Solving Spherical Mirror Homework: Focal Length & Point of Incidence

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SUMMARY

The discussion focuses on the relationship between the focal length (PF) of a spherical mirror and its radius (R). It is established that when the aperture of the mirror is small, the point of incidence (P') approaches the pole (P), leading to the conclusion that the focal length is half the radius, expressed as PF = R/2. The geometric reasoning provided clarifies that as P' nears P, the distances FP and FP' become nearly equal, reinforcing the formula PF = Fc.

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Homework Statement



In the attached picture ,
It has been explained in my book that
" I the apeture of the mirror is small, the point P'( ponint of incidence) is very close to the point P( pole) ,Then PF(focal length)= P'F
:. PF =Fc
or PF=1/2R"
I don't get it !
Plzzzz help!
 

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By geometry they have shown that Fc = FP'.
If P' is away from P, FP' cannot be equal to FP because it is not the radius of the spherical surface. But if P' is very close to P, FP is nearly equal ti FP'. And hence you can show that focal length of the spherical surface is R/2.
 

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