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2. Proposed By Huygens

4.  
 
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
 
 
   
V V
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
 
  
  
  
 

6.  
 
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'
 
 
 
 
 
  

7. 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 AC
n
Sin r AC AD
CBC
n
AD C
CSin i
n Constant
Sin r C
  
 
  

8. 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.

10. 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

11. 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.

12. 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.

13. 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.

14. 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.

16. 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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