How Anti-Reflective Coating Works: The Physics of Interference

Why a bare lens reflects light

When light meets the surface of a lens, part of it passes through the material and part is reflected back. The reason is the change in refractive index: light travels at a different “speed” in air and in the lens material. At every such transition, a percentage of the energy is reflected.

On an ordinary organic lens, about 4% of the light is lost at each surface — and a lens has two surfaces. The result is ghost images, blurring and those annoying reflections that hide your eyes in photographs.

The trick: destructive wave interference

This is where physics comes in. Light behaves like a wave. If you add a thin layer to the lens with a carefully chosen thickness and refractive index, you create two reflections: one from the top surface of the layer and one from the layer-lens interface.

• If the thickness of the layer is one quarter of the wavelength of the light, the two reflections come out in opposite phase.
• A wave at its “peak” meets a wave at its “trough” — and they cancel each other out.
• The light that is not reflected doesn’t disappear; it passes through the lens. So you see more clearly.

This cancellation is called destructive interference. It’s the same phenomenon that gives colours to a soap bubble or to a thin film of oil on water.

Why you see green or purple glints

The problem is that visible light doesn’t have a single wavelength — it spreads from violet (~400nm) to red (~700nm). A single layer can perfectly cancel only one wavelength. Manufacturers usually optimise it for the centre of the spectrum (green-yellow), because that’s where the eye is most sensitive.

The result: the edges of the spectrum — blue and red — are reflected slightly more. The residue that reaches your eye appears as a faint green, purple or blue glint. It’s not a defect; it’s the “signature” of a good coating.

Modern multi-coat coatings partly solve the issue: they stack several layers with alternating refractive indices, so they cancel reflections over a broader range of wavelengths. This way the residual glint becomes fainter and more neutral.

Where it helps in practice

AR coating isn’t just about aesthetics. It translates into less eye strain in front of screens, better night vision (fewer halos from lights) and more transparent lenses in photographs. It is often combined with other functional coatings in a “package” — for the full picture, see our guide to lens coatings and how polarization works with different physics.

If you want to combine anti-reflective technology with thinner high-index lenses, see the relevant page. You can also compare how different lens technologies behave in polarized vs photochromic vs mirror.

A quick summary of the physics

1. Light is a wave and reflects at every change in refractive index.
2. A layer of thickness λ/4 creates two reflections in opposite phase.
3. The reflections cancel each other out → more light passes through.
4. One layer doesn’t cover the whole spectrum → a coloured glint remains.
5. Many layers → broader coverage and a more neutral residual glint.