Discussion
A grating passes \( 600 \ \mathrm{nm} \) in first order. Which wavelength arrives at the same angle in second order?
Answer: B
Successive orders satisfy the grating equation at the same angle when:
\[ \begin{aligned} m\lambda &= \text{constant} \Rightarrow \lambda_{2} \\ &= \frac{600}{2} \\ &= 300 \ \mathrm{nm} \end{aligned} \]
giving 300 nanometres. Doubling rather than halving would place it outside the grating's range in that order. That the contaminating wavelength is always shorter, and so more energetic, is why an order-sorting filter is a long-pass one.
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