Define the term principal focus for a convex mirror.

What is the power of a concave lens of focal length $25\, cm$?

$(a)$ Define the optical centre of a spherical lens.
$(b)$ You are given a convex lens of focal length $30 \, cm$. Where would you place an object to get a real,inverted,and highly enlarged image of the object? Draw a ray diagram showing the image formation.
$(c)$ $A$ concave lens has a focal length of $20 \, cm$. At what distance from the lens should an object be placed so that it forms an image at $15 \, cm$ away from the lens?

How will you distinguish between a convex and a concave lens without touching them?

What type of image is formed by a plane mirror?

Define the following terms:
$(i)$ Angle of incidence
$(ii)$ Angle of reflection
$(iii)$ Plane of incidence