The answer to this question states: Light has to travel the distance between the two mirrors twice. So 2d + 2 = 58, d = 28
Can you please explain this?
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The question here, by the way is: "If incoming parallel rays, when reflected by the mirrors, converge 2 m to the left of the reflective side of the primary mirror, what is the approximate distance between the two mirrors?"
For this question, keep in mind what they mean by "incoming parallel rays...converg[ing]". This is simply the definition of the focal point -- the point at which parallel rays are all converged to. When we look at the diagram, that means the light has to travel a distance d between the "Primary Mirror" and the "Secondary Mirror" (an arbitrarily defined variable). Then, it has to go back a distance d, still converging, and travel another two feet past the Primary Mirror to converge at the "FOC/FOS/HRS" in the picture. You can see the light rays coming in -- just follow their path, and you'll see they go d + d + 2 while they converge, or 2d + 2 where d again is the distance between the Primary and Secondary Mirror.
If 2d + 2 is that distance, defined as the focal length, then it is equal to 58 m, as stated in the third paragraph. So d is 28.
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