OPTICS
MD. ABDULLAH AL ZAMAN
Lecturer (Physics)
Department of Textile Engineering
Northern University Bangladesh
Dhaka 1213
Proyashzaman@gmail.Com
01627041786
O
ptics is the branch of physics that studies the behavior and properties of light, including its interactions with matter and
the construction of instruments that are used in detecting the interactions. Optics usually describes the behavior of
visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation
such as X-rays, microwaves, and radio waves exhibit similar properties.
Behaviors or properties of light includes
1. Reflection 2. Refraction 3. Diffraction 4. Interference 5. Scattering 6. Dispersion 7. Polarization
There are four theories which guided the evolution of our understanding of the nature of light. The theories are,
1. Corpuscular theory
2. Wave theory
3. Electromagnetic theory
4. Quantum theory
1. Corpuscular theory: This theory was first postulated by the ancient Greeks and later it was supported by Isaac
Newton. This theory suggests that a luminous body continuously emits tiny light and elastic particles called
corpuscles in all direction. These particles are so small that they can even pass through the interstices or gaps and
when they are fell on the retina of our eyes they produces the sensation of vision.
2. Wave theory: Christian Huygens brought this theory. According to this theory from a luminous body light energy
is transmitted or distributed equally in all direction in the space in the form of waves in hypothetical medium
named ether.
3. Electromagnetic theory: This theory was provided by Maxwell. He showed that the electromagnetic waves
travel with the speed of light which is 3× 108 and hence drawn the most important conclusion that light wave
itself an electromagnetic wave.
4. Quantum Theory: According to Max Plank thermal radiation is emitted or absorbed intermittently by indivisible
amounts of energy packets called quanta (plural Quantum) and each quanta is associated with 𝒉𝞾 amount of
energy (E).
➢ HUYGEN’S PRINCIPLE:
Huygen’s principle introduces the idea of Wave front.
1. Every part on a wave front acts as a secondary source of disturbance. Secondary wavelets spread in all directions
from these new sources. The secondary wavelets are spherical and have the same frequency and velocity as the
original wave.
2. The surface which touches all the wavelets from the secondary sources, gives the new position of the wave front.
OPTICS
➢ FERMAT’S PRINCIPLE:
Fermat’s principle of least time states that when a light ray travel between two points, it follows, out of all possible paths
between those two points, a path that requires the least amount of time.
Suppose a light ray has to move from point P to Q. It can travel through any of the paths L 1 , L2 and L3 but it will only travel
through the path that will require least amount of time. For a homogenous medium light will travel in the straight line.
Figure 1: Fermat's principle of least time.
➢ IMPORTANT TERMS
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•
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Phase of a wave: The phase of a wave is defined as the measure of the variables associated with the wave at any
instant.
Frequency: The number of cycles a wave undergoes per second is called frequency.
Monochromatic waves: Waves having a single frequency and wavelength are called monochromatic waves. For
example sodium lamp is a source of monochromatic wave.
Coherent source: If two or more waves maintain a constant phase difference over a long distance and time, then they
are said to be coherent. Sources that produce coherent waves are called coherent sources.
Superposition of waves: When two or more waves overlap, the resultant displacement at any point at any time may
be found by adding the instantaneous displacement that would be produced at the point by the individual waves if
each were present alone.
➢ INTERFERENCE
The phenomenon of redistribution of light energy due to the superposition of light waves from two or more coherent
sources is known as interference.
Conditions for interference
1.
2.
3.
4.
The waves from the two sources must be of same frequency.
The two light waves must be coherent
The path difference between the overlapping waves must be less than the coherence length of the waves.
If the two sets of waves are polarized then their planes of polarization must be the same.
Young’s double slit experiment
Young’s double slit experiment utilizes monochromatic light source like sodium light with frequency 5893 Å(Angstrom).
In the figure, first, light passes through slit S and then passes through slits S1 and S2. Cylindrical wave fronts are produced
in the process. Dark and bright fringes are observed in a screen at a certain distance.
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OPTICS
Figure 2: Young's double slit experiment
➢ FRESNEL BIPRISM
It is a tool used by Fresnel to show interference phenomenon. A biprism consists of two prisms of very small refractive
angles joined base to base. In practice a, a thin glass plate is taken and one of its faces is ground and polished till a prism
is formed with an obtuse angle of about 1790 and two side angles of the order of 300.
Figure 3 (a) : Biprism and
monoprism
Figure 3(b): Biprism
construction
Figure 3(c): Biprism
➢ DETERMINATION OF WAVELENGTH OF LIGHT WAVES BY A BIPRISM
S1 and S2 are the virtual light sources produced from the Biprism structure for a light source S. Interference fringes are
produced and can be observed through a microscope.
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OPTICS
➢ MATH PROBLEM
1. Green light of wavelength 5100Å from a narrow slit is incident on a double slit. If the overall separation of
10 fringes on a screen 200cm away is 2cm, find the slit separation.
Solution:
Given that,
Distance of screen from the slits D = 200cm
Wavelength of green light λ = 5100Å
= 5100× 10−8 cm
Overall separation of 10 fringes = 2 cm
2
So the fringe width β= = 0.2 𝑐𝑚
10
Slit separation d=?
We know that,
𝛽=
Or,
d=
𝜆𝐷
𝜆𝐷
𝑑
𝛽
5100×10−8 ×200
Or, d=
0.2
Or, d = 0.05 cm. (Ans.)
2. In a Biprism experiment the eyepiece is placed at a distance of 1.2 m from the source. The distance between
the virtual sources was found to be 7.5× 10−4 m. Determine the wavelength of light, if the eyepiece is to be
moved transversely through a distance of 1.888cm for 20 fringes.
Solution:
We know the fringe width 𝛽 =
And also 𝛽 =
𝑙
Or, 𝑛 =
Or, 𝜆 =
𝑙
𝜆𝐷
𝑑
𝑛
𝜆𝐷
𝑑
𝑙𝐷
𝑛𝑑
Where given that
l = 1.888 cm = 0.01888m
d= 7.5× 10−4 m
n= 20
D= 1.2 m
So, 𝜆 =
𝑙𝐷
𝑛𝑑
=
0.01888×1.2
20×1.2
= 5900× 10−10𝑚 = 5900Å (Ans.)
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OPTICS
➢ DIFFRACTION OF LIGHT
The bending of light waves around the edges of the obstacles is called diffraction. If the slit or hole in the way of light is
large then the diffraction will be less but narrow slit of hole produced greater diffraction. There are basically two types of
diffraction
1. Freshnel diffraction
2. Fraunhoffer diffraction
A figure below presents the diffraction of light,
➢ POLARIZATION OF LIGHT
Light contains components vibrating in several directions (unpolarized light), some crystals or substances like tourmaline
or Calcite can direct the components in a certain direction (polarized light). The process of turning the unpolarized light to
polarized light is called the polarization of light.
➢ BREWSTER’S LAW AND APPLICATION
Brewster’s law states that the tangent of the angle at which polarization is obtained by reflection is numerically equal to
the reflective index of the medium. If 𝜃𝑝 is the angle and µ is the refractive index of the medium then,
µ = tan𝜃𝑝
Application of Brewster’s law:
1. Brewster’s law can be used to determine the refractive index of the opaque materials.
2. It helps in calculating the polarizing angle necessary for calculating total polarization of reflected light.
3. Brewster’s angle is utilized in transmitting light beam inside an optical fiber without reflection loss.
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