Professor Askew has a pair of \"ruby-optic\" binoculars, meaning that the regula

Question

Professor Askew has a pair of "ruby-optic" binoculars, meaning that the regular lenses of the binoculars have a thin film coating of some material (probably NOT actually ruby) on top of the glass. When exposed to full spectrum light, the light reflected is a beautiful red color. While this looks really cool, it's actually pretty poor for bird watching (which is why he owns them in the first place), where one would like to see bright vibrant colors of plumage. Why doesn't this work well? What does it mean for the reflected light off of the film/lenses to appear red?

Explanation / Answer

The coating of the film on the lenses is such that the width of it is integral multiples of the wavelength of red light (about 650 nm). This means that the reflected red light will be in phase with the incident red light leading to constructive interference thereby making red more dominant. This phenomenon is known as thin film interference.

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