Anti Reflection Coating

Introduction:

If a wire loop from a child’s blow-bubble tub is dipped in to the soapy solution, and then held vertically, a movement of light over the soapy film takes place. A number of colored bands will be seen if the light is reflected from the soapy surface. As the soap drains downward due to gravity, the appearance of bands changes. and we are left with an almost completely non-reflecting area at the top. After some time, the bubble bursts, because it has become too thin to support itself.

Aim:

If we were to coat the surface of glass lens with a thin film of magnesium fluoride, to the thickness of one quarter of the wavelength of light then in theory we would get a non reflecting surface. Provide that the refracting index of coating allows the two reflected rays to be of equal intensity, there would be no reflection.

Method:

It is not possible to get such type of material which will achieve this condition with low index glasses. Co complete extinction of the two rays will not occur.

An incident ray strike the first surface of lens, 4.3% light is reflected by the first surface and 4.3% light reflected by the second surface of the lens. Only 91.4% light can be transmitted through the glass (crown glass).

If thickness of the magnesium fluoride coating were to have a thickness equal to half the wavelength of the light, reflected rays along both paths would be in phase and reflection would be increased.

If, however the coating is only one quarter of a wavelength thick, the reflected rays will be half a wavelength out of phase and therefore tend to cancel each other. The result will be reduction of reflection and an increase in transmission.

The amount of light reflected at a surface, depends upon the refractive index of medium. A formula to calculate this was derived by French physicist, Fresnel

                                                (n1-n2/n1+n2)2

Where n1 and n2 are refractive indices of first and second substances respectively.

Spectacle crown glass would give the following result:

(0.523/2.523)2=0.043

This means that each surface of the lens would reflect 4.3 per sent at each surface. If the glass which has a hire index, say 1.7,the reflection would rise to 13.4 per sent by both surfaces.

Summery: the advantages of coated lenses are numerous. Car driving at dusk, or playing golf in fading light, and the fact that any spectacle wearer need no longer fear that his eyes will be partially obscured by unsightly reflection from windows or lights. Patient who requires high corrections, particularly deep minus lenses will find that the corrections have a less ‘strong’ look, and that the appearance of power rings is less noticeable. So anti-reflection not only has a definite technical advantage, for any lens power, but a decided cosmetic advantage too.