REFRACTION

Refraction of Light: The phenomenon of change in path of light when it passes from one medium to
another is called Refraction.

Cause of Refraction

The bending of light takes place when it passes from one medium to another because speed of light
changes from one medium to another. Speed of light is different in different media.

• Some examples of refraction

→ The bottom of swimming pool appears higher.

→ A pencil partially immersed in water appears to be bent at the interface of water and air.

→ Lemons placed in a glass tumbler appear bigger.

→ Letters of a book appear to be raised when seen through a glass slab.

Optically Rarer medium : A medium in which the speed of light is more is known as optically rarer
medium.

Optically Dense medium: A medium in which the speed of light is less, it is known as optically dense
medium.
 1. When a ray of light goes from a rarer medium to a denser medium, it bends towards the normal(at
the point of incidence).
• When a ray of light goes from air into glass, it bends towards the normal.
• Water is also an optically denser medium than air, so when a ray of light goes from air into
water, it bends towards the normal.
2. When a ray of light goes from a denser medium to rarer medium, it bends away from the normal(
at the point of incidence).
• When a ray of light goes from glass into air, it bends away from the normal.
• When a beam of light travelling in water enters into air, it bends away from normal.

Refraction Through A Rectangular Glass Slab

When light passes through a rectangular glass slab, it undergoes refraction at both the entry and exit faces:

1. Entry Refraction: As light enters the glass from air, it bends towards the normal due to the slower
speed of light in the denser glass.

2. Inside the Slab: The light travels in a straight line within the slab, still refracted but at an angle to
the normal.

3. Exit Refraction: Upon leaving the slab, the light bends away from the normal as it moves back into
the less dense air.

Key Points

• Parallel Faces: The entry and exit rays are parallel because the angle of incidence equals the angle
of emergence.

• Lateral Displacement: Although parallel, the light is displaced sideways from its original path,
depending on the slab's thickness, the angle of incidence, and the glass's refractive index.

In essence, while the light rays remain parallel, they shift from their initial path due to refraction.
Laws of Refraction:

I(i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point
of incidence, all lie in the same plane.

(ii) Snell’s law: The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for a
light of given colour and for a given pair of media.

sin i/sin r = constant

Refractive Index

• Refractive index of a medium(n21)

The relative refractive index of a medium 2 w.r.t medium 1 is the ratio of speed of light in medium 1 to the
speed of light in medium 2.

The refractive index depends on the nature of the material of the medium and on the wavelength (or
colour) of the light used.

If any two media are optically exactly the same, then no bending occurs when light passes from one
medium to another.

• Absolute Refractive Index of a Medium

The proportion of speed of light in vacuum to the speed of light in a material is called absolute refractive
index of medium.

• Optically denser-Greater the value of n less will be the speed of light.

• Optically rarer-Lesser the value of n greater will be the speed of light

REFRACTION BY SPHERICAL LENSES:

A lens is a piece of transparent glass bound by two spherical surfaces.

There are two types of lenses: (i)Convex and (ii)concave lens.

Rules for obtaining images formed by convex lenses:
Rules for obtaining image by concave lenses:
Image formation by concave lens:

SIGN CONVENTION FOR SPHERICAL LENS:

• 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:

m = h’/h= v/u
USES OF CONVEX LENS:

• Convex lens are used in spectacles to correct the defect of vision called hypermetropia(or long
sightedness)
• Convex lens is used for making a simple camera
• Convex lens is used as magnifying glass(or magnifying lens) ( by palmists, watchmakers etc)
• Convex lens are used in making microscopes, telescopes and slide projectors.

USES OF CONCAVE LENS:

• Concave lenses are used in spectacles to correct the defect of vision called myopia ( or short
sightedness)
• Concave lens is used as eye lens in Galilean telescope.
• Concave lens are used in combination with convex lens to make high quality lens system for optical
instruments.
• Concave lens is used in wide angle spyhole in doors.

POWER OF LENS:

• The power of a lens is a measure of the degree of convergence or divergence of light rays falling on
it.
• The power of lens is defined as the reciprocal of its focal length in metres.
P= 1/f

Where p is the power of lens and f is focal length of the lens in metres.

• A lens of short focal length has more power whereas a lens of long focal length has less power.
• The unit of power of a lens is dioptre (D)
• One dioptre is the power of a lens whose focal length is 1 metre.
• Power of a convex lens is positive
• Power of a concave lens is negative

POWER OF A COMBINATION OF LENSES:

If a number of lenses are placed in close contact, then the power of the combination of lenses is equal
to algebraic sum of the powers of individual lenses.

P = p1+p2+p3+…..

Example: If a convex lens pf power +4D, and a concave lens of power -10D are placed in contact, then
the resultant power will be: P=p1+p2

Where p1= +4D and p2= -10D

Therefore, P=+4+(-10)

P=-6D