WAVE-LIKE PROPERTIES OF LIGHT

A good starting point to learn about the behavior of waves is to study waves in a stretched spring or string. A wave pulse is generated when a stretched spring is given a shake at one end. This wave pulse travels along the spring to the other end and turns back to travel in the opposite direction. The spring is given a certain amount of energy during the shake. This amount of energy exists in the spring and is carried along the spring by the wave pulse. The spring is the medium for the wave pulse (energy) to travel along. The particles of the medium are displaced while the pulse is passing and they returned to their original positions after the pulse is through. They do not travel with the pulse. The transfer of energy from one place to another does not involve the net transfer of any material of the medium.

When the spring is shaken perpendicular to the length of the spring, the motion of a particle in the spring and the motion of the pulse are perpendicular to each other. This type of waves is categorized as transverse waves. Shaking a stretched spring along the direction of its length can also generate Wave pulses.

The motion of a particle in the wave is parallel to the motion of the pulse. This type of waves is categorized as longitudinal waves. Again the particles do not travel with the pulse. They are displaced when the pulse is passing through them.

PERIODIC WAVES: -

 If the shaking of the spring is done repeatedly, a periodic travelling wave is formed in the medium (the spring).

A ‘full’ shake produces a cycle of the periodic wave. The time interval for generating a cycle is the period T of the wave. The length of a cycle of the wave is called its wavelength l. The number of cycles generated in a unit time (second) is the frequency f of the periodic wave. Frequency and period of a wave are related according to

                        F = 1 / T

The highest point of a wave is the crest, and the lowest point the trough. Half way between the two is the equilibrium position where the spring is. The distance between a crest (or trough) and the equilibrium position is the amplitude of a wave.