# Question of light

1. Aug 17, 2011

### batballbat

i am having problem with the nature of light. The sun is lighting the earth. Suppose there solar system was emplty. Lets say we put the sun and earth in their respective places in a experiment. So initially it will take 8.3 minutes to illuminate the part of earth facing the sun. The earth rotates and the dark part will slowly face the sun. My question is does it take again 8.3 minutes to light the first fraction of the initially dark part.
Suppose we have a continuous source of water like a pipe and lets say its hitting a sphere ball. Initially it will take some time to reach the ball. And as the ball rotates the water wont take time to hit it (the back part).
So is the light we receive during sunrise was given 8.3 minutes ago or is it otherwise, that as soon as we face the sun, we receive the light?

2. Aug 17, 2011

### Staff: Mentor

It is both. As soon as we face the sun we receive the light that was given 8 minutes ago.

3. Aug 17, 2011

### Drakkith

Staff Emeritus
The water and light both have finite travel times. After you turn the hose on it will take a finite time for the water to initially hit the sphere, however you never turned the water off, so every molecule of water has had the same amount of flight time to get to the sphere. The situation with the Sun is identical.

4. Aug 17, 2011

### jewbinson

On this topic - if you shine a laser at the moon for 0.1s and then stop, will the light reach the moon, or do you have to hold down the laser till the light reaches the moon, as if the light behind other light particles is "pushing" the light in front of it.

I'm going to guess this post will be flamed for stupidity and I suspect I already know the answer, but I just want to make sure.

5. Aug 17, 2011

### Drakkith

Staff Emeritus
Each photon in the laser beam travels on its own and doesn't need anything to push it along. You would not need to keep the laser on to ensure it hits the moon.

6. Dec 25, 2011

### Danger

Jewbinson, you will never be 'flamed' on PF. As to your laser question, Drakkith nailed it down quite well. If it helps (hell, even if it doesn't) I'll offer an example that you might be familiar with. When you watch a fireworks display, the explosions are long over before you hear them. Because it is so incredibly fast, most people don't realize that the light from the shells also has a travel time, so the explosions are essentially over before you even see them. (There is a lot of overlap, though, because the travel time is less than the combustion time.)

Last edited by a moderator: Dec 25, 2011