Light is a form of energy. It satisfies law of conservation
of energy. According to this law, energy is neither created nor destroyed. It
just converts from one format to another format. Hence another format of energy
can be converted into light and the light also can be converted into other
formats of energies.
Light exhibits a wide variety of properties. They are
reflection, refraction, dispersion, interference, diffraction, polarization,
photo electric effect, Compton Effect, Stark effect and Zeeman Effect. To
explain all these properties of light, we have different theories of light.
If the size of the object interacting with the light is much
larger than the wavelength of the light with respect to the object, light
appears like a straight line and all the corresponding concepts were studied
under the topic called Ray optics.
If the size of the object is comparable to the wavelength of
the light, light appears like a wave and all the relevant topics are studied
under the name wave optics or physical optics.
The properties of light like interference, diffraction and
polarization can be explained only by assuming that light is travelling like a
wave.
Right from the beginning of the human evolution, human beings
are very much interested in understanding the properties of light. In this
process different theories are proposed. Each theory is having its own
advantages and failures and here we are going to have brief discussion of the
concepts.
Newton’s corpuscular theory
Among different theories, Newton’s corpuscular theory is
first one. According to this theory light is a stream of tiny particles called
corpuscles
.
This tiny particle carries energy by moving along straight
lines.
There are different colors of the light. According to this
theory, it is due to different sizes of corpuscles.
It is assumed that this tiny particles travels with enormous
velocity.
This theory is successful in explaining the concepts of
reflection and refraction.
Anyway this theory is having some issues. According to this
theory, velocity of the light is more in denser medium than the rarer medium.
But practically it is proved that velocity of the light is actually more in the
rarer medium and this theory failed to explain why.
As the light is leaving the source like particles, mass of
the source shall decrease but that is not happening.
This theory failed to explain how reflection and refraction
can happen simultaneously.
To address all these issues a new theory called Huygens wave
theory is proposed.
Huygens wave theory
According to this theory light is not traveling like a
straight line but it travels like a wave.
He assumed that light wave is mechanical in nature which
demands a medium for propagation.
Hence a invisible, highly elastic low-density medium is
imagined all-around and it is called ether medium.
He also proposed a principal called Huygens principle to
explain the wave propagation.
This theory is successful in explaining that velocity of the
light is more in the rarer medium than the denser medium and hence the problem
is solved.
This theory is also successful in explaining the phenomena
like interference and diffraction.
But this theory is also having some problems. It fails to
explain the concept of polarization.
It also failed to explain photo electric effect and all other
modern-day effects of light.
Experimentally it is proved that there is no imagined medium that
is filled over the entire space and hence there is no exact way to explain how
the wave is propagating from one place to another place.
To solve all these problems electromagnetic wave theory is
proposed was proposed by Maxwell.
Electromagnetic wave theory
According to this theory light propagates like a
electromagnetic wave and it doesn't need any medium for propagation.
A variable electric field generates a variable magnetic field
around it and the propagation of the wave is perpendicular to both of them.
Because it is a non-mechanical wave we don’t need any medium
for propagation itself.
The wave equations that we have written in waves and
oscillations are valid in explaining the propagation of electric and magnetic
fields.
Basing on the permittivity and the permeability of the vacuum
we can define the velocity of the light.
It is proved that velocity of light is constant and is
maximum in the vacuum. According to this theory, velocity of light in any other
is less than the velocity of the light in vacuum.
This theory is successful in explaining the concepts like
interference, diffraction and polarization.
At the same time this theory is unable to explain how the
photoelectric effect and Compton Effect are happening.
To address this problem Plank’s quantum theory is proposed.
Plank’s quantum theory
According to Plank’s quantum theory, light is not travelling
like a wave rather it is travelling like a small packets of energy called
quanta.
Energy is not emitted by the source continuously but
discreetly in the form of wave packets.
This theory is successful in explaining the modern-day
properties of the light like photoelectric effect and Compton Effect. But
simultaneously it is unable to explain the concepts like interference,
diffraction and polarization.
Hence we do not have a unified theory which can explain all
the properties of light. So it is assumed that light travels like a wave and
exhibits certain set of properties and when it interacts with someone, it interacts
like a particle and exhibits the modern-day properties. This concept is called
dual nature of the light.
Huygens principle
According to this principle, every point the primary wave
front behaves like a secondary source and propagates the light in the forward
direction.
Wave front is the locus of all the points that are in the
same phase. The wave front from a point source is spherical in nature where as
the wave front from the cylindrical source is cylindrical in nature. As we move
far away from the point source, the radius of the spherical wave front increases
to a larger value so that it appears like a plane wave front.
Let us consider a point source at a particular position.
Light starts from the source and it travels in all directions simultaneously
with the same velocity. After a specified time, the distance traveled by the
light is same in all directions from the source. The line joining of all the
points is called a wave front. Every point on this primary wave front behaves
as if like it is a source. Thus each point again propagates the light in the
forward direction.
Basing on this concept reflection and refraction are
explained successfully. When the light is reflecting, angle of incidence is
equal to angle of reflection. When the light is refracting, it obeys a law called
Snell’s law.
According to Snell’s law, the refractive index of the medium
to where the light is going with respect to refractive index of the medium from
where it is coming is equal to ratio of sin
angle of incidence to the sin angle
of refraction.
We can explain the refraction basing on this principle as
shown below. As the light Ray is moving from rarer medium to denser medium, it
moves towards the normal. The second light Ray travels more distance to reach
the boundary that is separating the two media. Basing on some simple
mathematical rules we can derive equation for the width of the refracted light
beam as shown below.
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