Difference between Light waves and Sound Waves

Question

Which of the following phenomenon is shown by light waves but not by sound waves?

a) Reflection
b) Refraction
c) Diffraction
d) Polarization

Solution

Reflection, refraction and diffraction are all properties of waves, and since light and sound both show wave nature, both of them will exhibit the phenomenon of reflection, refraction and diffraction. However, polarization depends on whether a wave is transverse or longitudnal in nature.

Light waves are electromagnetic waves, that are transverse waves, therefore light waves show polarization in all media. However, sound waves are elastic waves (or pressure waves) that are formed due to compressions and rarefractions in a media, which are longitudnal in nature. Therefore, since the vibrations in sound waves are along the direction of motion, it is not possible to polarize sound waves.

However, when sound waves are produced in solids, the compressions and rarefractions cause a shear strain in the solids. Unlike a fluid (liquid or gas), solids are resistant to compression, they respond to the shearing by trying to pull itself back. This produces a shear wave in the solid which is transverse in nature. This shear wave is produced along with the original sound wave, which is longitudnal. Therefore, in solids, sound waves have two components: the primary wave, which is longitudnal; and the secondary wave (or shear wave) which is transverse. Since there is a transverse component of the sound waves in solids, it is possible to polarize sound waves in solids.

Application

The phenomenon of polarization of light waves have numerous applications. For example, polaroids are used in sunglasses to reduce the intensity and the glare of sun rays. Polarized light is also used in liquid crystal display (LCD) screens. It is also useful for determining the shape and size of biological organisms.

An example of shear waves produced in solids are the S-waves produced during an earthquake. The seismic waves produced during earthquakes have two components: the longitudnal component called the P-waves, which can pass through any kind of medium (solid, liquid or gas); and the transverse component called the S-waves, which can travel only through solids. Since S-waves are transverse waves, their amplitude is much larger. Therefore they cause much more damage than the P-waves during the earthquake.

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