The Ultrasonic diffraction apparatus is used to study diffraction of light by ultrasonic waves. Ultrasonic sound refers to sound with a frequency greater than the human audible range (20Hz to 20 KHz). When an ultrasonic wave propagates through a medium, the molecules in that medium vibrate over very short distance in a direction parallel to the longitudinal wave. The apparatus consists of a graduated rail and rail carriages appropriately fitted with a laser head, an RF oscillator, a glass tank mount with crystal holder and a screen.
The ultrasonic waves generated by the transducer travels down the medium (liquid)and gets reflected at the bottom (flat glass plate) of the cell. The incident and reflected waves interfere and a stationary / standing wave pattern is formed. The pattern can be observed on a screen or a wall and we can plot diffraction pattern on a graph paper for further calculations.
The velocity of ultrasonic wave in a liquid,
V = ʋ ʌ
Where υ is the frequency of the crystal oscillator and Λ is the wavelength of ultrasonic wave.
Λ = n λ / Sin θ
Where n is the order of diffraction, λ is the wavelength of the laser used and θ is the angle of diffraction.
θ = tan-1 ( D / L )
D is the order length and L is the distance measured from the crystal oscillator to the detector.
The bulk modulus of the liquid,
β = ρV2
Where ρ is the density of the liquid and V is the velocity of the ultrasonic wave.
The compressibility of a liquid is the reciprocal of bulk modulus,
K = 1 / ρV2
Where ρ is the density of the liquid and is V the velocity of the ultrasonic wave.
Fig: Optical system for observation of diffraction by ultrasonic waves