My mommy told me a bedtime story. Is it true?

[Abitconfused]

Once upon a time there were ten sprinters lined up in single file behind the start line of a track on the inside. They belong to “Team Lens Reflection” and eagerly anticipate the sound of the starter pistol and an easy win. Oddly enough, these runners cannot run straight. They must run as if a wave right then left to the edges of the track to make progress. Lined up with similar eagerness behind the same start line on the outside of the track are another ten sprinters in single file. They belong to “Team Coating Reflection” and confidently anticipate the sound of the pistol and victory. They cannot run straight either. They must run like a wave left then right to the edges of the track to make progress. All runners are the same size and have the same stride length and frequency. How far do our runners get at the sound of the pistol? Right. They might as well never have left the start line. They hit each other and stop within a few strides. Similarly, lens coatings create a reflection wave that blocks a lens reflection wave.

[tao2]

Which begs the question...

"If ah hold a lens in my hand, put a rear end cap on it, look into the front of it - why don't ah see a completely black hole? "

[pnodrog]

First problem is they don't hit each other they are all traveling at the same speed and if their paths do cross they just pass through each other like ghosts...Your story does not even scratch the surface of the lens coating. Which of course you should avoid anyway...

Best not to worry as much as you do about the physics, just gain enough practical understanding to take the photographs you want.

[Dave Humphries]

Problem is that the ten runners do not have equal strides and pace frequencies: each a different colour from 'deep' red (with a long stride) to 'ultra' blue (short stride).

You're making me think - never a good thing

[James G]

if their paths do cross they just pass through each other like ghosts

And a most effective demonstration of this can be photographed. Try dropping two stones into a still pond and watch the ripples spread. Or a rain puddle! (not the best shot I ever took )

[Abitconfused]

Now the technical stuff. Light reflected from a lens is in the form of a wave, for our purposes, an undesirable wave. Because reflection is a wave, engineers can create an “anti-wave” to eliminate the reflection. They must create a reflected wave of opposite phase (up when the other wave is down). The anti-wave must have an equal frequency, an identical magnitude, and travel in the same direction as the undesirable reflected wave. The thickness of a lens coating must be exactly 1/4 of the wavelength of light for creation of an out of phase wave. When the reflected light wave from the surface of the lens meets the reflected light wave from the surface of the lens coating, 1/4 wavelength distant, they are 1/2 wavelength out of phase. They are 1/2 out of phase because some of the incoming light, having traveled through the 1/4 wavelength thick coating, is reflected back through the 1/4 wavelength coating and 1/4 + 1/4 is 1/2. The difference between the trough and crest of the reflected waves is one-half wavelength and they are traveling in the same direction. Collision is unavoidable and annihilation results. This “destructive interference,” cancels out both reflected waves. So-called “quarter wave coatings” are very effective. Reflections are gone. Eliminated. Almost none, anyway, 99.9% gone. Multi-layered lens coatings of different thicknesses on a quality lens eliminate reflection throughout all wavelengths of the visual spectrum. It works very, very well.

[tao2]

Now the technical stuff. Light reflected from a lens is in the form of a wave, for our purposes, an undesirable wave. Because reflection is a wave, engineers can create an “anti-wave” to eliminate the reflection. They must create a reflected wave of opposite phase (up when the other wave is down). The anti-wave must have an equal frequency, an identical magnitude, and travel in the same direction as the undesirable reflected wave. The thickness of a lens coating must be exactly 1/4 of the wavelength of light for creation of an out of phase wave. When the reflected light wave from the surface of the lens meets the reflected light wave from the surface of the lens coating, 1/4 wavelength distant, they are 1/2 wavelength out of phase. They are 1/2 out of phase because some of the incoming light, having traveled through the 1/4 wavelength thick coating, is reflected back through the 1/4 wavelength coating and 1/4 + 1/4 is 1/2. The difference between the trough and crest of the reflected waves is one-half wavelength and they are traveling in the same direction. Collision is unavoidable and annihilation results. This “destructive interference,” cancels out both reflected waves. So-called “quarter wave coatings” are very effective. Reflections are gone. Eliminated. Almost none, anyway, 99.9% gone. Multi-layered lens coatings of different thicknesses on a quality lens eliminate reflection throughout all wavelengths of the visual spectrum. It works very, very well.

And, in answer tae #2 ?

[Abitconfused]

Waves in a puddle that radiate randomly certainly can be compared to waves traveling out of phase, in the same direction, with the same magnitude, and frequency.

I challenge anyone to offer up a paragraph as comprehensive technically but more readable. I have not seen one online. And I am so poorly schooled.

[tao2]

Hi Ed,
as ah asked, if multi-coating is so effective, why aren't lenses opaque when we look at them?

[Abitconfused]

Perhaps it does not matter as the dynamic range of our vision still exceeds by a fair margin that of the camera. So a more blunt visual instrument, living in a darker world, cannot see what we see.

[tao2]

Perhaps it does not matter as the dynamic range of our vision still exceeds by a fair margin that of the camera. So a more blunt visual instrument, living in a darker world, cannot see what we see.

So...

multi-coating is unnecessary ?

[Abitconfused]

OK thinking now. The reflected light frequency is diminished by coatings the "incident" light is not.

[tao2]

OK thinking now. The reflected light frequency is diminished by coatings the "incident" light is not.

Aaah, so this can't be true then...

Because reflection is a wave, engineers can create an “anti-wave” to eliminate the reflection. They must create a reflected wave of opposite phase (up when the other wave is down). The anti-wave must have an equal frequency, an identical magnitude, and travel in the same direction as the undesirable reflected wave.

[Abitconfused]

I suspect you can see destructive interference in a wave tank in real time. And I suspect the 1/4 wavelength could be duplicated as well. But as to seeing the benefits, to do this dissasemble a lens scrape off all the coatings, I bet a Brillo pad would work, reassemble and compare before and after.