# Light travelling in space

1. The problem statement:

My problem is regarding doppler effect for light . I have read in text book that blue/red shift occurs in case of light when it travels from a star to earth .

Q no 1:
Now, in case of sound it is easy to under stand that sound wave comprise of rarefaction and compression , in this way as a result of continuous compression and rarefaction , the energy of sound waves get reduced and that reduction appears as wave length shortening.

But in case of light how wave length gets reduced?
Is energy lost during motion?

Q2:
If light travels travels from point A in space to point B, does it approach point B with a frequency and wavelength different from frequency and wavelength it had while initially moving?

HELP!

i don't think you understand the doppler effect properly

In sound the motion of the emitter respective to the receiver is what causes the Doppler effect. In the simplest terms: If something stationary emits a signal every second then the distance between the signals would be its velocity 'v' times one second. if the emitter then starts moving with velocity 'b' in the direction of the signal then the distance between each signal would be v-c times one second i.e. it's wavelength has been shortened. So it is the motion as well as the gravitational field of the stars that governs the redshift

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What I am saying is that as wavelength of a sinusoidal wave is the spatial period of the wave – the distance over which the wave's shape repeats , then frequency or number of waves passing through a point should change , not wavelength.

tiny-tim
Homework Helper
hi rocephin! Now, in case of sound it is easy to under stand that sound wave comprise of rarefaction and compression , in this way as a result of continuous compression and rarefaction , the energy of sound waves get reduced and that reduction appears as wave length shortening.

the sound wave does not lose energy

the number of wave-tops passing a stationary point per second is obviously less than the number passing a point that is moving towards the wave …

ie the frequency increases (blue-shift)

and if the point is moving away, the frequency decreases (red-shift) …

(same for any wave, including light)

it's just geometry! 