When you are on a train, you may notice that the window glass is like a mirror. When it is bright outside, it is just a clear windowpane, but when the train enters a tunnel or when it is dark outside, your face is reflected in the windowpane.
In fact, this is an important clue to the secret of how things appear in mirrors. The smooth surface of glass reflects light. However, during the daytime, the light coming from the other side of the glass is much stronger than the reflection, so the reflected light cannot be seen clearly.
However, at night, when it is dark outside, the light reflected from the surface of the glass becomes visible. This is why people's faces are sometimes reflected.
This can be easily seen by placing a black piece of paper and a white piece of paper behind the glass and comparing them. The reason why mirrors are so good at capturing images is similar to this. In a mirror, a small amount of silver is coated behind the glass.
This silver reflects almost 100% of the light. Of course, the surface of the glass also reflects light, but the reflection of the silver is stronger, so only what is reflected by the silver is clearly visible in the mirror.
Normally, we do not notice this, but if we look closely at the reflection in the mirror, we can see both the reflection on the surface of the glass and the silver reflection on the reverse side of the glass. In other words, the reflection in the mirror appears to be slightly double.