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 long rail and rail carriages appropriately fitted with laser head, an RF oscillator and a detector with translation stage. The ultrasonic diffraction setup uses laser as light source. As the laser beam is intense and monochromatic, we get clear higher order diffraction pattern.
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 laser head is mounted using a kinematic holder. This helps to direct the laser beam through the liquid and then to the detector conveniently. The diffraction pattern is scanned using a translation stage with freedom in X axis. The velocity of ultrasonic waves in liquids can be calculated from this experiment. This instrument is designed to give accurate and best results.
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
Fig: Graph shows distance Vs. detector current