Question

20. In a single slit diffraction experiment first minimum
for red light (660 nm), coincides with first maxima
of lambda. Then
(1) 3300 Å
(2) 6600 Å
(3) 4400 Å
(4) 1100 Å

Answer 1

In the experiment the first minimum for red light coinciding with the first maxima of lambda will be (3) 4400 Å

The first minima of the red light will coincide with the maxima of some other light .

According to Fraunhofer Diffraction -

b Sin Ф = n λ  

where, - wherein  b is the slit width  and Ф is the angle of deviation.

For first maxima -

b Sin Ф = 3λ/2  

For first  minima of red light -

b Sin Ф = λred

 Thus,

λred = 3λ/2

2λ/3 red = 4400 Å