1. An object placed 20 cm from a concave mirror forms an image that is 3 times a

Question

1. An object placed 20 cm from a concave mirror forms an image that is 3 times as large as the object on a screen. Find the radius of curvature of the mirror.

a. 30 b. 16.5 c. 40 d. 15

someone said it was 30 because s = -20, but i dont see why its not +20?

2.A hemispherical glass paperweight with R=10 cm and n = 1.5 has a bubble blemish located 8 cm from the surface (vertex). At what distance from that surface does the image of the blemish appear to the outside observer?

a. +3.2 cm b. +4.2 cm c. -7.3 cm d. -6.9 cm

someone said it was C but I also dont understand why

Thank you!

Explanation / Answer

m = - di/do = -3

di = 3 do

do = 20 cm and di = 60 cm

1/f = 1/di + 1/do

1/f = 1/20 + 1/60

f = 15 cm

and R = 2f = 30 cm ........Ans

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