Presented By:- Abhishek Chandra, Abhishek Singh, Akash Gupta, Abhishek Pandey, Amit Tiwari B.Sc.:-IIIrd Year

Transcription

1 Diffraction Grating Presented By:- Abhishek Chandra, Abhishek Singh, Akash Gupta, Abhishek Pandey, Amit Tiwari B.Sc.:-IIIrd Year

2 DIFFRACTION The Phenomenon of bending of light round the corners of an obstacle and their spreading into the geometrical shadow (of an object) is called Diffraction. The distribution of light intensity resulting in Dark and Bright Fringes(That is, with alternate Maxima and Minima)is called Diffraction Pattern. * Fresnel successfully explained that the diffraction phenomenon is caused by the interference of innumerable secondary wavelets produced by the unobstructed portions of the same wave front.

3 What is Grating? A Plane Diffraction Grating is an arrangement consisting of large no. of close, parallel, straight, transparent and equidistant slits, each of equal width a, with neighbouring slits being separated by an opaque region of width b. The spacing (a+b) between adjacent slits is called the Diffraction Element or Grating Element.

4 Formation of Multiple Spectra with Grating (a+b) sinθ= ± nλ From above equation, (i) For a particular wavelength λ, the directions of principal maxima of different orders are different. (ii) For a particular order n, the light of different wavelengths will be diffracted in different directions. (iii) Longer the wavelength, greater is the angle of diffraction θ. Since the wavelength of violet is least, therefore it is deviated the least and Red is deviated the most. The spectrum consists of white principal maximum of zero order having on either side of it the first order spectra, the second order spectra and so on. The spectra of each order consists of spectral colours in the order from violet to red.

5 Calculation of Wavelength The wavelength of the spectral line can be calculated by the formula:- (a+b) sinθ= nλ where, (a+b) is the Grating Element n is the order λ= wavelength of the light wave; For first order, n=1 For second order, n=2 *The maximum possible value of angle of diffraction is 90. *If the width of grating element is less than thrice the wavelength of light, then only two orders are possible, i.e., (a+b)< 3λ

6 Determination of Grating Element The grating element (a+b) is measured directly from the number of rulings per inch written on the grating by the manufacturer. If this number is N, then N (a+b)= 1 inch = 2.54 cm (a+b) = 2.54/N cm

7 Measurement of θ The cross wire of the eyepiece of telescope is focussed to a spectral line of a particular colour for first order along left side of direct image and its position is noted on both the verniers. Now the telescope is moved to the right of direct image and the cross wire is focussed to the spectral line of same colour and the readings of both the verniers is noted down. Similarly the readings for different colours and different orders of spectrum are noted down. The difference between the two readings of the same vernier gives 2θ for that spectral line.

8 Now when all the necessary things for the calculation of wavelength is calculated, the wavelength of the light of different colours can be calculated by putting in the formula (a+b) sinθ= nλ Dispersive power:- The dispersive power of a diffraction grating is defined as the rate of change of the angle of diffraction with the change in the wavelength of light used,i.e., dθ/dλ

9 Effects on spectrum Effect of Closeness of Rulings:- If (a+b) is small that is the lines on the grating are close together, the dispersive power will be large. That is, angular spacing between maxima becomes large. Effect of width of the ruled surface:- If the width of the ruled surface that is N(a+b) increases, the width of principal maxima decreases, that is, the maxima becomes Sharper. Advantage of increasing the number of Rulings on a grating:- If the number of rulings in a grating increases, the principal maxima become intense and sharp. Moreover, the secondary maxima become weaker.

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