The two laws of refraction are :
(i) The Incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.
(ii) The ratio of the sine of angle of incidence (i) to the sine of angle of refraction (r) at a point in a medium is constant. i.e., ‌

sin‌i

sin‌r

= Constant
Absolute refractive index: It is the ratio of speed of light in vacuum (or air) to the speed of light in the given medium.
Refractive index (n)
=‌

‌ Speed of light in vacuum or air ‌(c)

‌ Speed of light in medium ‌(v)

OR
The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it. It is also defined as the reciprocal of its focal length in metres.
The S.I. unit of power is dioptre (D).
Focal length of a convex lens is positive.
So,
F‌convex ‌‌=40‌cm=‌

40

100

m
‌ Power ‌‌=‌

1

‌ Focal length ‌

=‌

1

40 100

‌=‌

100

40

=+2.5D
So, lens having focal length 40‌cm is convex. Focal length of a concave lens is negative.
F‌concave ‌=20‌cm=‌

−20

100

m
So, Power
=‌

1

‌ Focal length ‌

=‌

1

−20 100

=‌

−100

20

=−5D
So, lens having focal length −20‌cm is concave.