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A beam of light parallel to the principal axis is focused on a screen with the help of lens, forming a circular spot with a diameter of A. If we put a plane - parallel glass plate of thickness h and refractive index n between the lens and the screen, the diameter of the bright spot on the screen increases. The diameter of the lens is d and focal length F. Find the new diameter in mm. (Take F=10 cm, A = 1 cm, n=1.5,d=2 cm, h=3 cm) |
Answer»  `A/(F-y)=(A')/(F-y+h(1-1/n))` `A'=A[1+h/(F-y)(1-1/n)]` `F-y=(AF)/d` `A'=A[1+(DH)/(AF)(1-1/n)]` `=1[1+(2xx3)/(1XX10)(1-1/1.5)]=1.2 CM = 12mm` 
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