Study material 12th Physics - Wave Theory of Light Part II
description
Transcript of Study material 12th Physics - Wave Theory of Light Part II
• Live Webinars (online lectures)
with recordings.
• Online Query Solving
• Online MCQ tests with detailed
solutions
• Online Notes and Solved
Exercises
• Career Counseling
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.
• Ask Your Doubts
• For inquiry and registration, call
9011041155 / 9011031155.