Question

For an object placed at a distance of 20cm from a
concave mirror, the image is formed at the same position What is the focal length of
the mirror​

Answer 1

Answer:

The answer is 10cm.

Explanation:

1. We know that, for an image to be formed at the same location with a concave mirror, the object must be placed at the center of curvature (C).

2. We also know that the center of curvature is twice the focal length of the mirror.

3. Then, the formula is f = C/2

   => f = 20/2

   => f = 10cm

Answer 2

the focal length will be 10 cm.... so,if an image is formed at the same position of the object which is placed, then the only possibility is for the object to be placed at center of curvature so the image is also formed at centre of curvature. so the image and object are both formed at 20 cm. so 20 cm is the radius of curvature of the mirror(radius of curvature-distance between the pole and centre of curvature). focal length is half of the centre of curvatre as it is the formula in all cases. so focal length is half of 20 cm and is hence 10 cm