Under the guidance of his teacher on a sunny day, a student took a concave mirror of large aperture in his ha…

Under the guidance of his teacher on a sunny day, a student took a concave mirror of large aperture in his hand and directed its reflecting surface towards the sun. After that he directed the light reflected by the concave mirror on a thick sheet of white paper held close to the mirror. He observed a bright circular spot of light on the paper. Then he moved the mirror back and forth gradually until he got the brightest and smallest spot of light on the paper. The teacher said this sharp spot of light is, in fact, the real and inverted image of the sun and the distance between the mirror and this image is the focal length of this concave mirror.