Huygens Principle, Longitudinal and Transverse Waves

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Huygens wave theory of light

Christiaan Huygens proposed the Huygens’ wave theory of light. According to this theory, every point of a luminous body acts as a source of the disturbance. This disturbance is due to the vibrations of atoms and molecules of the source of light. The disturbance is in the form of a longitudinal wave in a hypothetical medium called ether. He said that the wavelength of the wave is much lesser than the dimensions of objects we measure. This concept of Huygens proved the laws of reflection, laws of refraction, interference and diffraction of light.

Huygens’ statement was against the Corpuscular theory of light proposed by Issac Newton. According to corpuscular theory, light is made up of tiny particles. Huygens theory could not explain the phenomenon of polarization. However, this was overcome when Thomas Young and Fresnel suggested that the light waves are transverse and not longitudinal as mentioned by Huygens.

Huygens Principle

According to Huygens principle, every point on a wavefront acts as a secondary source of light. Each point on the wavefront emits spherical wavelets of light. These wavelets are semicircular in nature. The line tangents to all these secondary wavelets form a wavefront. These secondary wavelets travel with the speed of light.

Let us consider the example of dropping a stone into a still pond. We notice that circular rings are formed in the pond. All the particles inside the waves oscillate in the same phase. These waves of constant phase are called a wavefront. The speed with which the wavefront moves outside the source is called the speed of the wave. The energy of the wave travels in the direction which is perpendicular to the wavefront.

Types of Waves

One of the way to categories the waves is based on the direction of motion of the particles with respect to the direction in which the wave travels. Based on this the waves are of two types – longitudinal and transverse waves. The light wave is an example of a transverse wave and sound wave is an example of a longitudinal wave.

Transverse waves

It was proposed by Huygens that light waves are longitudinal waves but later found to be transverse in nature. Transverse waves are always characterized by the particle moving perpendicular to the direction of the wave.  Transverse waves have a sinusoidal waveform. Example of a transverse wave is the ripples on the surface of the water. Wavelength and frequency are important characteristics of the wave. Wavelength is the distance between two consecutive crests or troughs. Frequency is the number of vibrations per second.

Longitudinal Waves

A longitudinal wave is a wave in which particles of medium move in a direction parallel to direction that the wave moves.  An example of a longitudinal wave is the compression and rarefaction along a slinky. This can be produced by pushing and pulling the slinky horizontally. In the case of longitudinal waves, the wavelength is the distance between two consecutive compressions or two consecutive rarefactions.