Spherical mirrors with small apertures, the radius of curvature (R) is found to be equal to twice the focal length (f). Mathematically, this relationship is expressed as R = 2f. This implies that the principal focus of a spherical mirror ...

The aperture of a spherical mirror is the diameter of its reflecting surface, which has a circular outline. The aperture is the distance represented by MN in the figure. The reflecting surface is, by and large, spherical in shape.

The distance of the image from the mirror provides an approximate value for the focal length of the concave mirror. This distance represents the point where sunlight is effectively concentrated, leading to the ignition of the paper.

The mirror is responsible for concentrating sunlight. The bright spot on the paper is the image of the Sun formed at the focus of the concave mirror.

The paper starts to burn because the concave mirror converges sunlight at a sharp, bright spot on the paper, essentially creating an image of the Sun. This focused sunlight generates enough heat to ignite the paper.