# B Does water slow light down?

1. Apr 27, 2016

### hipokrytus

I know that light slows down once it crosses the border between air and water. But once the light is in water, is its speed constant or does it slow down, much like a person, who has jumped into water from a good height, penetrates water slower and slower due to water resistance?

2. Apr 27, 2016

### Zephyr007

According to http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html ,

"Light is slowed down in transparent media such as air, water and glass. The ratio by which it is slowed is called the refractive index of the medium and is usually greater than one.* This was discovered by Jean Foucault in 1850."

So... Light could slow down through various materials. But when it travels through a vacuum, its a no because of Einstein's theory of relativity where light is always constant. (I'm not really good in Physics but I guess I'm good in researching XD. But still, I'm trying D:).

3. Apr 27, 2016

### houlahound

If the water doesn't change it's physical properties the speed is constant.

4. Apr 27, 2016

### Staff: Mentor

The speed is constant unless the temperature or composition of the water changes.

5. Apr 27, 2016

### houlahound

Eg

6. Apr 27, 2016

### houlahound

Density change with depth, prolly sugar;

7. Apr 27, 2016

### Tom MS

Yes yes yes. It slows down absolutely. There have been experiments to make it slow down more. Measurements such as a Planck length assume light going through a vacuum. Everything points to the fact that light slows down. Saying "The speed of light is constant" simply means that as an object approaches the speed of light, it needs asymptotically infinite energy to continue accelerating. Therefore, there is this restriction on how fast things can go. In addition, if you were traveling at half the speed of light and you were able to view how fast a photon (not moving with you but moving in the same direction) was moving, the photon would still be going the speed of light! Newtonian mechanics say that the photon should be going slower, but Einsteinian mechanics say otherwise.

8. Apr 27, 2016

### jbriggs444

The question posed here is not whether it slows down. It is agreed that it does. The question is whether it slows down abruptly at the boundary and then moves at a new, lower but still constant speed or whether, instead, it slows down gradually as it travels through the water.

The answer is that it moves at a constant speed, barring unusual setups where the refractive index of the water varies through its volume.

9. Apr 27, 2016

### houlahound

If the water is uniform does the intensity vary with depth?

10. Apr 27, 2016

### jbriggs444

If water is illuminated from above, light will be absorbed as it passes deeper and deeper into the water. This has nothing to do with the light slowing down as it penetrates further (which it does not).

11. Apr 27, 2016