider this fact, let's construct ray diagrams.

Recall Snell's Law:

$$\mu = \frac{\sin{i}}{\sin{r}}$$

Here, we consider the ray from inside to outside water, so we get $\mu = \frac{3}{4}$. Red would give a value of $\mu$ slightly more than $\frac{3} {4}$, while violet would give a value slightly less than $\frac{3}{4}$. As a result, the value of $r$ is more for violet than for red.

Red will appear to bend less than the mean deviation and violet will bend more. Trace their backward paths, you get:

I hope this helps.

• Sorry your diagram makes little sense to me. You don't have a 'red' and 'violet' source on the bottom of the pool just one source (the black line). Nevertheless I think using the projections of the rays this analysis results in red been at the bottom and violet been at the top - counter to that shown in the image. – Quantum spaghettification Jun 3 '17 at 17:17
• My bad they aren't sources, they are backtracing of the violet and red rays. It shows an idea of where it's perceived to appear from. – Pritt Balagopal Jun 3 '17 at 17:19
• Why did you draw the backward paths this way? They should both be straight lines, producing an inverted (real) image where red is on the near side and blue on the far side. Why are you permitted to "refract" the dotted lines? – valen Jun 3 '17 at 17:19
• I second @valen that the rays should be straight. – Quantum spaghettification Jun 3 '17 at 17:20
• My bad, my bad, apologies, I'll fix up the diagram. – Pritt Balagopal Jun 3 '17 at 17:20