Re: is the intensity of light is affected when the source is moved ?

Your question is interesting and subtle.

As a light source moves toward an observer, two things happen:

     1.  The light gets closer, so that it will appear more intense.
     2.  The Doppler effect will shift the light to shorter wavelengths

The inverse square law says that as a source moves closer to an observer, the 
intensity increases as the inverse ratio of the square of the distance moved.  
This is easy to see for a source which emits light in all directions uniformly: 
 if it emits a constant number of photons, the number per unit area (the 
intensity!) crossing a spherical surface around the source depends on the area 
of the sphere, which is 4 X Pi X R^2 (4 pi r-squared).  As the source gets 
closer, R goes down, so the intensity goes up!

Also, since shorter wavelength photons have higher energy, if you do an 
experiment which measures photon energy you will observe higher values and the 
power per unit area seen by an observer will increase.

Good question... hope this helps your understanding of how light works!

Steve Guch

[Moderator note:  I think that the photon number will not change from one frame 
to another.  As Steve says, for an approching source, they are seen to increase 
in frequency and energy.]