# Does water slow light down?

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?

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:).

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

hipokrytus
Drakkith
Staff Emeritus
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?

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

hipokrytus
Eg

Density change with depth, prolly sugar;

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?
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.

jbriggs444
Homework Helper
Yes yes yes. It slows down absolutely. There have been experiments to make it slow down more.
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.

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

jbriggs444