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Assumptions of Newton’s Corpuscular Theory of Light:
- According to corpuscular theory, every source of light emits minute, perfectly elastic particles of negligible mass known as ‘Corpuscles’. That is light energy emitted by any source consists of tiny stream of particle. 2.
Explanation of Rectiliner Propagation of Light:
According to Newton, Corpuscles are unaffected by the force of gravity and they travel in straight line away from its source at very high speed. This explains rectilinear propagation of light.
Explanation of Sensation of Vision:
When corpuscles fall on eye, they produce sensation of vision.
Explanation of Different Colours:
The various colours of light are due to different sizes and masses of the Corpuscles. The brightness of a surface depends on the number of Corpuscles incident on it.
Explaination of Opacity:
The ‘ Corpuscles are so small that they can pass through the intermolecular space of transparent substances. However, size of Corpuscles are larger than intermolecular space of opaque substances, so that they are stopped by them.
Explaination of Reflection and Refraction:
To explain reflection and refraction of light, Newton proposed that Corpuscles are repelled by a reflecting surface whereas they are attracted by a refracting surface.
Explaination of Partial Reflection and Partial Transmission:
Partial reflection and partial transmission of light from a substance like glass was explained by using a ‘theory of fits’. According to this, theory, the Corpuscles are alternately attracted and repelled from such a surface.8. On the basis of Corpuscular theory, Newton was able to explain the phenomenon of rectilinear propagation, reflection and refraction of light.
Drawbacks of Newton’s Corpuscular Theory of Light:
- Newton’s Corpuscular theory predicted that the velocity of light is more in optically denser medium than that in rarer medium. Foucault’s experimental observation disproved it.
- On the basis of Corpuscular theory, Newton’s explanations for partial reflection and refraction was unsatisfactory.
- Phenomenon of reflection and refraction was not explained satisfactorily.
- Corpuscular theory fails to account for interference, polarization and diffraction of light.
- As Corpuscles were assumed as particles, there should be a small decrease in mass of a source which emit light continuously for a long time. There was no experimental evidence for it.
Maxwell’s Electromagnetic Theory:
- Maxwell proposed existence of electromagnetic waves. He obtained relation between magnetism and electricity.
- He assumed light to be in the form of electromagnetic waves due to the fact that the velocity of electromagnetic waves is equal to that of light.
- He observed that the electromagnetic waves do not require any medium for their propagation hence they can travel through vacuum. Thus the propagation of light in vacuum can be explained by assuming them to be electromagnetic waves.
Max Planck’s Quantum Theory:
- To explain black body radiations Max Planck proposed this theory.
- He proposed that the light is propogated in the form of light energy called quanta or photon.
- Each photon or quanta has energy given by E = h υ. Where ‘h’ is planck’s constant and ‘υ’ is the frequency of photon.
Huygene’s Wave Theory of Light:
- According to wave theory, light from a source is propagated in the form of longitudinal waves with uniform velocity in homogeneous medium. (Later it was proved that the light waves are transverse waves).
- These waves emitted by the source move in a straight in homogeneous medium.
- To explain the propagation of light through vacuum, Huygens assumed the existence of a hypothetical (imaginary) elastic medium called luminiferous ether. According to Huygene, ether particles are all pervading (present everywhere) and possess properties such as inertia, zero density and perfect transparency.
- On the basis of Huygens wave theory, Various colours of light are due to different wavelengths of the light waves.
- When light enters our eyes we get the sensationn of light.
Merits of Huygene’s Wave Theory of Light:
- On the basis of wave theory the phenomenon of reflection, refraction, diffraction, interference, polarization and total internal reflection of light could be explained.
- Wave theory correctly predicted that velocity of light in an optically denser medium is less than that in a rarer medium which is in agreement with experimental results.
- On the basis of wave theory partial reflection and refraction of light could be explained without making contradictory hypotheses.
Demerits of Huygene’s Wave Theory of Light:
- Wave theory assumes the existence of luminiferous ether. However, experimental attempts to detect the presence of ether particle were unsuccessful.
- Huygen’s theory could not explain the rectilinear propagation of light.
- The phenomena of photoelectric effect, Crompton effect in Modern Physics cannot be explained using wave theory.
Important Concepts in Wave Theory of Light:
- A wavefront is defined as the locus of all the points of a medium, which receives light wave disturbance simultaneously so that all these points are, in the same phase.
- A perpendicular drawn to the surface of a wave front at any point in the direction of propagation of light is called as Wave Normal.
Ray of Light :
- The direction in which light is propagated is called ray of light.
- Thus ray of light is always perpendicular to the wave front.
- From Huygens’ concept of wave front, it is clear that a wave front always moves parallel to itself in a homogeneous medium. This means that the wave front always carries light energy in a direction perpendicular to its surface.
- This can be represented by drawing a normal at any point in that direction. The direction of propagation of a wave front is given by a ray of light. Therefore, a wave normal represents a ray of light.
Types of Wave Fronts:
Spherical Wave front:
- Consider a point source of light S placed in air. The source will emit waves of light in all possible directions.
- If the velocity of light in air is c, then in time ‘t’, each wave will cover a distance ct. Therefore, at the end of time interval ‘t’ the light emitted, by, the source will reach at all points as a sphere with centre S and radius equals to ct.
- All the points on the surface of this sphere are in same phase. Such a spherical surface is called spherical. wavefront.
- Thus spherical wave front is given by a point source.
Plane Wave front:
- Consider a spherical wave front of very large radius. At a very large distance from a point source of light, the spherical wavefront is so large that a small part of its surface can be considered as a plane surface i.e. a plane wave front.
- Light from sun reaches the surface of earth in the form of plane wave fronts.
- Theoretically, plane wave front is obtained only at infinite distance from a point source.
Cylindrical Wave front:
- If a source of light is linear i.e. in the form of a slits the wave front originating from it has the shape of a cylinder. Such a wave front is called as cylindrical wave front.
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