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Chapter 4:
Clever Wavefront Sensors
The wavefront or phase of a beam is only meaningful when we compare it to a reference. Prof. Leslie B. Smythe needs a reference beam to measure how the atmosphere has distorted the phase. Being an astronomer, he decides to use one of the stars in the sky. This is very clever because (1) there are a lot of stars in the sky, and (2) they are very far away. Except for the closest star, our Sun, they are all so far away that they only appear as points of light. They don’t seem to have any apparent size. The light seems to leave the same point on the star at the same time. This phenomenon, called coherence, is going to be a great benefit for our wavefront sensor. The light travels out from the star equally in all directions. The wavefront of the light from this star, as it moves outward, is a giant sphere. Because we know precisely what it is, we can use this light as a reference. Did you ever think that the world was flat? You probably did before you became a highly educated person and realized that, even though you know the truth now, it seems flat. (At least it does on the surface.) So, perception is reality? No, it’s just that the area around you, on this large sphere Earth, is so small that when it is compared to the 4000 mile distance to the center of the sphere, it appears flat.
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