The experiments illustrating interference and diffraction of light  studied in previous chapters have shown beyond doubt that light like sound is some form of wave motion. These experiments do not reveal whether the light waves are longitudinal or transverse because phenomenon of interference and diffraction can occur with both longitudinal and transverse waves. We shall, therefore, investigate the nature of light waves and to begin with, we shall describe an important feature which distinguishes the two types. From the study of sound, we know that sound travesl in the form of longitudial waves and properties of such a wave motion are the same with respect to any plane through its line of propagation while a transervese wave behaves differently in different planes. The statement can be illustrated by a simple mechanical analogy given below :

Take a stretched rubber cord CD threading through two narrow slits S1 and S2 cut in card board pieces and placed parallel to each other in the vertical planes. End D of the cord is fixed. Now set up a longitudinal wave in CD by moving the end C forward and backward along the cord. Rotate any of the slits about CD  as axis. It will be found that this rotation does not effect the passage of the wave, ie. the wave passes through the first and second slits without being affected at all i whatever position the slits may be arranged. Thus, a longitudinal wave motion has the same properties with respect to all planes throughout its line of advance.