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White light enters a diffraction grating placed in front of a very wide viewing screen. Only the central white spot and the first order spectrum are visible on the screen. How many lines per centimeter must the diffraction grating have? I tried using the equation $d\sin{\theta} = m\lambda$, but I don't know what value I would use for $\lambda$. |
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It actually says 'white spot', so that is most likely the entire visible spectrum (about 400-800 nm). Look at the two extreme values of this spectrum, consider how large the angle $\theta$ can be at most and keep in mind that you see the (entire) first order spectrum. |
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