Reflection by a Plane Mirror

Published on Jul 15, 2016

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We all use the mirror daily to check how we look or appear. The mirror shows us our reflection, standing erect exactly as we appear. The only difference is that when you move your right hand, your reflection moves the left hand. This is known as lateral inversion or sideways inversion.

Reflection and refraction can be understand studied under geometrical optics, a sub-branch of optics, which is concerned with the particle nature of light. In geometrical optics, light is modeled to move in a straight line and interact with matter. A single line of light is called a ray of light and a collection of rays is termed as a beam of light. Geometrical optics is also called ray optics.

Reflection is not scattering

In order for light to get reflected, it has to bounce off a polished surface, otherwise simple scattering of light will take place. In case of the latter, the light particles will just be bounced off randomly, leading to illumination but not a proper reflection. A mirror is a highly polished surface. It allows the light particles to be reflected while following the two laws of reflection:

  1. The angle of incidence is equal to the angle of reflection.
  2. The incident ray, the reflected ray and the normal to the mirror at the point of incidence – all lie in the same plane.

If you consider a plane mirror, the normal to the mirror is an imaginary line that we can draw perpendicular or at 90° to the surface of the mirror. The incident ray is the ray of light that approaches the surface of the mirror. Point of incidence is the point on the surface of the mirror where the incident ray makes contact with the surface. An imaginary line perpendicular to the surface of the mirror drawn at this point is called the normal to the mirror at the point of incidence. The ray of light after reflection is called reflected ray.

First law of reflection

The angle of incidence is the angle that the incident ray makes with the normal to the mirror at the point of incidence. Similarly, the angle of reflection is the angle that the reflected ray makes with the normal. According to the first law these angles are always equal. By an extension, you can also say that the angles that the incident and the reflected rays make with the surface of the mirror are also equal. It is basically a consequence of the fact that light rays always travel along the path between two points which requires the shortest time.

Second law of reflection

The second law of reflection is fairly simple. Given the incident ray, the reflected ray and the normal at the point of incidence, you can find a plane in which all of the three lie. Think of a “plane” as a flat surface of a table top. You can always find such an imaginary surface on which all the three components appear to lie.  These two laws of reflection can be used to explain every phenomenon of reflection that takes place, whether the mirror is plane or spherical.

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