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WAVE THEORY OF LIGHT

Proposed By Huygens

Reflection of light

o

PA'N    i   angle of incidence and    

NA'D    r  angle of reflection     

 In triangles A'BC and A'DC,       

   side A'C is common.          

 m A'BC m  A'DC  90           

   ABC       ADC

From the

geometry of the figure,

it can be proved that,

PA'N    BA'C and  

NA'D  DCA'      

  BA'C  DCA'   

i.e. NA'D   PAN   

i.e. r    i

Refraction of Light

PA'N    i    angle of incidence and,

N'A'D    r   angle of refraction

From the geometry of figure, it can be proved that,

PA'N    BA'C

i.e.    i       BA'C

Also,  N'A'D    DCA'

i.e.   r        DCA'

In right angled triangle, ABC, sin i =BC/AC

In right angled triangle, ADC, sin r =AD/AC

According to Snell’s law, the ratio of sine of angle of incidence to sine of angle of refraction is always constant. This constant is called refractive index (n) of the given pair of media.

1

2

1

2

Sin i BC ACn     

Sin r AC ADCBC

 n AD C

CSin i n Constant

Sin r C

Polarization According to wave theory of light, light is a

transverse electro-magnetic wave. Electro-magnetic waves are generated by varying electric and magnetic fields. These fields are mutually perpendicular and also perpendicular to the direction of propagation of the wave. Out of these two vectors, electric vector decides the optical characteristics of light, hence, called optical vector or light vector. When the variations in the electric field are taking place in one plane only, the light is called plane polarized or linearly polarized.

The propagation vector and electric vector

from a plane. The plane in which the electric

vector varies, is called plane of vibration

and the plane perpendicular to ii is called

plane of polarization.

An unpolarized light can be polarized in

different ways. Scientist Malus developed

The simplest method to polarize light by

simultaneous reflection and refraction. When a

beam of unpolarized light is incident on a plane

glass surface, part of the light is reflected and

the remaining part is refracted, i.e. transmitted

through the glass. The reflected light is partially

polarized. At a certain angle of incidence, the

reflected light is completely polarized.

In 1892, Sir David Brewster

discovered that,

when light is incident on a transparent

medium at a polarizing angle qP, the

reflected light is plane polarized, in the

plane of incidence. In this situation, the

reflected and refracted rays are

perpendicular to each other.

He also proved that, the tangent of

angle of incidence at which complete

polarization takes place by reflection, is

numerically equal to the refractive index

of the refracting medium.

If qP is the polarizing angle, ‘n’ is the refractive index of the refracting medium and r is the angle of refraction, from the figure, r = 90 - qP .n= Sin qP /Sin r = Sin qP /Sin (90- qP) = Sin qP /Cos qP

= Tan qP This is known as Brewster’s law. As the refractive index of a medium depends on wavelength of light, polarizing angle also depends on the wavelength.

Polaroids Certain films or crystals have

property to absorb one of the refracted rays and transmit the other. This property is called dichroism and the material is called dichroic.

A sheet of such material is known as polaroid.

Uses of Polaroid

1. In motor car head lights to remove headlight glare.

2. In three dimensional movie cameras.

3. To produce and analyze polarized light.

4. It is used as filter in photographic cameras.

5. In window of aeroplanes to control amount of light.

6. In polarizing sunglasses (goggles) to protect the eyes

from glare of sunlight .

7. They are used to improve colour contrast in old oil

paintings.

8. They are used in calculators , watches, monitors of laptops

which have LCD screens.

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