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Theory of Light

Theory of Light

Introduction to light

Generally, light is a electromagnetic radiation of visible wavelength. Visible light ranges from the wavelength of 400 to 700 nm (nanometer). But in scientific context, light refers to whole electromagnetic spectrum of any wavelength. Theory of light describes how and why light shows its properties and nature .Light is composed of elementary particle called photon. It is produced by different luminous sources like fire, light bulb, etc.

Light shows different properties like reflection, refraction, diffraction, interference, photoelectric effect, etc. Optics is the branch of science which deals with these kinds of optical phenomena.
Ray optics deals with the geometric of the light like reflection, refraction, rectilinear propagation of light etc. while physical optics deals with the production, emission, propagation of light like interference, diffraction and polarization of light.

Different theories of light

From the very ancient time, people have been presenting different views about light. It has become one of the interesting field of research in modern world also. Scientists have made many discoveries regarding the light.

Ancient theories of light

In the ancient time, some people had purposed some theories in light . Earlier people considered it as one of the elements of nature. Later on, some assumptions were made as:

  • Light is composed of extremely small particles (though not mentioned clearly).
  • It travels at the speed of infinity.
  • Light always travels in straight path.
  • Colors of light are made up of darkness and brightness.

However, due to lack of further supports and proofs, these assumptions are negleted now.

These assumptions were based upon two different perspectives. They were tactile theory of light and emission theory of light.

  • Tactile Theory of Light:

    This theory was based upon the ability of eye to ‘touch’ objects. According to it, eye sends out streamers towards the object. When these streamers come into contact with an object, we would be able to see the object.
    But this theory lacked enough support and evidence.
  • Emmision Theory of Light:

    It is opposite to tactile theory of light. This theory is explains that the objects sends out light signals which is received by our eyes and we see the objects. This theory was accepted by all and all the modern theories are based upon it.

Corpuscular Theory of Light

This theory was given by the great scientist, Sir Issac Newton in 1678 AD. It supports the particle nature of light. Some of his assumptions on light were:

  • Light is composed of tiny, light(not heavy) and perfect elastic particles called corpuscules, emitted by luminious sources.
  • The size of corpuscules determine the colour of light.
  • The bouncing back of these corpuscules from the surfaces causes reflection of light.
  • The particles present in denser medium attract the corpuscules which cause them to accelerate. It explains the refraction of light.

He also explained that the white light is made up of seven colours (i.e. Violet, Indigo, Blue, Green, Yellow, Orange and Red) by the process of diffraction through triangular prism. Similarly, he made different optical instruments and contributed a lot in the field of physical optics. He also published a book named “Opticks” in 1704 AD.

His theory could not explain the wave nature of light like interference, polarization, diffraction etc. But this theory successfully explained particle nature of light. Since photoelectric effect was not discovered at that time, this theory was discarded.

Huygen’s Wave Theory

This theory was purposed by Christian Huygen in 1678 AD. It explains light as a longitudinal wave in which particle vibrate simple harmonically along the direction of propagation of wave. For the transmission of the longitudinal wave, a medium is necessary. So Huygen assumed that a hypothetical ether medium was present in the space which is responsible for transmission of light waves.

This theory could explain reflection, refraction, dispersion and total internal reflection of the light. Moreover, Frensnel and Young explained interference phenomenon on the basis of this wave theory.

However, this theory failed to explain the phenomena of polarization of light. Also, the existence of ether could not be verified by the experiments

Maxwell’s Electromagnetic Theory of Light

James Clerk Maxwell in 1860 AD mathematically proved that light is the electromagentic wave which travels with the speed of 3×108m/s in vacuum. He used the following equation to find out the speed of light.

Equation of Maxwell's Electromagentic Theory of Light

According to this theory of light, light is a form of electromagentic waves in which the electric and mangnetic field vectors oscillate transversely in the mutually perpendicular directions. Such wave needs no material for the propagation. So, the ether hypothesis is no longer required.

But the phenomena like compton effect, emission and absorption of light etc could not be explained by this theory. Hence, Physicists started further study of light naure.

Quantum Theory of Light

This theory considers light as a discrete packet of waves called quantum. Each quanta (singular of quantum) of light consists of a photon which carries energy equal to the product of plank’s constant and its frequency.
i.e. E = hν
where, h = 6.62607004 × 10-34 m2 kg / s
A photon travels with very high speed ( 3 × 108 m/s in vacuum). Higher will be the frequency of light, more will be the energy carried by the photon.
Quantum theory of light successfully explains the phenomena of photoelectric effect, compton effect, etc.

Conclusion

The experimental observations show that light sometime shows the behavior of wave and sometime it behaves as particle. E.g. light showed particle nature during interference while it showed particle nature during photoelectric effect.
In 1924 AD, de-Broglie suggested that a particle moving with the momentum ‘p’ has an associated wavelength of λ = h/p
i.e. λ = h/(mv)
However, light is found to have no rest mas. So the light never remains stationary and always travels with very high speed.

Hence we can conclude that light has dual nature of wave and particle. Photoelectric effect can be explained on the basis of quantum theory of light while interference, diffraction and polarization can ve explained on the basis of wave theory.


Finally, we have come to an end. You can leave your suggestions, queries or reactions below 😊.

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