What is the relation between the radius of curvature $(R)$ and the focal length $(f)$ of a spherical mirror?

$(a)$ What happens to a ray of light when it travels from one medium to another having equal refractive indices?
$(b)$ State the cause of refraction of light.

The laws of reflection are true only for plane mirrors and not for curved mirrors.

The magnification of a real image formed by a concave mirror is always ...

$A$ student focused the image of a candle flame on a white screen by placing the flame at various distances from a convex lens. He noted his observations:

Observation	Distance of flame from the lens $(cm)$	Distance of screen from the lens $(cm)$
$(i)$	$60$	$20$
$(ii)$	$40$	$24$
$(iii)$	$30$	$30$
$(iv)$	$24$	$40$
$(v)$	$15$	$70$

$(a)$ From the above table, find the focal length of the lens without using the lens formula.
$(b)$ Which set of observations is incorrect and why?
$(c)$ In which case will the size of the object and the image be the same? Give a reason for your answer.

What type of mirror is used to obtain a real image?