one question i can't get the answer

katchum · Post by **katchum** » Thu Jan 22, 2004 5:19 pm

I'm studying for exams and this is something I am wrestling with.

Two air planes fly to each other at 0.8 times light speed.

What's the speed of the second air plane, seen by the pilot in the first air plane?

And what 's the length of that second air plane seen by pilot one?

Is it 1,6 times light speed? I don't think so, because then you would get a negative length contraction.

Anyone here who can help me? Chess players should have such interests I hope.

I still have 3 days until exams.

tellymetwise · Post by **tellymetwise** » Thu Jan 22, 2004 9:07 pm

Proper speed formula:

(Speed1 + Speed2)/(1 + (speed1 * speed2)/c^2) = 1,6c / 1,64 = 0,98c

Length = Length_0 * sqrt(1- (v/c)^2)

katchum · Post by **katchum** » Sat Jan 24, 2004 11:53 am

So it is similar to: you are watching a plane flying inside of a plane at the time you are outside of the two planes.

But thanks, I thought it could be this formular.

katchum · Post by **katchum** » Sat Jan 24, 2004 12:23 pm

By the way which light is faster in glass? red light or blue light?

I would think red light. Because: n-red=1.330 and n-blue=1.342
So speed-glass=speed-air / n

Funny enough light speed in vacuum is stationary.

Another question.

speed of light in water = wave length in water x frequency

So when I use a red laser beam under water, it should shift into a blue laser beam.

Follow my theory.

wave length of red light in air = 600 nm
speed of red light in air = 300000km/s
speed of red light in water = 300000km/s / 1.5 = 200000km/s

This is a change of 2/3

so: wave length changes from 600 nm to 400 nm because frequency doesn't change.
And 400 nm is blue light.

So why doesn't it work? If i use the red laser when i'm in bath then i still see that red laser in my eyes.

Ok I can believe this is boring, so don't bother with it...

It would be too late anyway, still one day.

Post by **gmiller** » Sat Jan 24, 2004 5:07 pm

Only the speed changes, not the frequency. The peaks of the waves will be closer together, but since it's moving slower, they arrive at your eye at the same frequency.