Question

Red light of wavelength $625nm$ is incident normally on an optical diffraction grating with $2\times {10}^5$ lines/m. Including central principal maxima, how many maxima may be observed on a screen which is far from the grating?

• A. $15$
• B. $17$
• C. $20$
• D. $18$

Answer 1

The correct option is B $17$

For principle maxima in grating spectra $\frac{sin\theta }{N}=n\lambda$
where, n = (1, 2, 3) is the order of principle maximum and 0 is the angle of diffraction.
$n=\frac{1}{\lambda N}=\frac{1}{6.25\times {10}^{-7}\times 2\times 2\times {10}^5}=8$
Number of maxima $=2n+1=2\times 8+1=17$