Study material 12th Physics - Wave Theory of Light

WAVE THEORY OF LIGHT

Proposed By Huygens

Prepared by:- Prof. Dnyanesh Vaidya

What is the Nature Of Light ?

Newton Corpuscular Theory

Light consists of very Tiny, Elastic, Rigid particles known as Corpuscular”.

These corpuscles on emission from the source of light travel in straight line with high velocity When these particles enter the eyes, they

produce image of the object or sensation of vision.

Corpuscles of different colours have different sizes.  Reflection and refraction of light are because of

repulsive and attractive forces respectively.

Demerits of the theory

Velocity of light in denser medium is greater than velocity of light in rare medium.

Newton unable to explain simultaneous reflection and refraction

Newton unable to explain polarization and interference concept.

Scientist unable to find out relation between size and colour at larger level. At nano level its true.

Huygens’ Wave theory of light (1678)Assumtions

Light is propagated in form of Longitudinal waves. These waves are emitted by the source of light

and travel in straight lines with a uniform velocity through a homogeneous medium.

When light enter our eyes it creates an optical impression on the retina. Hence, we get the sensation of light.

Different colours are due to different wavelengths of light waves.

Light waves are mechanical waves. For propagation of these waves, a hypothetical medium called “luminiferous ether” is present everywhere. Light travels through ether in form of waves.

Merits of wave theory of light

The phenomena like reflection, refraction, polarization, simultaneous reflection and refraction, total internal reflection, diffraction etc can be successfully explained with this theory.

According to Huygens’ theory the speed of light in denser medium is less than the speed of light in rarer medium. This conclusion is in perfect agreement with the experimental findings.

Demerits of wave theory of light.

The existence of so called luminiferous ether, assumed by Huygens, was not confirmed.

Rectilinear propagation was not explained by the theory. It was then justified by Fresnel.

Diffraction was explained much later.

This theory could not explain photoelectric emission.

Wave front It can be defined as the locus of all

the points of the medium to which the wave reaches simultaneously, so that all the points are in the same phase.

Types of wavefronts : Spherical Wave front :-

Plane wavefront :-

3. Cylindrical wavefront :-

Wave normal & A ray of light A perpendicular drawn to the surface

of the wave front at any point is called a wave normal. This is in the direction of the propagation of light at that point.

Huygens’ principle :-

Every point on a wave front acts as a secondary source of light, sending out secondary waves. The envelop of all these secondary waves, at any later instant, gives the new wave front at that instant.

If the nature of the wave front at any instant is known, we can determine the nature and the position of the wave front at any later instant by Huygens’ construction, based on Huygens’ principle.

Huygens’ construction of spherical wavefront

1. Consider a known position of a spherical wave front, at time t = 0 as PQRS

2. According to Huygens’ principle, as soon as the wave front is formed, every point on

this wave front will act as a secondary source,

and will start emitting secondary waves. 3. To determine the position of the wave front

after a time t, we have to draw spheres with every point on the surface PQRS as centre and radius equal to ct, where c is the velocity of light.

4. These spheres will represent secondary wave fronts. Draw a tangential surface P’Q’R’S’ to these spheres.

5. The surface P’Q’R’S’ represents the position of the wave front after time t. It is also a spherical wave front.

Huygens’ construction of plane wavefront :

Consider a known position of a plane wave front, at time

t = 0 as PQRS According to Huygens’ principle, as soon as the wave

front is formed, every point on this wave front will act as a secondary source, and will start emitting secondary waves.

To determine the position of the wave front after a time t, we have to draw spheres with every point on the surface PQRS as centre and radius equal to ct, where c is the velocity of light.

These spheres will represent secondary wave fronts. Draw a tangential surface P’Q’R’S’ to these spheres.

The surface P’Q’R’S’ represents the position of the wave front after time t. It is also a plane wave front.

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