Ncert Exercises & Solutions

Consider sunlight incident on a slit of width $10^{4}$ $\overset{\circ}{A}$ . The image seen through the slit shall

1.	be a fine sharp slit white in colour at the centre
2.	a bright slit white at the centre diffusing to zero intensities at the edges
3.	a bright slit white at the centre diffusing to regions of different colours
4.	only be a diffused slit white in colour

(a) Hint: The diffraction of light occurs when it encounters an object of size comparable to its wavelength.

Step 1: Find the size of the slit.

Given, the width of the slit = $10^{4}$ $\overset{\circ}{A}$

= $10^{4} \times 10^{- 10} m = 10^{- 6} m = 1 μm$

Step 2: Find the wavelength of the light.

The wavelength of (visible) sunlight varies from 4000 $\overset{\circ}{A}$ to 8000 $\overset{\circ}{A}$ .

As the width of the slit is comparable to that of wavelength, hence diffraction occurs with maxima at the centre. So, at the centre all colours appear i.e., mixing of colours forms a white patch at the centre.

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