Question

Define focus
and focal length of a lens.

Answer 1

• Focus is that point on the principal axis where all the rays that are parallel to the principal axis converge after reflection or refraction.
• For the convex mirror, it seems that the light rays are diverging out from the focus.

• The Focal length is the distance between the pole and focus.

• The focal length for a concave mirror is negative.

      The focal length for a convex mirror is positive.

Answer 2

From the given question the correct answer is:

Focus:  

• All the rays that are parallel to the principal axis converge after reflection or refraction at the specific point on the principle axis is called Focus.
• The light rays are diverging out from the focus in the convex mirror.

Focal length

• The Focal length is the distance between the pole and focus.
• The focal length for a concave mirror is negative.
•      The focal length for a convex mirror is positive.