FEW FACTS ON LIGHT

Different colours of light bend at different angles when they cross the boundary of two medium .

Speed of light is different in different media. It changes when light enters from medium to another medium.

Layers of atmosphere have different optical densities.

So light bends when it moves from layer of the atmosphere to the other.

Speed of light is less in water and glass than that in air.

A medium in which speed of light is more than that of the other medium is called optical rarer medium.

FEW FACTS ON LIGHT

A medium in which speed of light is less than that of the other medium is called optical denser medium.

A few optical devices are:

REFRACTION OF LIGHT WHAT IS REFRACTION?

Refraction is the bending of a wave when it passes from one medium to another. The bending is caused due to the differences in density between the two substances.

Hence, we can define,

“Refraction is the change in the direction of a wave passing from one medium to another”.

Some important terms Regarding Refraction of Light Interface : A plane surface separating two different medium is

called interface.

Incident Ray: A ray of light travelling in the first medium and striking the interface is incident ray.

Normal : A perpendicular drawn on the interface at the point of incidence is called Normal.

Angle of incidence : The angle made by the incident ray with the normal.

Refracted Ray : A ray of light travelling in the second medium after bending is called Refracted Ray.

Angle of Refraction : The angle made by the refracted ray with the normal.

Causes of Refraction

Refraction is caused due to the fact that speed of light is different in different mediums.

Lets see the speed of light in different media.

If the difference in the speeds of light in the two media is more, then the bending of light will also be more.

Why does the change in speed make the ray of light change its direction? A ray of light prefers to travel between two surfaces of a

medium following a path which can be covered by it in the least possible time.

Medium Speed of light

1. AIR 3 x 10 m/s

2. WATER 2.25 x 10 m/s

3. GLASS 2 x 10 m/s

Two cases of Refraction

Two cases of Refraction A ray of light is going from an optically rarer medium

to an optically denser medium.∠푟 < ∠푖

A ray of light is going from an optically denser medium to an optically rarer medium

∠푟 > ∠푖

Refraction in a parallel sided glass slab

What do you understand by lateral displacement? Lateral Displacement is the perpendicular distance

between the emergent ray from a glass slab and the incident ray produced forward.

Factors on which lateral displacement depends : Angle of incidence, I

Thickness of slab , t

Speed of light in glass (depending upon the quality of glass)

If angle of incidence I or thickness of slab t is more , lateral displacement d will also be more and vice versa.

If the speed of light is more in the slab, then the lateral displacement is less and vice versa.

Angle of incidence = Angle of emergence

i = e

Absolute Refractive Index The absolute refractive index is defined as the ratio of the

speed of light in vacuum and in the given medium.

The absolute refractive index should never be less than 1.

Absolute refractive index = ( )

i.e,

Refractive index is a ratio of same physical quantities, i.e, velocities, so it has no units.

A medium having lower value of refractive index is optically rarer than the medium having higher value of refractive index.

푛 =푐푣

Factors on which Refractive Index Depends on,

Nature of the optical medium

Colour or wavelength of light

Temperature of medium.

Relative Refractive Index The value of refractive index when light travels from one

medium to another medium is known as the relative refractive index.

Absolute refractive index 푛 of a material or medium 2 with respect to refractive index 푛 of another medium 1 is called relative refractive index.

It is denoted by 푛 .

푛 = = 푥 = =

푛 = =

Relative Refractive Index

In the same way, refractive index of medium 1 with respect to medium 2 is given by,

푛 = =

If we multiply both the equations we get,

푛 푥 푛 = 푥 = 1

푛 =

Relative refractive index is also represented as 푛 표푟 푛

푛 means refractive index of medium 1 with respect to medium 2

푛 means refractive index of medium 2 with respect to medium 1

Laws of Refraction of Light LAW1 : the incident ray, the normal at the point of incidence

and the refracted ray, all lie in the same plane.

LAW 2 : the frequency of light does not change when it travels from one medium to another.

Where,

Frequency is v

Wavelength is 휆

Velocity is u

Here frequency v is constant and wavelength or colour and velocity may change during refraction.

u = 휆v

Laws of Refraction of light LAW 3 (Snell’s Law ) : For a pair of media, the ratio of Sine of angle of

incidence to the sine of angle of refraction is constant.

If a ray of light passes from air to another medium, then this is a constant is the refractive index n of the medium.

= constant

= n = 푛