Question

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

Answer 1

The rays from sun coming from infinity are parallel to principal axis after reflection they converge at a point, known as focus. Therefore, focal length (F) of concave mirror is 15 cm. 

And we know that same size, real and inverted image is formed by concave mirror when object is placed at centre of curvature = 2×Focal length (F). So to form same size image object will be placed at 15x2 = 30cm.