Question

A 2.0cm high object is placed at a distance of 20cm from a spherical mirror which forms a real image at 40cm from the mirror. Calculate the focal length of the mirror and size of the image. Is the mirror converging or diverging.

Answer 1

Remember

Convex mirror never form real image. Thus mentioned mirror is Concave.

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Mirror Formula

➵ 1/f = 1/v + 1/u

➵ 1/f = 1/(- 40) + 1/(- 20)

➵ 1/f = - 1/40 - 1/20

➵ 1/f = (- 2 - 4)/80

➵ 1/f = - 6/80 or - 3/40

➵ 1/f = - 40/3 or - 13.3 cm

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m = - v/u = h2/h1

➵ - (- 40)/(- 20) = h2/2

➵ - 2 × 2 = h2

➵ - 4 = h2

Hence new image is 4 cm (magnified) and - sign shows that it's a real image.

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1. Concave Mirror

2. f = 13.3 cm

3. Image height = 4 cm