Question

A concave mirror having focal length of magnitude 20 cm forms a real image at a distance of 60 cm from it. The object distance (in cm) is?

• A. -15
• B. -30
• C. +30
• D. +15

Answer 1

Answer:

B. -30

• Using the mirror formula:

• 1/f = 1/v + 1/u, where f is the focal length, v is the image distance, and u is the object distance.

• Given f = -20 cm (concave mirror)

• v = -60 cm (real image).

• Substituting these values, we get: 1/-20 = 1/-60 + 1/u.

• We get 1/u = 1/-20 + 1/60 = (-3 + 1)/60 = -2/60 = -1/30.

• Therefore, u =-30 cm.