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9.An object is 4cm high. It is desired to form a real image 2cm high at 60cm from the mirror. What type of mirroris needed and what should be its focal length?{Ans: - 40 cm)

Answer»

Height of object ho = 4 cm

Focal lenght of convex mirror = ? cm

Object distance u = ? cm

Image distance v = 60 cm

Image height hi = 2 cm

Magnification m = hi/ho = 2/4=1/2

v/u = 1/2

60cm/u = 1/2

u = 120 cm

mirror formula: 1/v + 1/u = 1/-f

1/-f = 1/v + 1/u

= 1/60 + 1/120

= 3/120 = 1/40

⇒ f= -40 cm



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