A cell phone that is 8.90cm tall is placed in front of a convex mirror. The imag

Question

A cell phone that is 8.90cm tall is placed in front of a convex mirror. The image of the phone is 7.80cm tall and is located 14.8cm from the mirror. What is the magnitude of the mirror's focal length? a.) 105cm b.) 120cm c.) 175cm d.) 85cm e.) none of these 11.) 13 points] When a person stands 40cm in front of a cosmetic (concave) spherical mirror, the upright image is twice the size of the object. What is the magnitude of the focal length of the mirror? a.) 10cm b.) 20cm c.) 40cm d.) 80cm e.) none of these

Explanation / Answer

10)

object distance p = ?

image distance q = -14.8 cm

object height ho = 8.9 cm

image height hi = 7.8 cm

magnification hi/ho = -q/p

7.8/8.9 = 14.8/p

object distance q = 16.9 cm

1/f = 1/p + 1/q

1/f = 1/16.9 - 1/14.8

focal length f = -119 cm

magnitude of focallength f = 120 cm

OPTION (B)

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11)

object distance p = 40 cm

image distance q = ?

object height = ho

image height = hi = 2*ho

magnification m = hi/ho = -q/p

2 = -q/40

object distance q = -80 cm

1/f = 1/p + 1/q

1/f = 1/40 - 1/80

focal length f = 80 cm

OPTION (d)

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