# A Very Light Problem

#### Velocity

This question really has me stumped..i would appreciate it if anyone could help me out

White light containing wavelengths of 400nm to 750 nm is shone normally onto a diffraction grating of 3000 lines/cm. Calculate the width of the spectrum that appears as the second antinode on a screen that is 100 cm away.

Is there a specific formula I can use to solve the question? If so then please help.. thanks

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#### Tom Mattson

Staff Emeritus
Gold Member
The condition for maxima/minima is the same as that of a 2-slit apparatus. For adjacent slits separated by a distance d, we must have:

dsin&theta;=m&lambda; for maxima and
dsin&theta;=(m+1/2)&lambda; for minima.

In both cases, m is an integer.

You use the information given about the grating to determine d and you use geometry to determine the displacement y of the antinodes on the screen. Note that your expression for y will depend on the wavelength &lambda;.

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