Can a plane mirror be called a spherical mirror?

The image formed by a lens is always erect and diminished. What is the nature of the lens?

What is the absolute refractive index of common glass?

An object is placed at a distance of $20 \text{ cm}$ in front of a concave mirror of focal length $10 \text{ cm}$. Where will the image be formed?

The absolute refractive index of any medium is always $.....$

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