Rays from Sun converge at a point 15 cm in front of a concave mirror. Where an object should be placed so that the size of its image is equal to the size of the object?

Rays from Sun converge at a point 15 cm in front of a concave mirror. Where an object should be placed so that the size of its image is equal to the size of the object? Correct Answer 30 cm in front of the mirror

Concept:

Concave Mirror

• A concave mirror is a type of converging mirror. It converges parallel rays of light falling on it on a point known as focus. 
• The center of the mirror is known as the pole. Distance from pole and focus is focal length.
• The radius of the mirror is twice that of the focal length. 

Explanation:

Image Formation when the object is at a radius

• If the object is at radius, the image formed by the concave mirror is inverted, real, and of the same size as the object.
• Here, focus f = 15 cm
• The size of its image is equal to the size of the object. So, the distance of the object must be twice of focal length (or at radius).
• So, image must be formed at r = 2f = 2 × 15 cm = 30 cm
• The correct option is 30 cm from the mirror.

Additional Information

The ray diagram of image formation by the convex mirror at different object positions is shown below.

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