The Science behind coatings of instruments and spectacles

Hi! We are looking at non-reflective coatings! These guys admit more light into cameras and other optical instruments.

The blue-violet colour produced by a non-reflective coating

Explanation of the appearance:

On axis, the lens looks dark – which means that little light is being reflected, so most is being admitted into the camera – which in turn is the chief purpose of non-reflective coatings: they make the instrument more sensitive to light.

How does the coating create such an effect?

The key idea is destructive interference

Destructive interference refers to waves canceling one another out.

If there is no coating, then light will only be reflected at the boundary between

– lens and air

If there is coating, light will be reflected at the boundary between

– air and coating,

– coating and lens. (see picture below)

 

The purpose of the coating is to provide another layer for incoming light to be reflected so that the reflected waves will cancel each other out.

see video: http://www.youtube.com/watch?v=1bGx0t50Gu0

if little energy is reflected due to destructive interference, then most of the energy must be transmitted into your eyes, or into the your camera!

Why do lenses appear to have a blue-green tinge?

 The key thing to understand is the angle of incidence.

When the angle of refraction in the coating is θ, the pathlength difference is λ/(2n cos θ): longer by a factor of 1/cos θ. So the destructive interference is more complete for longer wavelengths – towards the red end of the spectrum – and the destructive interference is less complete for blue and violet. This explains why the lens in the photo above appears to havea blue-violet tinge – which provides a simple way of recognising such coatings.



These photograhs show the effects of non-reflective coatings on spectacles. In the photo at left, we see less light reflected from the spectacles with the coating (and, as explained above, the destructive interference is more effective at the red end of the spectrum, because the photo is taken off axis).

 Practical benefits of having coating on your spectacles

The non-reflective coating allows more light into the spectacles – the eye receives more light. Another effect is that another observer can see the wearer’s eyes better, because less reflected light from the spectacles is added to that reflected from the wearer’s eyes.

source:
http://www.animations.physics.unsw.edu.au/jw/light/non-reflective-coatings.html

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