Does light ever slow down?

Question

When light travels from the sun it travels at the speed of light, it takes around 8 minutes to travel to earth. So if i was one light year away from the sun, does it take 1 year and 16 minutes (8 minutes to travel to earth and 8 back) to travel from the earth?

Does light slow down due to it entering the earths atmosphere, bouncing off the earths surface etc?

Answer 1

As you've no doubt heard, "The speed of light is constant." What you may not have heard is that "The speed of light is constant for a given medium the light is traveling in." This is important because the speed of light depends on what medium it is traveling in. v = c ~ 186,000 miles/sec in a vacuum. It can travel much slower than that in other media, but never faster.

For all intents and purposes, the speed of light in our atmosphere is essentially the speed of light in a vacuum. But it is significantly slower in water, for example. This is why that pencil you stick in a glass of water seems to break where it enters the water. You are actually getting the reflected light from under the water a bit after the light on the pencil above the water.

If you were 1 ly from the Sun, you would be way outside our Solar system. By way of example, Earth is 8 light minutes from the Sun. So the light would take 8 minutes to reach you to give you a tan on the beach, but then continue on to that point outside our Solar system 1 light year away.

Answer 2

Can Light Slow Down

Answer 3

It would take between one year + 8 minutes down to one year minus eight minutes depending on whether the earth was on the near side or the far side at that time of year. And light slows down in matter (or seems to by taking frequent pitstops), but that doesn't have anything to do with what you're asking I think.

edit--no, it doesn't slow down enough to affect this problem. In fact, the 8 minutes doesn't affect the 1 light year enough to affect the problem. 1 light year doesn't really mean exactly one it means give or take a light month or two.

Answer 4

The speed of light in a vacuum is very close to 3x10^8 m/s. In a material it is 3x10^8 divided by the refractive index of the material. In air this makes little difference, but in diamond the speed is less than half that in a vacuum.

In your question, it depends whether the Earth is on the near side of the Sun or the far side. On the near side it takes 1ly - 8minutes, far side 1ly + 8minutes.

Light takes about 500 seconds to get from the Sun to Earth, this is the same as 500/(365.25*24*60*60) of a light year... or 0.00001585 of a light year... so in the grand scheme of things this isn't a great deal.

Answer 5

Yes, it can, but maybe not in the way you're thinking. Light travelling in space won't get slowed down - it will travel at the speed of light. There's nothing to stop it - even bouncing off something doesn't change it's speed, just maybe it's energy.

But if it enters a dense medium, like water, it CAN slow down. All at once - it doesn't really decelerate, it just moves at a different velocity, smaller than the speed of light.

Answer 6

OK this is how I saw your problem visually...
if that's right then my answer is relevant..

A....................B...C
Person is at A, 1 ly away is B, sun, and "8 mins travel time away" is earth, C
if you're measuring time it takes for a light ray that leaves B, goes to C, and comes back towards B and to A, then yes, it would take 1 yr and 16 mins, but that doesn't mean light slows down.

Answer 7

It seems to.