What is Reflection of Light? Definition, Laws and Types

Studying Physics Topics can lead to exciting new discoveries and technological advancements.

What do you Mean by Regular Reflection? Compare Regular and Diffused Reflection?

Any luminous body is a source of light. The sources of light may be of two kinds—

1. self-luminous source and
2. non- luminous source.

The sun, the stars, electric bulbs, burning candles, etc. are luminous sources. Non-luminous sources themselves become visible when light from a luminous body falls on them. The moon and the planets are non-luminous bodies. These are visible as light from the sun falls on them and is reflected.

Reflection, absorption and refraction of light: Light travels in a straight line in a homogeneous medium. When it is travelling in one homogeneous medium and meets the surface of another homogeneous medium, the following effects may occur:

1. A portion of the light falling on the surface of separation returns to the first medium. This phenomenon is called reflection of light.
2. A part of the incident light is absorbed by the second medium.
3. If the second medium is transparent or translucent then a portion of the incident light penetrates into the second medium and undergoes a change of direction at the surface separating the two media and continues to travel along a straight line. This phenomenon is called refraction of light.

Reflection of light: When a ray of light passing through a medium is incident on the interface with another medium then, a portion of light returns to the first medium. This phenomenon is called reflection of light.

The surface from which the reflection of light takes place is called the reflector. Amount of reflected light depends on the following two factors.

i) Direction of incident light: The more obliquely the incident light falls on the reflector, the more is the amount of reflected light.

ii) Nature of the first and second medium: It is found from experiment that if light is incident from air to glass normally, about 4% of incident light is reflected. But if light from air is incident on a plane mirror normally, about 80% of incident light is reflected. Again if light is incident from air to glass the amount of light reflected is more than the amount of light reflected when light is incident from air to water. On the other hand, if the reflector is black, most part of the incident light is absorbed by the reflector. As a result there is negligible reflection from a black body or surface.

Some Definitions

In Fig., M₁M₂ is a reflector. A ray AO is incident on the reflector at the point O and is reflected along OB. AO is the incident ray and OB is the reflected ray. O is the point of incidence.

Angle of incidence: The angle which the incident ray makes with the normal to the reflector at the point of incidence is called the angle of incidence. ON is the normal drawn on M₁M₂ at O. Hence, ∠AON is the angle of incidence.

Angle of reflection: The angle which the reflected ray makes with the normal to the reflector at the point of Incidence is called the angle of reflection. ∠BON is the angle of reflection.

Laws of Refleciton

Reflection of light obeys the following two laws.

1. The Incident ray the reflected ray and the normal to the reflecting surface at the point of incidence, all lie on the same plane.
2. The angle of incidence is equal to the angle of reflection.
   In Fig. ∠AON = ∠BON.

Normal incidence: In Fig., the ray of light AO is incident normally on M₁M₂. So the angle of incidence is zero. According to the law of reflection, the angle of reflection is also zero. Thus the ray retraces its path into the first medium.

Types of Reflection

Reflection can be of two types depending on the nature of the surface of the reflector.

1. regular reflection and
2. diffused reflection.

Regular Reflection

If a parallel beam of rays is incident on a smooth plane reflector, then it is reflected wholly as a parallel beam [Fig.]. This type of reflection is called regular reflection. Such reflectors are plane mirror, upper surface of undisturbed water, polished metal surface, etc.

In case of smooth plane surface, the normals drawn at the points of incidence of the beam of rays are parallel to each other. So in case of regular reflection, if the incident rays are parallel, the reflected rays are also parallel to each other. The image of an object is formed due to regular reflection [vide section 1.7]. So, we see our image in a plane mirror.

It will be interesting to note that only the part of the reflector which reflects parallel rays into the observer’s eyes appears brighter to the observer than the other part of the reflector.

Diffuse Reflection

A parallel beam of rays after reflection from a rough surface do not remain parallel more. Sh type of reflection of light is called diffused reflection. In this type of reflection, which occurs on the surface of every visible object each ray of the incident parallel beam is reflected in its own way without being parallel to one another. This is because the normals at the points of incidences are not parallel to each other. Hence no image is obtained, Fig.

However it is because of diffuse reflection the objects in our surroundings are visible. When a beam of rays falls on the rough surfaces of those objects, it is scattered in all directions. So, whatever may be the position of the observer, quite a few rays will invariably enter the eves. As a result, the observer sees the object more or less distinctly. In this case the reflector looks almost equally bright from all directions but no image of a source is seen.
Reflection from any plane, curved or rough surface, follows the two laws of reflection.

Comparison Between Regular and Diffuse Reflections

Regular Reflection	Diffuse Reflection
1. Takes place on a smooth surface.	1. Takes place on a rough surface.
2. Can form images.	2. Cannot form images.
3. To an observer the section of the surface from where the reflected rays are com-ing to his eyes appears brighter in comparison to the other portions of the reflector. For different positions of the observer different portions of the reflector appear comparatively bright.	3. Reflected rays enter the eyes of the observer from all portions of the reflector. So, the observer sees the entire reflector equally bright. The brightness does not depend on the position of the observer.
4. The two laws of reflection hold good.	4. The two laws of reflection hold good.
5. The reflected rays are similar to the incident rays. For examples, If the incident rays are parallel to each other, the reflected rays will also be parallel to each other.	5. The reflected rays are not identical to the incident rays.