This document discusses key concepts in optics including: - The focal length and power of convex and concave lenses, with power expressed in dioptres when focal length is in meters. - The mirror formula relating the focal length of a spherical mirror to its radius of curvature, and how magnification is calculated. - How light bends when passing from a less dense to more dense medium or vice versa. - The refractive index being the ratio of light's speed in a vacuum to its speed in a medium. - The lens formula relating focal length to power, with power measured in dioptres.

1. 10th standard

2.  All measurements are taken from the optical centre of the
lens.
 Focal length of a convex lens is positive and that of a concave
lens is negative.

5.  SI unit of power of a lens is ‘dioptre’. It is denoted by the
letter D.
 If f is expressed in metres, then, power is expressed in
dioptres.
 Power of a convex lens is positive and that of a concave lens
is negative.

6.  Mirror formula
 The focal length of a spherical mirror is equal to
half its radius of curvature. R = 2f or f = ½ R
 The magnification produced by a spherical
mirror is the ratio of the height of the image to
the height of the object.

9.  Denser medium to a rarer medium – bends away
from the normal.
 Rarer to a denser medium – Bends towards the
normal.
 Refractive index of a transparent medium is the
ratio of the speed of light in vacuum to that in
the medium.

10.  Lens Formula
 Power of a lens is the reciprocal of its focal
length.The SI unit of power of a lens is dioptre.