A diaphram with round aperture, whose radius `r` can be varited during the experiment, is placed between a point source of light and a screen. The distance from the diaphram to the source and the screen are equal to `a = 100 cm` and `b = 125 cm`. determie the wavelength of light if the intensity maximum at the centre of the differaction pattern of the screen is observed at `r_(1) = 1.00 mm` and the next maximum at `r_(2) = 1.29 mm`.

Explanation

Suppose maximum intensity is obtained when the aperture contains `k` zones. Then a minimum will be obtained for `k + 1` zones. Another maximum will be obtained for `k = 2` zones. Hence
`r_(1)^(2) = k lambda (ab)/(a + b)`
`r_(2)^(2) = (k + 2) lambda(ab)/(a + b)`
Thus `lambda = (a + b)/(2ab) (r_(2)^(2) - r_(1)^(2)) = 0.598 mu m`
On putting the values.