What is the focal length of the objective of an astronomical telescope as compared to that of the eyepiece?

State the relation between object distance,image distance,and focal length of a concave or convex mirror. $A$ concave mirror produces a two times magnified real image of an object placed at $10 \ cm$ from it. Find the position of the image.

Define linear magnification produced by a spherical mirror. Also,give its relation.

$(a)$ 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.

$A$ candle is placed at a distance of $10 \ cm$ from a plane mirror. At what distance from the mirror will the image of this candle be formed (in $cm$)?

$A$ virtual,magnified and erect image can be formed by a concave mirror.