Diffraction Grating Experiment
Aim: To determine the wavelength of prominent spectral lines of mercury by plane diffraction grating in
first and second order spectrum.
APPARATUS: Spectrometer, mercury lamp, plane diffraction grating, readings lens, sprit level et.
THEORY AND FORMULA:
The grating spectrum formed in the direction 𝜃 is given by
(e + b) sin 𝜃 = ( )sin e b n
where (e + b) = grating element (cm) = angle of diffraction n = order of the spectrum
PROCEDURE: Before performing the experiment, the following adjustments are made.
1. Setting the Grating for Normal Incidence
(a) The spectrometer is well adjusted for parallel beam using the usual procedure of seeing a far object.
(b) The position of the telescope is adjusted in such a way that the image of the slit is focussed on the vertical crosswire of the telescope. In this position, the collimator and the telescope are in the same line. The reading is noted on the circular scale.
(c) The telescope is now turned through 90° and clamped.
(d) Prism table is now rotated through an angle of 45° or 135° as the case may be, so that it becomes normal to the incident light. The slit of the collimator is also adjusted, so that sharp spectral lines parallel to the rulings of the grating are formed. The prism table is clamped in this position.
2. Determination of Diffracting Element ( 𝒆 + 𝒃 ) On every grating, number of rulings (lines) per inch
are marked by the manufacturer. Thus grating element will be ( 𝑒 + 𝑏 ) = 1 𝑖𝑛𝑐ℎ 𝑁 = 2.54𝑐𝑚 𝑁 N = number of lines on the grating, 2.54 cm = 1 inch.
3. Determination of diffracting Angle ( 𝜃 )
(a) When the source of light emits radiations of different wavelengths, the beam gets dispersed by gating and in each order, a spectrum of constituent wavelengths is observer.
(b) The telescope is now turned to get the first order spectrum. The cross wire is adjusted on the line for which wavelength is to be determined (say red). The position of the telescope is fixed and the readings of the vernier are recorded.
(c) The telescope is then turned on the side and the crosswire is adjusted on the same coloured line of the first order spectrum. The reading of the two verniers are again recorded.
(d) The differences of the readings of the same vernier gives twice the angle of diffraction for that lines in the first order. The average of two vernier is taken. Engineering Physics Lab Manual Department of Physics Manipal University Jaipur
(e) Procedure (b) to (d) is repeated for other lines and for other orders.
APPARATUS: Spectrometer, mercury lamp, plane diffraction grating, readings lens, sprit level et.
THEORY AND FORMULA:
The grating spectrum formed in the direction 𝜃 is given by
(e + b) sin 𝜃 = ( )sin e b n
where (e + b) = grating element (cm) = angle of diffraction n = order of the spectrum
PROCEDURE: Before performing the experiment, the following adjustments are made.
1. Setting the Grating for Normal Incidence
(a) The spectrometer is well adjusted for parallel beam using the usual procedure of seeing a far object.
(b) The position of the telescope is adjusted in such a way that the image of the slit is focussed on the vertical crosswire of the telescope. In this position, the collimator and the telescope are in the same line. The reading is noted on the circular scale.
(c) The telescope is now turned through 90° and clamped.
(d) Prism table is now rotated through an angle of 45° or 135° as the case may be, so that it becomes normal to the incident light. The slit of the collimator is also adjusted, so that sharp spectral lines parallel to the rulings of the grating are formed. The prism table is clamped in this position.
2. Determination of Diffracting Element ( 𝒆 + 𝒃 ) On every grating, number of rulings (lines) per inch
are marked by the manufacturer. Thus grating element will be ( 𝑒 + 𝑏 ) = 1 𝑖𝑛𝑐ℎ 𝑁 = 2.54𝑐𝑚 𝑁 N = number of lines on the grating, 2.54 cm = 1 inch.
3. Determination of diffracting Angle ( 𝜃 )
(a) When the source of light emits radiations of different wavelengths, the beam gets dispersed by gating and in each order, a spectrum of constituent wavelengths is observer.
(b) The telescope is now turned to get the first order spectrum. The cross wire is adjusted on the line for which wavelength is to be determined (say red). The position of the telescope is fixed and the readings of the vernier are recorded.
(c) The telescope is then turned on the side and the crosswire is adjusted on the same coloured line of the first order spectrum. The reading of the two verniers are again recorded.
(d) The differences of the readings of the same vernier gives twice the angle of diffraction for that lines in the first order. The average of two vernier is taken. Engineering Physics Lab Manual Department of Physics Manipal University Jaipur
(e) Procedure (b) to (d) is repeated for other lines and for other orders.
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