The Diffraction Grating
To measure the wavelength of light with a diffraction grating.
The two types of diffraction gratings are the transmission and reflection gratings. They are made by ruling on a piece of glass or metal a number of evenly spaced lines with a fine diamond point. Diffraction phenomena can be analyzed in terms of Huygens’ principle, according to which every point on the wave front of a wave should be considered as a source of wavelets.
Consider the transmission grating shown in Figure 1. The wavelets from the slits will interfere constructively at certain fixed angles.
If the point P is the first order maxima away from the central maximum at O, then it is clear that the wavelets from succeeding slits differ in path by . For the second order, this difference is 2. In general, the maximum from the grating satisfy the equation,
where d is the slit separation.
The experiment set-up was simple to arrange. After we derived and got a better understanding of the theory and method of the diffraction grating from the professor, we placed the diffraction grating approximately 1 meter away from our screen. The light source was placed such that the light passes through the diffraction grating to project onto the screen. Now, we measured the distance from the first order maxima from the center order maxima. Similarly we measured distances of each color inside the first order maxima to the center maxima. This way, we would obtain the adjacent and opposite sides of our angle from the center to the respective order maxima. Recording all these distances as well as the grating constant, we continued to calculate wavelengths to match the figures given in advance.
Also given that the diffraction grating shoots 600 lines/mm. This means there is 1/600 of a line per mm. This equation represents d as in the separation of the slits. Colors
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