Red light of wavelength 625 nm is incident normally on an optical diffraction grating with `2xx10^(5)` lines/m. Including central principal maxima, how many maxima amy be observed on a screen which is far from the grating ?

Options

A. 15
B. 17
C. 8
D. 16

Answer

B

Explanation

Here,
Wavelength, `lamda=625" nm "=625xx10^(-9)m`
Number of lines per meter, `N=2xx10^(5)` For principal maxima in grating spectra
`(sintheta)/(N)=nlamda`
where `n(=1,2,3)` is the order of principal maxima and `theta` is the angle of diffraction
The maximum value of `sintheta` is 1
`:.n=(1)/(Nlamda)=(1)/(625xx10^(-9)xx2xx10^(5))=8`
`:.` Number of maxima `=2n+1=2xx8+1`
`=17`