A plane mirror reflects light

Mirror, mirror, why are you mirroring me?

Whether for applying makeup, shaving, examining our wrinkles and gray hair or on and in the car to keep an eye on the traffic, mirrors are a helpful tool for us in everyday life! But why can we even see ourselves in it?

The principle

A mirror has a very smooth surface. It consists of a thin aluminum or silver layer and a layer of glass in front of it, which serves as stabilization and protection. In order to create a mirror image, light is also necessary in addition to a smooth surface, because if the light beam hits us, part of the light is reflected back onto the mirror, i.e. the light beam is reflected while the rest is swallowed. The smooth metal layer ensures that, on the one hand, as much light as possible is reflected and, on the other hand, this also happens in an orderly manner, i.e. the light beam is reflected symmetrically to the perpendicular (an imaginary line perpendicular to the mirror surface, see illustration) (angle of incidence = angle of reflection). The sum of all reflected light rays then create the mirror image when they hit our eyes (see figure below, red lines).

Our brain is also involved

Our brain is also significantly involved in the appearance of the mirror image. Because the mirror kinks the light rays that hit the surface, because, as already mentioned, they fall back on us at the same angle as they hit. However, our brain cannot process this information, thinks away the kinks and extends the light rays through the mirror (see figure below, blue lines). Therefore it seems as if we are in or behind the mirror.

The one-way mirror!

Crime fans are probably familiar with the scenes in which a suspect is asked for questioning by a police officer. This is usually done in a brightly lit room with a large mirror that also appears to be a typical mirror from the interrogation room. But of course everyone knows that there are observers on the other side and can see through him into the room.

How does it work?

With this type of mirror, it has to be very bright in the interrogation room so that it can reflect enough light into the room, while it has to be very dark on the observer side so that there is no mirror image for these people due to incident light. The one-way mirror also has special properties, for example the metal layer is so thin that it can also let through (transmit) part of the incident light. Because a small part of the light penetrates to the observer side, they can watch the interrogation.

In short: The mirror must have a high degree of light reflection and a low degree of transmission!

Can animals actually see each other in the mirror?

Some yes, others (probably) no. Whether or not an animal can see itself in the mirror can be determined, for example, with the “rouge test”. To do this, a red dot is painted on the animal's forehead or back and then a mirror is held up to it. If the animal tries to wipe away the colored point, this is an indication that the animal can see itself. This has been found, for example, with:

Of course, it can also be that the animal sees itself in the mirror, but does not react to the red dot. In this case the test is unfortunately ineffective ...

We also have more to offer on the subject of mirrors, light and reflection:

  • Reflection on the plane mirror - for beginners:


  • Reflection on the concave mirror - for advanced users:


  • Reflection on the curved mirror - for experts:


Additional information can be found here: