11/09/2014 -- Why can't you see out of a lit room at night?

posted Nov 16, 2014, 6:31 PM by Patrick Poole   [ updated Nov 16, 2014, 7:58 PM ]
Sometimes physics helps us discovery new planets or new forms of matter, and those applications are important and exciting. The nice thing about learning how the world works, though, is that it applies to relatively mundane occurrences as well. There’s actually some interesting physics to be discovered in something as simple as whether or not you’re able to see out of a dark room at night. The next time you hear something scary outside your window when it’s dark, you can use what you learn here to peek out successfully.

Light behaves in some ways like a particle and in some ways like a wave

The interaction of light with matter is one of the most interesting and complex areas of physics, and many aspects of it are still studied today. Light can be thought of both as a particle (photons) and a wave (light wave, or traveling and oscillating electric and magnetic fields), and which of these ideas is more correct has been disputed by famous scientists for the past few centuries—Newton was sure light was a particle, and later scientists like Fresnel were proponents of the wave theory, etc. Today we know light exhibits properties of both waves and particles, and we can use the mathematical language of either of these when it is most convenient to the problem at hand. Reflection and transmission makes a lot of sense if you think of light as a traveling wave, so that’s how we’ll discuss it here.

Fresnel reflection

When light hits a different material than the one it is currently in, some light reflects and some transmits

Light waves are oscillating electric and magnetic fields that travel through space. Unlike other waves, light does not need a medium to pass through—ocean waves travel through water, and sound waves travel through the air, but light waves can travel through air, water, or empty space. When light moves from one medium to another, such as from the air inside a room to the glass of a window pane, the nature of the atoms that constitute that new medium have an effect of the traveling light.

It turns out this difference in speed is due to the light wave having to move by jostling the atoms inside the glass—it’s a lot easier to jostle air atoms than glass atoms, so it’s easier to travel through air than glass, and so the index of refraction of air is less than that of glass. We’ll talk more about this atom jostling by light in a later post.

When light first hits the air/glass interface, some of the light gets reflected and some goes through, or is transmitted. Additionally, some of the light can be absorbed by the glass, but usually windows are made not to absorb too much visible light. How much light is reflected or transmitted depends on the index of refraction of the two materials (the air and the glass in our case) and on the angle the light is hitting the interface (you can see this if you have a glass of water outside and you look at the water surface from different angles—at some angles you’ll have a brighter glare than others). Typically windows don’t reflect very much light—only about 4% or so will bounce back, depending on the angle. By the way, this effect is due to Fresnel reflection, which we mentioned previously.

If it’s dark outside, much more room-light is reflected back off a window than outside light transmits through

If you are inside a brightly lit room looking out at the dark night, there are two sources of light for you to see: one is the small (4%) of inside light that is being reflected off of the window, and the other is most of (96%) the light passing into the room from outside. At night the small amount of reflected room light will still be much more than the light entering from outside, and so your window will look like a mirror. If it’s very dark outside, the window will be a mirror regardless of what angle you view it from. One way to still see outside is to hold your face right up against the window and block the room light by placing your hands like binoculars around your eyes, but the real way to see outside is to simply turn out the inside lights!

Inside--turn out those lights!

Now imagine you are looking in from outside. Your eyes will see a small amount (4%) of the outside light reflected from the window, but you’ll see 96% of the room light that transmits through—so it’s very easy to see inside of a lit room at night.

Outside--don't be a creeper
Bonus physics--Anti-reflection coatings
AR coatings ar cool
Sometimes even this 4% reflection from a glass window is not acceptable. For example, the lenses used in laser science are often given a special coating that prevents nearly all light from being reflected—it’s possible to have as much as 99.9% transmission with these special coatings. This is important for scientists who care about looking at very small amounts of light—like astronomers who coat their telescope lenses—or scientists who deal with very large amounts of light—in a laser, reflections from lenses can focus light onto a sensitive mirror and damage it. Antireflection coatings are more prevalent than this though—if you wear glasses, the odds are good they have been treated with an anti-reflection coating as well. You can see an example of the effect this sort of coating has in the picture here. We’ll talk about the science of thin film coatings in a later post (it turns out it has to do with the wave nature of light, as well).

Thanks to Carl Milner for the crescent moon picture and Zaereth via Wikipedia for the AR coating picture.