# Speed of Light

1. Apr 10, 2010

### wilha

We are able to see things around us due to the reflection of light, but while standing in front of a mirror we see our own reflection. And the moment we move our reflection moves as well

My question: Since light reflects off us, then into the mirror, and back to our eyes, does it mean that our reflection is technically slower than our movements and the difference is so minute that it goes unnoticed to our eyes ?

2. Apr 10, 2010

### Antiphon

Yes.

3. Apr 10, 2010

### wilha

Thanks, i asked my teacher the same question, he didn't have a clear answer.

4. Apr 11, 2010

### Born2bwire

The speed of light though is almost 30,000,000 m/s. So while the reflection is slower, the time lag is only noticeable over very large distances or with very sensitive methods of measuring. For example, the light emitted by the Sun takes around 8 minutes to reach the Earth in an often quoted example.

5. Apr 12, 2010

### onlyyou200548

i agree with Born2bwire. but the speed of light is actually almosl 300,000,000 m/s

6. Apr 12, 2010

### Born2bwire

Dammit, I should have stuck with scientific notation.

7. Apr 12, 2010

### sophiecentaur

I assume we're talking non-relativistic speeds here.
The word "slower" could be confusing and the issue should be cleared up. There will be a time lag of everything you observe of about 3ns for every metre away the object is. That doesn't mean the object will appear to be going slower - it will appear to start late and arrive late on its journey but the time taken will be the same so you would measure the same speed.
The frequency of light that you see will not be reduced, either, unless you are getting the doppler effect because the motion is away from you.

8. Apr 12, 2010

### Lightfuzz

To be more specific, the speed of light is 299,792,458 m/s.

9. Apr 12, 2010

### sophiecentaur

Oh, I'm sure you can be more accurate than that.

10. Apr 12, 2010

### f95toli

No, you can't. The speed of light is defined to be exactly 299,792,458 m/s

(maybe you were joking, but I figured it was worth this out)

11. Apr 12, 2010

### sophiecentaur

Which reminds me of a friend at School who told us he had calculated pi to 40 sig. figs.. We were very impressed until he showed us how he'd done it. He'd divided 22 by 7 and carried on till he done it to 39 places.

I bet he still wakes up in a sweat, some nights, remembering the withering reception he got from his 'best mates'.

12. Apr 12, 2010

### f95toli

Amusing story...But what made you think of that?

13. Apr 12, 2010

### sophiecentaur

sig figs

14. Apr 12, 2010

### DaveC426913

:rofl: :rofl:
There's an hour he'll never get back!

15. Apr 12, 2010

### silentbob14

yeah your reflection is kind of not only slower but also it's in the past ;]

16. Apr 12, 2010

### ejr

This looks a new excuse for narcissism, oh sorry I was doing a physics experiment! I wasn't REALLY just staring at myself.

17. Apr 12, 2010

### map19

You are all wrong. Unless you're in a vacuum with a front-silvered mirror.
Speed of light in air is 0.9997c
in the mirror glass it's about 0.66c.

read the classic SciFi by Bob Shaw where slow glass takes months or years to pass light.

Last edited: Apr 12, 2010
18. Apr 12, 2010

### sophiecentaur

Please explain why it will appear "slower".