Question

Find the focal length of a concave mirror that produces four times larger real image of an object held at 5cm from the mirror.

Answer 1

Answer:

6.67 cm

Explanation:

Given

• h (image) = 4 x h (object)

Finding magnification

• m = h (image) / h (object)
• m = 4 x h (object) / h (object)
• m = + 4.0

Finding image distance

• m = - (image distance / object distance)
• 4 = - (v / 5)
• 4 = -v/5
• v = -20
• v = -20 cm

Mirror formula

• 1/f = 1/v + 1/u

Solving

• 1/f = 1/-20 + 1/5
• 1/f = -1 + 4/20
• 1/f = 3/20
• f = 20/3 cm
• f = 6.67 cm