It's and if, then statement!
This question is poorly stated, but I assume you mean what conditions are needed. It would have to be cold outside, correct?
Answer:
are able to see/observe
Explanation:
Humans are not able to see most wavelengths in the universe--but there is a select range that is visible to our eyes. This (which is usually shown on an electromagnetic spectrum diagram/chart/depiction by a small portion of rainbow) is the visible light spectrum
when the two waves interfere with eachother to make a dark spot the periodic difference of the two waves is π . the wave length for 2π is 600nm
. ie. for π difference it is 300nm