Night-driving: Newsletter Challenge (05/31/05)

My understanding is that glass in the rear view mirror is out of parallel and the back surface is actual silvered. In normal daytime position you are seeing the reflection using the back silvered surface which is around 85%. When you tilt the mirror, the silvered surface is reflecting the ceiling (which is dark at night), but the front surface (which is not silvered) is now reflecting the road. The front unsilvered glass surface is around 5% reflecting which is enought to see whats behind you, but at a much reduced level so the headlamps won't blind you.

Why does clear glass reflect at night?

It is due to something called the fresnel reflection. When light encounters a dielectric material, (glass is a dielectric), it will generally reflect some energy and transmit some energy. The amount of energy transmitted and reflected is related to the property of the material called the index of refraction. The fresnel equations determine how much light is reflected and transmitted at an interface based on the index of refraction. For most glass, the amount reflected at an interface is 4-5% thus even bare glass produces a reflection. The amont reflected also depends on angle. If you take a sheet of clear glass and look at it at near grazing incidence instead of directly through it, you will notice it becomes a pretty good mirror. For a thorough explanation of the fresnel equations, see http://scienceworld.wolfram.com/physics/FresnelEqu ations.html.

That's a great explanation, Thanks!

In a dimming rear view mirror you've got two reflecting surfaces--one with high reflectance, one with low. During the day you use the high reflector. At night the dimmer button swings the low reflector into place, dimming glare from headlights behind you. The trick is that the two reflecting surfaces are the front and back of the same piece of glass. Said glass is specially ground so that the back surface is slightly tilted relative to the front one. In other words, the glass looks wedge-shaped from the side. The back surface is coated with silver, making it highly reflective. The front surface isn't coated, but it's still slightly reflective, like all glass. Because the two surfaces are out of parallel, any time you look at the rear-view mirror, you're seeing two different reflections simultaneously. During the day with the mirror tilted into the normal position, the silvered surface shows you the road behind you. The non-silvered surface, meanwhile, shows you the back seat of the car--but it's so dim you don't notice it. At night the situation is reversed. When you flip the dim button, the silvered surface tilts so it's showing you the car's ceiling, which is so dark you don't notice it. But now the non-silvered surface is showing you the road. Because the headlights of the cars behind you are so bright, the non-silvered surface reflects enough light to let you see what's behind you. But it's not so bright that you're blinded.

As written in the 6/07 issue of Specs & Techs from GlobalSpec:

Most mirrors are glass with a back reflective coating. This protects the reflective coating from scratches and tarnish. There are front-reflective mirrors but they are rather rare, expensive and usually used in optical systems where you don't want "ghost" reflections from the partially reflecting front glass surface. However, in the case of rear view mirrors, this is exactly what you want to exploit. The trick here is that the two glass surfaces of the mirror are not parallel — the glass is actually a shallow wedge when viewed from the side. When you use the mirror during the day, you have it adjusted so the back silvered surface provides the reflection to your eyes. At night you pivot the mirror, using the factory installed adjuster, by the wedge angle so that the front surface of the glass now directs the reflections to you. Regular clear glass reflects approximately 4% of incident light (in air) from the front surface, so in this position about 4% of the light coming from the headlights behind you makes it to your eyes. The rest of the light (minus losses) is directed up to the headliner of the car (assuming you flip the mirror up to make the adjustment). Pretty simple and beautifully effective. Things have gotten even more interesting in recent years — there is now an automatic dimming mirror based on electrochromic technology where a voltage is applied to a thin layer of a nickel-magnesium alloy that changes the reflective properties of the mirror. The voltage is controlled by the difference in light readings between a backward-facing sensor (which "sees" the headlights in the rear) and another ambient light sensor that detects how dark it is outside. (This second sensor prevents the mirror from darkening just because it's bright in the car during the day.) Cool stuff!