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What is the Difference Between Transverse and Longitudinal Waves?
Definition: A wave which propagates along the direction of motion of the vibrating particles of the medium is called a longitudinal wave.
Example of longitudinal wave:
A thin and long spring AB is taken whose spring constant is very small [Fig.]. The end B is fixed to a rigid support and the other end A is held in such a way that the spring is in streched condition. Now the end A is made to oscillate back and forth so that a wave moves along its length to the end B. Some coils of the spring come close to each other creating compression and some other coils move away from each other creating rarefaction. Compressions and rarefactions are formed alternately along the length of the spring and they propogate towards the end B. Since the wave motion is directed from A to B, and during compressions and rarefactions the coils of the spring oscillate parallel to the length of the spring, this wave is a longitudinal wave.
Actually, a longitudinal wave travels through a medium in the form of periodic compressions and rarefactions.
Let the point O be the equilibrium position of a particle in a material medium [Fig.]. The medium may be imagined to consist of many layers of equal thickness. In Fig., a few layers on the right side of O have been shown. Now, some energy from outside is supplied to the layer at O so that the layer oscillates along the line AB. When the layer moves from A to B due to oscillation, it exerts pressure on the layers in front of it. So, those layers get compressed due to the property of compressibility of solid, liquid and gaseous media. Thus compression takes place in the region CD of the medium due to the motion of the layer at point O from A to B.
The opposite incident happens at the time of motion of the layer from B to A, i.e., the layers of the region CD gel rarefied due to the decrease in pressure. By that time, the previous compression reaches the region DE by compressing the next layers leaving a rarefaction behind. So, a complete oscillation (ABA) generates a compression and a rarefaction. These compressions and rarefactions are not confined to a region, but move through the medium due to the property of compressibility, thereby producing a wave.
Again, if we think of any array of layers, ¡t is found that the layers alternately get compressed and rarefied parallel to the direction of wave motion. It is evident that density and pressure in the medium increase in the zones of compression and decrease in the zones of rarefaction. It is the process of propagation of sound wave in air. It is clear that sound wave is a longitudinal mechanical wave.
Nature of the medium: All longitudinal waves are mechanical waves. This type of waves cannot propagate without any medium. Longitudinal waves can propagate through any solid, liquid or gaseous medium. These media revert to their original conditions, after the external driving oscillation (like ABA) is withdrawn. Till that, instant, compression or rarefaction passes to the next parallel layers continuously in the direction of the force applied.
Transverse elastic waves can be formed only in solids, not in liquids and gases. It is because liquids and gases have negligible compressibility and hence cannot sustain shearing stress as they have no definite shape. A solid has a definite shape and it opposes any force exerted to change its shape, i.e., a solid substance can sustain shearing stress. So, if one of its layers oscillates, its adjacent layer is also forced to oscillate in the same direction.
Light waves, radio waves, etc., are not mechanical waves. These are electromagnetic waves. Elasticity of the medium has no relation with the electric and magnetic fields. So electromagnetic waves can propagate through vacuum, as well as through solid, liquid or gas. Actually, the waves set up on the surface of water are not elastic waves; they are mechanical waves produced due to earths gravity.
Difference between transverse and longitudinal waves:
| Transverse wave | Longitudinal wave |
| 1. It propagates in a direction perpendicular to the direction of motion of the vibrating particles of the medium. | 1. It propagates along the direction of motion of the vibrating particles of the medium. |
| 2. In case of a material medium, crests and troughs are formed alternately. | 2. In case of a material medium, compressions and rarefactions are formed alternately. |
| 3. In unit time period of the vibrating particles, a crest and a trough are formed. | 3. In unit time period of the vibrating particles, a compression and a rarefaction are formed. |
| 4. In case of a transverse wave, the distance between two consecutive crests or troughs is called the wavelength. | 4. In case of a longitudinal wave, the total length of a pair of consecutive compression and rarefaction is called the wavelength. |
| 5. Transverse electric waves are formed only in solids. | 5. Longitudinal elastic waves are formed in solid, liquid and gaseous media. |
| 6. Transverse waves can be polarized. | 6. Longitudinal waves cannot be polarized. |