Aim: To determine the wavelength of given semiconductor laser light using diffraction by calculating grating constant.
Apparatus: Laser source, diffraction grating (500 LPI), image screen, meter scale.
Formula: (i) Wavelength of laser source, λ= (d 〖sinθ〗_m)/m nm
(ii) Angle of diffraction, θ_m=〖tan〗^(-1) ((2x_m)/f) deg
where, d = 1/N is the grating constant (m)
N is the number of lines/inch on the grating element
m is the order of the diffraction
xm is the distance of the mth order diffraction from the central maximum (cm)
f is the distance between grating and the screen (cm)
Procedure:
The laser is placed on a study table and switched on. At exactly 150 cm (f =150 cm) away on the path of the laser a screen is placed. The leveling screws of the laser are adjusted such that the laser beam exactly falls on the centre of the screen. The grating is now placed on the stand close to the laser source and the equally spaced diffraction pattern is observed. The total numbers of spots are counted. The central direct ray is very bright in the picture as the order increased the brightness decreased. The centre of the spots of the diffraction pattern are marked on the screen using a pencil and after marking all the diffraction pattern, the image screen is removed and the distances between consecutive order of diffraction is measured using a scale and tabulated. Using the equation, θ_m=〖tan〗^(-1) ((2x_m)/f) , diffraction angles are calculated for different order diffractions and noted in table.
Number of lines per unit length (N) of the given grating is 500 Lines per inch, then the grating constant (d) is determined by using the formula
d= 1/N
The wavelength of the laser light is calculated for different order diffractions using the formula, λ = (d sin Ɵ_m)/m and tabulated in table. The average value of the wavelength is calculated.