www.maxim-ic.com(Intensity is proportional to square of amplitude )vs (Intensity is inversely proportional to square of radius)
The two are not related except that they are referring to intensity.
First case : Intensity at a location is proportional to amplitude of wave at the same location. No possibility of confusion.
Second case: Do not use the relationship if possible. Use only Intensity is equal to Power divide area. If the power of source is the same then only I is proportional to radius square (area of the sphere)
Caution: Intensity of the point source has no meaning. It should be intensity at a certain distance from the point source
If intensity at certain distance from the point source is known. Then the power on a surface area S is equal to IS.Take note that all of the area S must be of the same distance from the point source. This will mean that S must be a curved surface!If S is not curved than it got to be very very small such that its intensity over the whole surface is constant.
Point to ponder: Imagine the projector as a point source and the screen is flat on the wall. Do you have the same intensity throughout the screen. A good projector may make it hard to detect. Maybe OHP may be a better illustration.
