A light ray, as we’ve seen earlier, is an infinitely thin beam of light, whose wavelength is approximated to be zero. So long as we’re dealing with objects and openings whose dimensions are much larger than the actual wavelength of the light used, we can safely use the approximation of the light ray to get fairly correct results. In what follows, we will briefly describe how we can put the concept of light rays to use, to understand simple optical phenomena involving reflection and refraction of light.
Reflection at a plane mirror
When you look at yourself in the mirror every morning, you naturally begin examining your appearance without giving a second thought to the question of how exactly you are able to see yourself in the mirror in the first place. A more careful inspection of what’s happening when you’re looking at your face in the mirror, can be quite fascinating. Firstly, to see yourself in the mirror, the room/place you’re in must be fairly lit. This external light first falls on your face and is reflected in different directions. Let’s consider the light that is reflected in the direction of the mirror – this light again undergoes reflection at the mirror and the reflected light is what enters your eyes and gives you a perception of your appearance. The reflection of light at the mirror happens according to certain rules. There are basically two laws.
Speaking only in terms of single rays: the ray that is incident on the mirror, the ray that is reflected back, and the “normal” to the mirror surface, all lie in the same plane.
Recall that a plane can be uniquely defined by two lines – give us two lines and we will tell you the plane in which they lie. And with the plane defined this way, we can very easily decide whether or not a given third line lies in the same plane. In stating the first law, we have three “lines” – the incident ray, the reflected ray and the normal. While the incident and reflected rays are very much real, the normal is a geometric construction, an imaginary line drawn for our reference. It is drawn perpendicular to the surface of the mirror, passing through the point at which the incident light ray is incident on the mirror (see the image below).
Now, what the rule says is, when a light ray is incident on the plane mirror, it forms a unique plane with the normal. With this plane defined, now the reflected ray has to lie in the same plane (the direction in which the reflected ray can travel is confined to this plane).
The angle of incidence equals the angle of reflection ()
What this law says is that the reflected ray is not just confined to the plane of the incident ray and the normal, but also is confined to travel in a particular direction after reflection. What this direction is, is ascertained by measuring the angles at which the incidence and the reflection happen. The angle of incidence is the angle formed between the incident ray and the normal. Similarly, the angle of reflection is the angle formed between the reflected ray and the normal. Therefore, the law 2 says that the reflected ray gets reflected at a particular angle with the normal which is equal to the angle made by the incident ray with the normal.
In conclusion, how a ray gets reflected from a plane mirror completely depends on how the incident ray is incident. Both, the plane in which the reflection must happen and the angle at which the reflected ray must emerge, are decided by the direction from which the incident ray is incident on the mirror.
An important special case is when the angle of incidence equals zero degrees. This is the case of “normal incidence” of light. As the second law says, the angle of reflection will also be zero. Therefore, the light ray falling normally at a plane mirror, reflects back along the same path that it came through.
And as you go on increasing the angle of incidence from zero, the angle of reflection too increases, and at every instance will be equal to the incidence angle.
Up next, we’ll see how reflection of light happens at mirrors that may not be plane, and may be curved.