Nature and Propagation of light Class 12 Physics | Notes

Nature and Propagation of light

Syllabus: Content to read

1. Huygen’s principle
2. Reflection and Refraction according to wave theory

Wave Optics

The branch of optics which deals with the production, emission and propagation of light, it’s nature and study of phenomenon of interference, diffraction and polarization is called wave optics.

Theories of Light

Newtons Corpuscular theory:-

Sir Isaac Newton proposed that light is made up of small discrete particles called "corpuscles"(little particles) which travel in a straight line with finite speed. When these particles enter the eyes, they produce image of object or cause sensation of vision. Corpuscles of different colors have different size.

Huygens’s wave theory:-

According to this theory, light propagates from the source in the form of wave. For the propagation of wave, a medium is necessary. So, it was assumed that all space including vacuum is filled with medium called ether, which has the property of both elasticity and inertia. From the source of light, periodic disturbance is produced, which travel in the form of waves and the energy is equally distributed in all direction. Huygens’s assumed that light waves are longitudinal. On the basis of this concept the phenomenon of reflection, refraction, diffraction, interference can be explained.

Maxwell's electromagnetic theory:-

Maxwell suggests that light is not a mechanical wave but is an electromagnetic wave. It propagates as electric and magnetic field oscillation with speed 3 x 108m/s. Also, these waves are transverse in nature.

Planck's Quantum Theory:-

According to this theory light travels in the form of small packet of energy called photons. The energy of each photon is
E= h f Where h= Planck's constant and f=frequency of light
Einstein explains photoelectric effect on the basis of this theory.

Dual Nature of theory:-

According to this theory light has dual nature i.e. it possesses both characters of wave and particle so we see that in phenomena like interference, diffraction and polarization, light behaves as a wave while in photoelectric effect it behaves as a particle.

Electromagnetic spectrum

Electromagnetic spectrum is the distribution of electromagnetic radiation of all possible frequencies and wavelengths.

Electromagnetic spectrum is classified are as follows:
1. Radio waves: Frequency of these waves ranges from few Hz to 109 Hz. Radio waves emitted by radio stations. Radio waves are also emitted by stars and gases in space.
2. Microwaves: Frequency of these waves ranges from 109 Hz to 3.0 x 1011 Hz. The wavelength of microwaves is greater than 1 mm and less than 30 cm. It is used in cell phones, cooking food, radar communication, etc.
3. Infrared: Frequency of these waves ranges from 3.0 x 1011 Hz to 4.3 x 1014 Hz. The wavelength of infrared is 1 nm to 700 nm. It is used in TV remote control, to treat muscular strain, etc.
4. Visible: Frequency of these waves ranges from 4.3 x 1014 Hz to 7.5 x 1014 Hz. The wavelength of visible light is 400 nm to 700 nm. Our eyes detect visible light. Fireflies, light bulbs, and stars all emit visible light.
5. Ultraviolet: Frequency of these waves ranges from 7.5 x 1014 Hz to 5.0 x 1015 Hz. The wavelength of ultraviolet rays is 400 nm to 60 nm Ultraviolet radiation is used to kill bacteria and viruses, to study structure of molecules, to detect forgery.
6. X-ray: Frequency of these waves ranges from 5.0 x 1015 Hz to 3.0 x 1018 Hz. The wavelength of X-ray is 60 nm to 10-8 nm. It is used to see fractures in bones, foreign matter in body like bullets, to detect hidden object at airport. Gamma ray: Frequency of these waves ranges from 3.0 x 1018 Hz to 3.0 x 1022 Hz. The wavelength of Gamma rays is 0.1 nm to 10-5 nm. It is used to treat cancer, sterilizing to produce nuclear reaction, etc.

Wave front and wavelets:

Wave front is the locus of all adjacent points which are equidistance from the source of light and are vibrating in same phase. Every point on the wave front acts as a source of disturbance, these disturbance from the points are called wavelets.

Types of wavefront

Depending upon the source of light, there are three types of wave fronts are:
i. Spherical wave front:
It is produced by the point source of light. This is because all such point which are equidistance from the point source lie on sphere.

ii. Cylindrical wave front:
When the source of light is linear in shape (e.g. a slit) then all points’ equidistance from the source will lie on the surface of cylinder and wave front is cylindrical.

iii. Plane wave front:
As the spherical or cylindrical wave front advances, its curvature decreases progressively. so a small portion of such wave front at a large distance from a point source or linear source will appear a plane and hence termed as plane wave front.

Huygens principle:

Huygens assumed that a source of light emits light in the form of waves.

Huygens’s principle is based on following assumptions

I.              Each point on a wave front acts as a new source of disturbance. The disturbance from these points are called secondary wavelets. These wavelets spread out in all direction in medium with the velocity of light.

II.            The new wave front is tangent plane or tangent envelope to all secondary wavelets from the secondary source.

 Reflection on the basis of wave theory

fig. Reflection on the basis of wave theory

Let us consider a plane wave front AP incident on a reflecting surface XY at an angle of incidence i. SA and QP are the incident rays perpendicular to AP.

At first the wave front arrives at point A and then at B,C,D.....of the reflecting surface .Thus different point on the surface XY becomes source of secondary wavelets at different instant of time.

When the disturbance from point P on the incident wave front reaches at point E on the reflecting surface, the secondary wavelets from the point A,B,C,D on the reflecting surface have acquired radius AA',BB',CC' and DD' respectively. The tangent plane A'E touching all the secondary wavelets represent reflected wave front and the lines A'R' and ER perpendicular to the reflected wave front represents reflected rays.

Therefore, N'ER= r = angle of reflection
Now in right angled triangles APE and AA'E
i) AA'=PE (Distance traveled by light in same time)
ii) AA'E=APE=90°
iii) AE=AE is common side.

 

Hence the triangles are congruent therefore,

Tr. PAE= Tr. AEA'

Or,  i = r

This verifies the law of reflection

Further if we consider the incident plane of wave front is parallel to the plane of paper ,the plane of reflected wave front and plane of normal all lie in that plane. So the incident ray normal to the reflecting surface and the reflected ray all lie in the same plane. This proves another law of reflection.

Refraction of light on the basis of wave theory:

fig. Refraction on the basis of wave theory

Let us consider a plane wave front AP incident on an interface XY separating two media.SA and QP are incident ray ,i is the angle of incidence.

At first the wave front arrives at A and then at point B, C, D and E. Thus, different point on the surface XY becomes as source of secondary wavelets at different instant of time.

When the disturbance from P reaches at E on the surface XY, the secondary wavelets from A, B, C and D have acquired radius AA', BB', CC' and DD' respectively. The tangent plane A'E drawn from A' is refracted wave front and A'R' and ER are reflected rays.

Then,  

∠REN' =r= angle of refraction

Let c and v be the velocity of light in air and in denser medium respectively.PE and AA' are distance covered by light in air and in denser medium respectively in same time.

is the refractive index of denser medium with respect to rarer medium. This is Snell’s law. It verifies first law of refraction.

Further the incident wavefront, refracted wavefront and refracting surface lie in one plane. Hence incident ray reflected ray and the normal to the surface also lie in one plane .This proves another law of refraction.