# Bandgap Spacing in Photonic Crystals

I am doing some self-study on photonics and have encountered the following question:

We know that amorphous electronic crystals such as amorphous silicon have a bandgap. Can amorphous photonic crystals also have a bandgap? Roughly how large would the spacing, d, have to be for a bandgap centered around ~ 600 nm?

Here is my solution: Since $n\lambda=2dsin(\theta)$, if $\lambda=600 nm, \theta=\pi/4, n=1,$ then $d=4.24 \times 10^{-7}$.

Could somebody check over my work?

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What exactly is an amorphous photonic crystal? One which has no long-range order? – BandGap Nov 13 '12 at 13:52
Also, isn't the Bragg formula used for constructive interference? If so you'd have to take d*0.5 – BandGap Nov 13 '12 at 14:00