Laws of reflection of light

Laws of reflection of light

First law : The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.

Second law : The angle of reflection (∠r) is always equal to the angle of incidence (∠i).

i.e.,      ∠r = ∠i

(For normal incidence, i = 0, r = 0. The ray is reflected back along normal).

A ray of light striking the surface normally retraces its path.

When a ray of light strikes a surface normally, then angle of incidence is zero i.e., Ði = 0. According to the law of reflection,
∠r = ∠i, ∴ ∠r = 0 i.e. the reflected ray is also perpendicular to the surface. Thus, an incident ray normal to the surface (i.e. perpendicular to the surface) retraces its path as shown in figure.

Normal Incidence

Laws of reflection are also obeyed when light is reflected from the spherical or curved surfaces as shown in figure (a) and (b)

Reflection from curved surface

Regular and Irregular Reflection :

Regular Reflection – The phenomenon due to which a parallel beam of light travelling through a certain medium, on striking some smooth polished surface, bounces off from it, as parallel beam, in some other fixed direction is called Regular reflection.

Regular reflection
Regular reflection

Regular reflection

Regular reflection takes place from the objects like looking glass, still water, oil, highly polished metals, etc.

Regular reflection is useful in the formation of images, e.g., we can see our face in a mirror only on account of regular reflection. However, it causes a very strong glare in our eyes.

Irregular reflection or Diffused reflection :


Irregular or diffused reflection
Irregular or diffused reflection

Irregular or diffused reflection : The phenomenon due to which a parallel beam of light, travelling through some medium, gets reflected in various possible directions, on striking some rough surface is called irregular reflection or diffused reflection.

The reflection which takes places from ground, walls, trees, suspended particles in air, and a variety of other objects, which are not very smooth, is irregular reflection.

Irregular reflection helps in spreading light energy over a vast region and also decreases its intensity. Thus, it helps in the general illumination of places and helps us to see things around us.

Note : Laws of reflection are always valid no matter whether reflection is regular or irregular.