A slit of width `a` illuminated by red light of wavelength `6500 Å`. The first minimum will fall at `theta=30^(@)` if `a` is

Options

A. `6.5 xx10^(-4)mm`
B. `1.3` micron
C. `3250Å`
D. `2.6xx10^(-4)cm`

Answer

B

Explanation

According to principle of diffraction,
`a sin theta=n lamda`
where, n=order of secondary minimum
`therefore a sin 30^(@)=1xx(6500xx10^(-10))`
or `a=1.3xx10^(-6)m`
or `a=1.3` micron