Question

Why does multiple slit experiment have unequal fringes?

What is the effect of adding more slits?

What is the effect of increasing the width of the slit in a double slit experiment?

Answer 1

In the multi slit experiment:

1.Each of the slit would produce a diffraction pattern,and the width of this pattern is controlled by the width of the slit and wavelength of the light.

2.The light waves from each of the slits superpose and produce an interference pattern.

3.The intensity of these fringes produced by the diffraction pattern produced by each of the slits depends on the number of slits.Intensity decreases as the order of fringes iincrease. Grating intensity gives a peak intensity which is proportional to square of number of slits.Therefore by increases the number of slits intensity increases and the Maxima becomes more narrower sharp and bright.

4. The condition for nth principle maxima

n*wavelength=d*Sin theta

For a 2 slit interference angle theta is so small (<5 degrees) and therefore Sin theta ~ theta

Hence n* wavelength= d* theta ,which results in fringes appearing equally spaced.

But for a multislit experiment (n>2) where diffraction grating is used because the slit separation is small compared to the 2 slit arrangement, the angles at which they are Maxima are LARGE.

So the small angle approximation cannot be made and this the fringes produced are thus unequal.

5.The IntesityI of light produced is directly proportional to the width of the slits in Double slit,therefore by increasing the width between the slits intensity of the light increases.