PDA

View Full Version : Questions about light



Trevor.L.James
2007-Jan-10, 03:06 AM
So why doesn't light slow down?
It is affected (bent) by gravity and has some particle attributes so why would it not be slowed by friction or gravity and loose its momentum as it rockets on for millions of years? What gives it the energy to keep it up?
If it does slow down what does it become?

morlankey
2007-Jan-11, 09:26 AM
Strangely enough, the speed of light (in a vacuum) is the only thing that remains constant in the universe. Even time isn't constant.

However, the speed of light can change depending on the medium. It travels slower in glass for instance.

A photon, I believe, is pure energy and has no mass. This is why it doesn't need any external 'push' to keep it going.

swansont
2007-Jan-11, 02:22 PM
A photon, I believe, is pure energy and has no mass. This is why it doesn't need any external 'push' to keep it going.

Things only need an external push when there is some force that would tend to slow it down. The misconception that an object travelling at a constant speed must be pushed to maintain that speed is not uncommon. But without a mechanism to bleed off energy, no extra source is necessary.

The thing is that if a photon loses energy for some reason, you will still have a photon, and it will still travel at the same speed. That's how photons behave.

jamesabrown
2007-Jan-11, 03:27 PM
Suppose a photon is traveling through space at the speed of light, then encounters a medium that slows it down (like glass, or a planet's atmosphere) but then continues through the medium and back out into the vacuum of space. Would the photon's velocity increase back up to the speed of light? Or does it continue on at its reduced speed?

While I'm thinking about it, what exactly stops a photon from continuing on? They can pass through the molecules of glass or air or water, but at a slower rate, and yet they can't pass through the molecules of rock or even the back of my eyeball. Neutrinos are supposed to zip right through us without even slowing down, but photons can't. Why not?

morlankey
2007-Jan-12, 05:41 AM
While I'm thinking about it, what exactly stops a photon from continuing on? They can pass through the molecules of glass or air or water, but at a slower rate, and yet they can't pass through the molecules of rock or even the back of my eyeball. Neutrinos are supposed to zip right through us without even slowing down, but photons can't. Why not?

Neutrinos are so illusive because they are un-affected by things like electric or magnetic fields. The only way we can detect them is by hoping that they'll accidently collide right into the nucleus of an atom. This happens rarely because atoms are mostly empty space (if you think of a football field being the radius of the electron cloud, the nucleus would be about smaller then a tennis ball).
Light, however, is attracted to electrons. So, instead of passing right through an atom, it will be atracted by it's free electrons.
Some things have a very stable structure with regards to electrons (for example, pure crystals) and, thus, the photon finds it difficult to find free electrons to collide with. So it passes through.

Suppose a photon is traveling through space at the speed of light, then encounters a medium that slows it down (like glass, or a planet's atmosphere) but then continues through the medium and back out into the vacuum of space. Would the photon's velocity increase back up to the speed of light? Or does it continue on at its reduced speed?

The amount of energy in a photon remains constant and is governed by it's frequency. When it passes through something dense, for instance glass, it's wavelength decreases because it's speed decreases. It's frequency remains the same, however. Once it exits the glass, it's wavelength increases again and it accelerates back to the speed of light.

I'll try and think of a simile. Imagine, say, driving on a road with the accelerator pedal pressed to the floor. Then you suddenly hit some sand at an angle. Your left tyre hits the sand first and slows down. This causes the car to turn left until the right tyre hits the sand. Now the car is going in a straight line at an angle compared to where it entered the sand. Your accelerator is still pressed to the floor, your engine working just as hard, however, you are going much slower then when you were on the road. Then you hit the road again. You accelerate and your car turns and ends up driving in the same direction as it was originally.

Amber Robot
2007-Jan-12, 05:35 PM
They can pass through the molecules of glass or air or water, but at a slower rate, and yet they can't pass through the molecules of rock or even the back of my eyeball.

They can, but it depends on the wavelength of the photon. There are wavelengths of light that can't pass through the molecules of glass or air or water. Ever gotten an X-ray? They can pass through your whole body, except for the bones.

Standard glasses tend to be opaque in the ultraviolet, and special materials are needed to create optics that work in the near-ultraviolet. Similarly for the infrared.

The wireless internet works because certain wavelengths of light can go through the walls of buildings and houses.