Reflection of Light and Laws of Reflection


Reflection and refraction are the two important phenomena that occur with light rays when they are incident at a boundary seperating two optical media. As we have discussed already in rectilinear propagation of light rays, light travels in straight line as long as the medium remains uniform.

But this is not always the case when a light beam passes through the interface between two adjacent media. The light rays may suffer a deviation from its original path. The deviation produced may also be of continuous nature provided the medium has continuously varying index of refraction (with respect to position) and this may result into a curvilinear propagation of light as well.

The deviation in the path of a light ray is mainly caused by the two optical phenomena - reflection and refraction. In most instances, both the events i.e. reflection and refraction occur together and simultaneously. Here, in the present article we keep our focus on reflection of light only.

Reflection of Light

When a beam of light is incident on the boundary seperating two media, the boundary throws partly or wholly the incident rays back into the medium of incidence. This phenomenon is known as reflection of light.

The boundaries or interfaces causing reflection are called reflectors and they may be plane or curved, smooth or rough, transparent or opaque.

In the above figures, O is the point of incidence, AO is the incident ray, OB is the reflected ray and ON is the normal drawn to the reflecting surface.

Terminology related to Reflection

Angle of Incidence : The angle which the incident light ray makes with the normal at the point of incidence is termed as the angle of incidence. A general notation for the angle of incidence is 'i'. In the above figures, ∠AON is the angle of incidence.

Angle of Reflection : The angle which the reflected light ray makes with the normal at the point of incidence is known as the angle of reflection and it is generally denoted by 'r'. In the above figures, ∠NOB is the angle of refraction.

Glancing Angle : The angle made by incident ray of light with the plane reflecting surface is called glancing angle. It is denoted by 'g'. If 'i' be the angle of incidence then, the glancing angle, g = 90° - i.

The Two Laws of Reflection

Whenever the reflection of light rays occurs, it always happens by following some rules or laws. These laws are called the laws of reflection. There are two laws of reflection:
  1. The incident ray, the reflected ray and normal to the reflecting surface at the point of incidence, all lie in the same plane. This unique plane is called the plane of incidence or the plane of reflection.
  2. The angle of incidence is equal to the angle of reflection i.e. i = r.

Some Important Points

  • If ∠i = 0, ∠r = 0 i.e. if a ray of light is incident normally on a boundary, after reflection it retraces its path. This is what we have already studied as ''principle of reversibility of light rays''.
  • None of the frequency, wavelength and speed of the electromagnetic radiation (light wave) changes due to reflection. However, intensity and hence amplitude usually decreases (I ∝ A²).
  • The two laws of reflection (which we saw above) are expressible as a single mathematical vector equation as follows: r = e - 2n(e.n) where r is the unit vector along the reflected ray, is the unit vector along the incident ray and is a unit vector parallel to the normal drawn on the reflecting boundary. The above relation implies that, once the direction of incident ray and the orientation of the reflecting surface are fixed, the direction of reflected ray becomes predetermined (based on the two laws of reflection).
  • Whenever reflection takes place, the component of incident ray parallel to the reflecting surface remains unaltered, while the other component perpendicular to the surface of reflection reverses in direction.
  • The laws of reflection are valid for any type or nature of reflecting surface be it plane or curved, regular or irregular etc.

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