Question

1 point
11. Focal length of a concave
mirror depends upon
a) distance of the objects
O b) distance of the image
O
c) both, the distance of object and
image
O
d) neither, the distance of the object
nor the distance of image.

Answer 1

Option d

Explanation

For any spherical mirror,

focal length is equal to half of the radius

of curvature.

f= R/2

So, the focal length of a

concave mirror depends upon the radius

of curvature of the mirror.

Answer 2

The focal length of a concave mirror depends upon "neither, the distance of the object nor the distance of the image". (option d)

• The focus or principal focus of a concave mirror is a point where all the light rays from infinity coming parallel to the principal axis converge and intersect.
• The point at which the principal axis intersects the mirror is called the pole.
• Now, the distance between the principal focus and the pole is called the focal length of the concave mirror. The focal length is half the radius of curvature and, given as $f=\frac{R}{2}$, where R = radius of curvature.
• Hence, the focal length of a concave mirror only depends on the radius of curvature.
• Thus, the correct answer will be 'option d'.

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