Amplitude of light and intensity

[hangover]

Homework Statement

Light is emitted from a point source. How does amplitude of light change when light has traveled from R to 2R?

Homework Equations

Energy=(1/2)kA^2 where k is constant and A is amplitude
Intensity=(Energy/time)/area

The Attempt at a Solution

My teacher said that:
As area=4piR^2, intensity of light decreases when light travels to 2R. Therefore the amplitude of light decreases. Isn't the energy be constant while intensity changes? I am confused as he said that amplitude of light decreases as light moves further from the source?

What is the cause of the phenonmenon that light becomes dimmer in further distance? Decrease of intensity or decrease of amplitude or both?

Thanks for answering my question.

 

[Fightfish]

It is the decrease in amplitude that results in the decrease in intensity. (since frequency is constant)
The energy of the light does not decrease when it spreads out usually, but becomes spread over a larger area. (for a point or isotropic source, think of the wavefronts as spherical shells) This is the reason why the intensity (and amplitude) detected further away from the source decreases, because the power received at that point is reduced as the wave spreads out and distributes its energy over a larger region.