An object is placed 30 cm before a concave mirror of focal length of 20 cm to get a real image. What will be the distance of the image from the mirror?

Question

Question

This question was previously asked in

RRB ALP CBT I 30 Aug 2018 Shift 3 Official Paper

Answer 1

The correct answer is 60 cm.

Key Points

In a spherical mirror, the distance of the object from its pole is called the object distance (u), and the distance of the image from the pole of the mirror is called image distance (v).
Focal length (f) is the distance between the principal focus and the pole of the mirror.
- The image distance is given by the formula, \(\mathbf{\frac{1}{v} = \frac{1}{f}-\frac{1}{u}}\)
In the given question, the object distance is given as 30 cm and the focal length is 20 cm.
- So, the image distance is, \(\mathbf{\frac{1}{v} = \frac{1}{-20}-\frac{1}{-30} = \frac{-3+2}{60} = \frac{-1}{60}}\)
- So, the image distance, v = -60 cm (real image). Hence option 1 is correct.

Additional Information

The principal focus is the point on the principal axis at which light rays parallel to the principal axis, after reflection from the mirror.
Concave mirrors are used in torch lights, searchlights, vehicle headlights, shaving mirrors, etc.