(II) Two (and only two) full spectral orders can be seen on either side of the central maximum when white light is sent through a diffraction grating. What is the maximum number of slits per cm for the grating?
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The white light is used. The white light lises range of wavelengths from 4000A0 to 7000A0. The red light spread the widest violet spread the smallest. So two fully spectral orders of red light fills the screen completely.
θ is the spreading angle.
The maximum value that the sinθ can take in maximum spreading is 1