Huygen’s Principle and Interference of Light

light waves

What is Huygen’s Principle?

Huygen’s Principle is also called as Huygen’s-Fresnel principle. According to Huygen’s Principle, each point on the given wavefront is a source of secondary wavelets or secondary disturbances. Further, the secondary wavelets travel in the forward direction at the same speed as the source wave. The new wavefront will be a line tangent of the secondary wavelets. Huygen’s Principle can be explained using the given formula.

s = vt


s is the distance,v is the speed and t is the time

From the equation, we can understand that the wavefront emits wave with the speed equal to v. The wavelets emitted are semicircular in shape and it occurs after a time t. The new wavefront is the tangent to the wavelets. Huygen’s Principle explains concepts of reflection, refraction diffraction and interference. However, Huygen’s  Principle failed to explain a phenomenon like polarization, photoelectric effect, emission and absorption of light.

What is Interference of light?

Interference of light is the phenomenon of modification in the intensity of light due to the superposition of two or more waves. When light waves from two or more coherent sources superpose the distribution of energy will not be uniform. At some points, two crests or two troughs superpose which increases the amplitude to a maximum resulting in maximum intensity. This type of interference is called constructive interference. At certain points, a crest and a trough superpose and the amplitude decreases to the minimum or zero resulting in minimum or zero intensity. This type of interference is called destructive interference.

What are the conditions for interference?

The source of light much be coherent. Two sources are said to be coherent if the waves emitted from them have the same frequency and a constant phase difference. Laser light is an example of coherent light. The source must be small enough to be considered as a point source. The two sources must be kept near enough to produce wide fringes. If the interfering waves are polarised, then they must have the same plane of polarisation.

What is the Principle of Superposition?

According to the Principle of Superposition when two or more waves meet at a point the resultant wave will have a displacement equal to the algebraic sum of the displacement of the individual wave.

Young’s Double Slit Experiment

Young’s demonstrated the phenomenon of interference through an experiment called Young’s Double Slit Experiment. In this experiment, two coherent sources are created from a single source. Lasers lights are commonly used as the source. The light passing through the slits are made to fall on a screen. When the width of the slits is adjusted carefully interference pattern can be observed on the screen

Real-life Examples of Interference

The light from the sun is incoherent but small portions of light in smaller areas are coherent. One of the examples of interference is the colours on the soap bubble. Another example is the light reflected from the thin film of oil floating on water.

Kelly Passarelly
Kelly Passarelly is a professional writer and blogger that loves to write about different topics.

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