What is Refraction ?

Bending of the light rays as it passes from one medium to another medium is known as refraction of light.

Light coming from the fish refracts (changes direction) when it hits the surface. A person above the water sees the apparent position of the fish closer to the surface than the real position of the fish.

What happens when light travels from one medium to another?

When light passes from one medium to another medium, it changes its direction.

Refraction through a rectangular glass slab

When the light is incident on a rectangular glass slab, it emerges out parallel to the incident ray and is laterally displaced. It moves from rarer to denser medium and then again to the rarer medium.

Lateral displacement depends on :

Refractive index of glass slab
Thickness of the glass slab

To know more about Law of Refraction or Snell's Law of Refraction, visit here.

According to Snell’s Law:

Light bends towards the normal when moving from rarer to denser medium at the surface of the two media.
Light bends away from the normal when moving from denser to rarer medium at the surface of contact of the two media.

Refractive Index

Refractive Index (n) is the ratio of speed of light in a given pair of media

The value of refractive index is dependent on the speed of light in two media. v₁ is the speed of light in medium 1 and v₂ is the speed of light in medium 2.

n = Velocity of light in medium 1/Velocity of light in medium 2

The refractive index of medium 2 with respect to medium 1 is represented as n₂₁.

n₂₁ = (speed of light in medium 1)/(speed of light in medium 2) = v₁/v₂

→ n₁₂ means refractive index of second medium with respect to first medium.
n₁₂ = v₂/v₁

Absolute and Relative Refractive Index

Refractive Index of one medium with respect to another medium is called Relative Refractive Index.
When Refractive Index is taken with respect to vacuum or air, it’s known as an Absolute Refractive Index.

Note :

a) If medium 1 is vacuum or air, then the refractive index of medium 2 with respect to vacuum is known as absolute refractive index of the medium.

n_m = (Speed of light in air)/(Speed of light in medium ) = c/v

where ,

c is the speed of light in air,

v is the speed of light in other medium and

n_m is the refractive index of the medium.

b) Refractive index of one medium is reciprocal of other’s refractive index in a given pair.

n₁₂ = 1/n₂₁

c) If refractive index of medium 1 w.r.t. air is given as ₁n^air, and

If refractive index of medium 2 w.r.t. air is given as ₂n^air.

Then, refractive index of medium 1 w.r.t. medium 2 = (₁n^air)/(₁n^air)

d) Refractive Index of diamond is the highest till date. It is 2.42. It means speed of light is 1/2.42 times less in diamond than in vaccum.

e) Optically denser medium: Out of two given media, the medium with higher value of refractive index.

Optically rarer medium: Out of two given media, the medium with lower value to refractive index.

When light enters obliquely from a rarer to a denser medium, it bends towards the normal.
When light enters obliquely from denser to a rarer medium, it bends away from the normal.

f) Refractive index of a medium does not depend on physical density.

To know about Total Internal Reflection, visit here .

Refraction at Curved Surfaces

When light is incident on a curved surface and passes through, the laws of refraction still hold true. For example lenses.

Refraction by Spherical Lenses

Lenses are defined as transparent materials which are bounded by two surfaces, out of which one or both can be spherical.
When both the two spherical surfaces bulge outwards, it is known as convex lens. They converge the light rays.
When the two spherical surfaces bulge inwards, they are known as concave lens. They are known as diverging lens.

Convex Lens	Concave Lens
Thin from corners	Thick from corners
Thick at center	Thin at centre
Converging	Diverging

Important terms related to Spherical Lenses

Pole (P): The midpoint or the symmetric centre of a Spherical lens is known as its Optical Centre. It is also called as the Pole.
Principal Axis: The imaginary straight line passing through the Optical Centre and the Centre of Curvature.
Paraxial Ray: A ray close to Principal Axis and also parallel to it.
Centre of curvature (C): The centres of the spheres that the spherical lens was a part of. A spherical lens has two Centre of Curvature..
Focus (F): It is the point on the axis of a lens to which parallel rays of light converge or from which they appear to diverge after refraction.
Focal length: Distance between Optical Centre and Focus.
Concave lens: Diverging lens
Convex lens: Converging lens

Rules for Image Formation by Convex lens and Concave lens

Ray Diagram of Images Formed using Convex Lens

Ray Diagram of Images Formed using Concave Lens

Uses of Spherical Lens

Applications such as visual aids: spectacles, binoculars, magnifying lens, telescopes.

Sign Convention for Spherical Lenses

• Sign conventions are similar to the one used for spherical mirrors, except that measurements are taken from optical center of the lens.

• Focal length of convex lens = Positive

Focal length of concave lens = Negative

Lens Formula

1/v - 1/u = 1/f

Magnification

The ratio of the height of an image to the height of an object is defined as Magnification.

m = h_i/h_o= v/u

Where,

u is object distance,

v is image distance and

f is focal length.

Magnification is represented by m,

h₀ is the height of the object and

h_i is the height of the image.

Power of a Lens

The Degree of Convergence or Divergence of light rays is expressed in terms of Power.

So,

Power of a lens is the reciprocal of its focal length i.e 1/f (in meter).

Power (P) = 1/v - 1/u = 1/f

Power of a lens is represented by letter P.

The SI unit of Power of a lens is Dioptre (D). 1 D = 1 m^-1

Note;

One dioptre is the power of lens whose focal length is one meter.
Power of convex lens = Positive
Power of concave lens = Negative
Power ∝ 1/(focal length or thickness)
Power of a lens combination (P) = P₁ + P₂ + P₃ .........
CBSE Grade 10 Science Chapter 10 - Light - Reflection and Refraction . Part 1 ( Reflection ) visit here .

CBSE Grade 10 Science Chapter 10 - Light - Reflection and Refraction . Part 2 ( Refraction )