Light slows as it passes through water or glass. How can it return to light-speed when it exits?

Great question-- it's one that baffled physicists from Newton until the late 19th century.

First and foremost, don't expect light to behave like a normal object. It just doesn't. The rules that govern how light works aren't the same ones that govern the world as we generally perceive it. In fact, the strange nature of light was a major driving force in the development of both quantum mechanics and general relativity.

The equations that describe light are called Maxwell's Equations of Electrodynamics. They are based on they way that electrical charges and magnets work (remember that light has both electrical and magnetic field components) Maxwell's Equations tell us that the speed of light depends on the permittivity and permeability of space. Permittivity and permeability basically measure how easily a magnetic or electric field can penetrate a material. Since light is a combination of electric and magnetic fields, it's not too surprising that the speed of light depends on the permittivity and permeability of the material it's in.

It turns out that glass has a different permittivity and permeability than vacuum, so light must travel at a different speed in glass than vacuum.

Imagine a swimmer swimming first in a pool of water and then in a pool of honey. The swimmer will swim much faster in the water than the honey. Light is sort of the same, making it move faster in an empty vacuum than inside of a material.

Hope this helps.

Try looking at it this way. Suppose you were running with some velocity from location A to get to location B. Now, exactly in the middle of your chosen path, you come across an obstacle in the form of a huge crowd of people. Now, while crossing these people, your speed is the same, but just your motion is very restricted due to very low throughput. The moment you cross the crowded area, you regain your original speed.

Similarly, the amount of energy in the light particle is the same, but the velocity is low - that could probably be attributed to collisions with the molecules of the matter. The compensation for the decrease of velocity at constant energy can probably seen in the form of a decrease in the wavelength - because in the equation

Velocity = frequency x wavelength,

a decrease in velocity can only be compensated with a corresponding decrease in wavelength at constant frequency.

ericy919's attempt was commendable, but unfortunately incorrect. The phenomenon is also observed at normal incidence, when refractions are minimal. The light is still travelling in a straight line.

There are two things happening at the microscopic level while a photon of light is traveling through water (for example):

(1) Sometimes it is traveling at light speed in the tiny spaces between the water molecules.

(2) And sometimes it collides with, and has its energy absorbed by, a water molecule in its path. When this happens there is a momentary delay until the excited water molecule releases it's excess energy in the form of a photon similar to the one that excited it.

The emitted photon always travels at light speed when it is between particles. It is the little delays occasioned when the photon collides with, is absorbed by, and re-emitted from the molecules of the water that result in the average speed observed at the macroscopic level to be less than what light speed is defined to be in a perfect vacuum.

That is an interesting question, I don't know the answer for certain, but i will take a guess, I may be wrong so don't take it to heart.

I think that the light does not slow down in water, it just reflects off of a lot more, so it is going around and around inside the water, but still going as fast, like if you walk from one side of the room to another, but walk in circles the entire time, it takes more time, but the same speed. I think that is called the angle of refraction. I hope that helps.

Yes, the speed of light is slowed when passing through ANY medium depending on the density of the medium. Light is made up of photons and photons are massless - this was proven by Higg's mechanism. and just because it's slowed down, that doesnt mean that it gained mass. although photons may have a non-zero mass in some superconductors but that's off-topic.

Light is either a wave or a particle. It is considered to be energy, since it has no mass when it behaves like a wave. It is not an object, just energy traveling. A photon has an initial energy. When it is traveling in vacuum, it has no obstacles, nothing to go through. So with its initial energy, it can travel at high speed. When it is traveling through air, there are some molecules that it has to go through. When it is going through a molecule, it has to spend more energy than vacuum because it is going through a molecule. When it is water, there are more molecules to go through but their formation isn't as tight as solid, so it is a little slower than air, but faster than solid. When it is traveling through a solid, there are a lot of molecules to go through that are tightly together, so it takes more and more energy to pass them. Photons still have the same energy, but they are spending more energy to go through molecules, so they use 70% of the energy to speed in glass, when they can use 100% in vacuum.

Here is an analogy: You are driving on a highway. The road is very good, no holes, no bumps so you can go very fast. When you are going through a neighborhood, there are speed bumps, so you have to slow down every time. You are spending more effort to go through them, you have to apply the brakes, then put the pedal on the metal again. When there is no road, you are going very slowly, because there are holes on the road, stones that you go over and so on. But in every case, it is the same driver, same car. Everything is the same except the road.

Light can gain its speed back, because it is not an object. It has the same energy, traveling in whatsoever material. But it has to use more and more energy to go in glass, then in air. So when it gets to air again, it has the same energy, it speeds up.

Light is both a Wave and a Particle. Light is known to have wave-particle duality.

Meaning if you perform a wave test on light your results will indicate light is a wave. Likewise, if you perform a particle behavior test, your results will indicate that light is a particle.

The short answer is that light's not an object, so it has no inertia.