# I Light refraction in water

1. Jan 18, 2017

### Sirfrijole

As light travels through water, it's wavelength is shortened to compensate for the decrease in speed. This explains why there is little to no red light at depth. My question is: a) do these wavelengths eventually reach ultraviolet and beyond, for example at extreme depth are there only x rays present, and b) do these rays still carry enough energy to cause cell damage like they do normally?

2. Jan 18, 2017

### Orodruin

Staff Emeritus
No it does not. This has to do with attenuation, not refraction.

3. Jan 18, 2017

### sophiecentaur

This link has information about how much the various wavelengths of EM waves are absorbed by water. In the second diagram down, the graph shows a minimum of absorption in the blue region but absorption rises quite fast in the UV direction. You wouldn't need a great depth of water to cut out UV to an insignificant level. X Rays are another matter - but they are not present in appreciable quantities, basically because of the spectrum of sunlight. Atmospheric Ozone does a good job of screening us from UV (but not enough to do without your Factor 20 cream!!
PS Try to avoid home brewed ideas in PF contributions - someone will always pick you up on it. (Very touchy about such things, here)

Last edited: Jan 18, 2017
4. Jan 18, 2017

### Orodruin

Staff Emeritus

5. Jan 18, 2017

### sophiecentaur

6. Jan 18, 2017

### Drakkith

Staff Emeritus
The energy per photon does not change when light travels through a medium, even though the wavelength changes. Its frequency remains the same as does its energy. Also consider that light has to pass through your eye to reach your retina, so the index of refraction of the water is irrelevant since light just goes back to the "normal" refractive index when it enters your eye (normal meaning the same as during your day-to-day life outside of water).

7. Jan 19, 2017

### _PJ_

This cannot be right, surely?
I mean sure if the frequency remains constant, then the energy must, but if wavelength changes, frequency must change or otherwise, the speed does.

I've long held that light "slowing down" in a medium is a myth because light, being massless can ONLY ever travel at c, the medium is irrelevant.There are delays due to absorption/emission and an increase in distance travelled due to scattering.

8. Jan 19, 2017

### Orodruin

Staff Emeritus
The speed of a light wave in a medium is given by $c/n$, where $n$ is the index of refraction.
You would simply be wrong. If you knew anything about how light is described either clasically or in QED, you would understand why. Hint: The dispersion relation changes in the presence of a background medium.

I do hope that you realise that light slowing down in a medium is a well tested experimental fact. In fact, it was known long before Einstein's work that light in a moving medium is dragged along with the medium to some extent, see eg https://en.wikipedia.org/wiki/Fizeau_experiment .

This is a typical example of "lies we tell kids". In reality, the index of refraction is a result of the well understood phenomenon of coherent forward scattering.

9. Jan 19, 2017

### Staff: Mentor

10. Jan 19, 2017

### ZapperZ

Staff Emeritus
You need to dig down and figure out what is meant by speed of light. It is the group velocity of light! This is what we measure, and this is what has been measured to slow down in ordinary medium. So it isn't a myth! By invoking photons, you are applying apples to explain oranges.

Zz.

11. Jan 19, 2017

### Drakkith

Staff Emeritus
To quote from the article:

At first glance this appears to support PJ's stance that a delay in emission is responsible for light traveling slower than c in a medium. What am I missing?

12. Jan 19, 2017

### ZapperZ

Staff Emeritus
Again, it has to do with what we mean as the "speed of light". While the speed of photon may be constant at "c", this is NOT the "speed of light" that we usually measure. We do not measure the speed of a photon. We measure the "pulse" of light, and this is the speed that we measure.

So to say that a variation in the speed of light in a medium is a myth is false based on what we actually measure.

Zz.

13. Feb 16, 2017

### _PJ_

I must apologise for saying "absoption/emisison".
This was utterly incorrect and a result of sheer laziness on my part. I oversimplified and used the wrong words - which actually made reference to an old conjecture that minute delays due to absorption/emission might slow the ovberall path of light through 'solid matter'.
Obviously, this is not sufficient. One only needs to consider that a vast majority of photons would not interact with any electrons (certainly not nucleons) in even the most dense matter.

I am sorry for my miswording to dredge up this old idea and insist this was not at all what I meant, nor the actual reason for my "long held belief". What I ought to have described, was that my point remains that light travels at c. Within different media, you will always obtain a result of overall displacement / proper time resulting less than this value because of the interactions within the electromagnetic fields which can be expressed as a virtual sea of photons.

An inaccurate but visual (no pun) analogy is that of figuring the journey for a distance from a map. The map will show only 2 dimensions, not the topographical height, so the actual time recorded for journey will always be longer than planned for a constant speed of the car.

And we measure only the overall displacement divided by the proper time we measure, there's no way to observer photons travelling, for any measurement, they exist only having travelled. (or at a location, not having travelled at all)