To determine the wavelength of given laser source by plotting diffraction minima’s.
Track and screen from the Basic Optics System, Laser source, Single Slit Disk, Screen white paper, Graduated Scale.
Since the work of Thomas Young, about 1800, light has been thought of as a wave. One can, therefore, speak of the amplitude and phase of a light wave at any point in space. As like any other wave, two or more light waves may interfere at any point to give either an increase or decrease in wave amplitude at that point. When a light wave encounters an obstacle, the light interacts with the material of the obstacle. As a result, the amplitude and phase of the wave is partly changed. The modified part of the wave may then interfere with the rest of the wave, producing a pattern of light and dark. These effects are not usually noticeable because we deal with obstacles large compared to a wavelength, and we do not closely examine the shadows cast by such objects. In that case, a ray picture is quite adequate. If we use a very small obstacle, or look carefully at the shadow, we will see the effects of interference between various parts of the wave. In the laboratory, it is possible to make a small slit. When the obstacle is illuminated by a small light source a screen placed near the slit will show the expected shadow pattern of a bright line on a dark background. As the screen is moved away from the slit, the pattern becomes more complicated, due to the interference of the parts of the wave that interact with the slit edges. At very large distances, one sees an array of bright lines, spaced at regular intervals. Laser light is much more coherent than light from conventional sources. So that one may observe interference effects even when the path difference between the interfering rays is much greater than 109 wavelengths. Figure -1 is schematic of the apparatus used to observe this effect.
Fig. 1. Arrangement for observing diffraction with slit and point source An exact calculation of the diffraction pattern for the situation we have been considering more generally the angle to the maxima (bright fringes) in the interference pattern is given by
d sin θ = mλ (m = 0,1,2,3,. )
λ= a
sin θ/mWhere d is the slit separation, λ is the wavelength of the light, and m is the order 0 for the central maximum, 1 for the first side maximum, 2 for the second side maximum,. counting from the center out?. Since the angles are usually small, it can be assumed that
sin θ≈tan θ
Least count =1 ????????????/???????? ???????????? ???????? ???????????????????????????? ????????????????????
To find the slit width of the first slit.
S. No | M.S.R. | V.C. | MSR+(LC) VC |
1 | |||
2 | |||
3 | |||
S1 =---------------------cm
To find the slit width of the second slit.
S. No | M.S.R. | V.C. | MSR+(LC) VC |
1 | |||
2 | |||
3 | |||
To determine the wavelength of laser source
Slit width ‘a’ cm | Order of diffraction (m) | ‘r’ cm | sin ???? ????= √????2 + ????2 | ???? sin ???? ???? = ???? |
S_{1} | ||||
S_{2} | ||||
The wavelength of given laser light is--------------------
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