Ultrasonic Grating - Debye-Sears Method

Debye-Sears Method

• This method is used to determine the wavelength of monochromatic light using acoustic or ultrasonic grating. This method utilises the concept of Piezo electricity to obtain ultrasonic grating.

The phenomenon of Diffraction of light using ultrasonic grating was first observed by Debye and Sears in 1932. When ultrasonic waves are propagated in a liquid, the density varies from layer to layer due to periodic variation of pressure. If under this condition, monochromatic light is passed through the liquid at right angles to the waves, the liquid behaves as diffraction grating. Hence such a grating is called acoustic grating. This acoustical grating behaves in the same way as that of a normal ruled grating. Hence these can be used to determine the wavelength of monochromatic light as well as the speed of waves depending on wavelength.

If is the wavelength of monochromatic light, and be the wavelength of ultrasonic waves, then applying the principle of diffraction, we get

Where is the angle of diffraction.

Thus we can calculate either or accordingly. Here, we need not worry about the grating element since the nodes themselves act as slits, hence the distance between two slits will be equal to the wavelength only.

Generally, we use this method to determine the velocity of ultrasonic waves using Monochromatic sources like sodium vapour lamps. This is done usually by using piezo electric crystals like Quartz, Tourmaline, Rochelle salt etc..and producing mechanical stress along their mechanical axes using an R.F oscillator. By adjusting the frequency of the oscillator, we can determine the velocity of the ultrasonic waves by using

where is the frequency of the oscillator.