-5 -4 -3 -2 -1 0 1 2 3 4 5 Distance (mm) 0 0.2 0.4 0.6 0.8 1 Intensity (a.u.) Fi

Question

-5 -4 -3 -2 -1 0 1 2 3 4 5 Distance (mm) 0 0.2 0.4 0.6 0.8 1 Intensity (a.u.) Figure 2: The diffraction pattern of a double slit. (a) Explain why multiple intensity maxima occur on the viewing screen. (b) Figure 2 shows the pattern for a double slit with a slit separation of 0.400 mm. What wavelength is used to illuminate the slit? (c) Consider the double slit that was used to create the pattern in the previous section. Describe how the pattern would change if extra slits were added (keeping a constant slit spacing). Ensure you discuss the intensity, location and shape of the peaks. Part B (Advanced) Consider viewing a distant point source of monochromatic light. Ideally the image formed on the retina of the eye would be an extremely small spot. Discuss how diffraction by the aperture of the eye limits the sharpness of the spot formed on the retina. Estimate the size of this sp

Explanation / Answer

Given intensity pattern of diffraction slits on a viewing screen

a. there are multiple maxima intensity peaks because of diffraction. So when the monochromatic light passes through the interference slits which are closely spaced then the light begans to behanve as if it is coming from two different sources in phase, ( or multiple sources depending on the number of slits). Due to this, at different points on the viewing screen, we might get constructive / destructive/ out of phase interference of light from different sources and hence we get multiple maximas on viewing screen.

b. from figure 2

2nd minima is at y = -2 mm

so using the formula for youngs double slit diffraction for destructive interference

y = 3*lambda*D/2d

given, d = slit seperation = 0.4 mm

D = 1 m

so 2*10^-3 = 3*lambda*1/2*0.4*10^-3

lambda = 5.33*10^-7 m is the wavelength of light used

c. if extra slits are added, the formula for the maxima and minima locations on the viewing screen remains the same

so the relative seperation of maximans and minimas does not change but, we get multiple secondary maximas and minimas inside the primary maximas and minimas we get to see in the double slit experiment. Moreover, the intensity distribution of the intesity pattern on theviewing screen changes with maximum light falling into the central maxima

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