Why Do Windows Reflect At Night? The Physics Explained

Have you ever tried looking out from your window at night when it’s dark outside, while having your interior lights on? You may notice that you can’t see very well outside and the windows appear almost as if they were mirrors. So, why do windows seem like they become more reflective at night?

In short, windows become more reflective at night because the light coming from indoor lighting gets reflected off the window. In fact, windows reflect also during the day, but at night, the reflections are much more visible as there is little to no sunlight coming from outside.

In this article, we’ll be discussing this phenomenon in detail and it begins by actually understanding what happens when light hits a window. The surprising answer is that a window, or glass in general, is not actually completely transparent.

Do Windows Actually Reflect Light?

When most people think of a window (or glass), they think of something that’s transparent. However, this is not completely true because even glass is not perfect. So, does a window then actually reflect light?

A window reflects a small percentage of the light that hits it. When light transitions from air to glass, most of it gets refracted, however, a small amount will also get reflected. This is why it is possible to see a fuzzy reflection of yourself in a window, even during the day if you look closely.

Generally, any time there is a transition of light moving from one material to another (in this case, from air to glass), there will be both reflection and refraction happening (refraction is the phenomenon where light travels through the material but its direction, wavelength and speed will change).

Fundamentally, this occurs because no material acts as “perfectly transparent” (well, apart from possibly dark matter, but nobody knows yet). At least such a material has not been discovered so far.

This, of course, applies to glass as well, meaning that no window is perfectly transparent, letting all of the light through it. Some of it, albeit a fairly small amount, will get reflected.

Now, understanding this fact is key in order to understand what happens to a window when it’s dark. In particular, why a window acts almost like a mirror when it’s dark outside.

How Does a Window Turn Into a Mirror At Night?

A window does not turn into a mirror at night anymore than it does during the day. However, at night, there is very little sunlight from outside, meaning that it is simply easier to see the light coming from inside that gets reflected off the window. This makes the window act as if it were a mirror.

Let me explain this in a bit more detail. First of all, you may notice that during the day, it is much harder to see your reflection in the window (you can, but you have to look very carefully).

This is simply because of the overwhelming amount of sunlight that comes from the outside, which makes it very hard to see the small amount of reflected light that comes from the inside.

Based on this, you could probably imagine what happens at night when it’s dark outside. Now there would be little to no sunlight from the outside (also assuming there isn’t much moonlight or other outside lights on).

This would make the reflected inside light pretty much the only light at play here. This, on the other hand, would mean that you could see the reflected light much more easily as there is no outside light coming in.

Interestingly, if you have almost no light coming from the outside and some bright lights on inside, you’ll notice that you can’t really see outside at all. Now turn off the inside lights and you’ll notice that you can now see much better outside even if it’s dark.

So, really the key idea here is quite intuitive; the more light there is coming from a certain direction, the easier it is to see in that direction and the harder it is to see in the opposite direction.

Is It Possible For Someone To See Inside Your House At Night?

Have you ever been inside at night and someone walks past your window? If you have, you may have thought to yourself whether or not they could see inside your house from the outside. So, can someone actually see inside your house at night?

It is possible for someone to see inside your house at night, but only if you have lights on inside and there aren’t any outside lights on. If, on the other hand, you turn the indoor lights off and have some bright outdoor lighting, people will probably not be able to see inside your house at night.

So, in case you want more privacy at night, this would be a simple physics-based way to do so; just have your indoor lights off and get some bright outdoor lamps.

You’ll be able to see outside pretty easily since there is a lot of light coming from the outside in, but someone looking from outside won’t be able to see inside your house very well since there will be a lot of reflection as seen from the outside.

Of course, if you really want some privacy, just get some curtains on your windows.