An optical flat test performed with both green (532 nm) and red (650 nm) light. The red and green are harmonically opposites, so the fringes fall between each other in certain locations, yet they overlap in others, forming yellow fringes. The wavelengths are nearly harmonically opposite of each other, being roughly λ/5 shorter. A true harmonic-opposite is λ/4 shorter (532 is opposite of 665, and 650 is opposite of 520). The lower inset shows how the waves would overlap and interfere with each other if they were true opposites, directly corresponding to the fringes at the bottom.