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What is Regular and Diffuse Reflection of Light
In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction. In this case, parallel incident rays remain parallel even after reflection and go only in one direction (see Figure). Regular reflection of light occurs from smooth surfaces like that of a plane mirror (or highly polished metal surfaces). For example, when a parallel beam of light falls on the smooth surface of a plane mirror, it is reflected as a parallel beam in only one direction as shown in Figure. Thus, a plane mirror produces regular reflection of light. Images are formed by regular reflection of light. For example, a smooth surface (like that of a plane mirror) produces a clear image of an object due to regular reflection of light. A highly polished metal surface and a still water surface also produce regular reflection of light and form images.
This is why we can see our face in a polished metal object as well as in the still water surface of a pond or lake. A polished wooden table and a marble floor with water spread over it are very smooth and hence produce regular reflection of light. The regular reflection of light from a smooth surface can be explained as follows : All the particles of a smooth surface (like a plane mirror) are facing in one direction. Due to this the angle of incidence for all the parallel rays of light falling on a smooth surface is the same and hence the angle of reflection for all the rays of light is also the same . Since the angle of incidence and the angle of reflection are the same (or equal), a beam of parallel rays of light falling on a smooth surface is reflected as a beam of parallel light rays in one direction only (see Figure).
In diffuse reflection, a parallel beam of incident light is reflected in different directions. In this case, the parallel incident rays do not remain parallel after reflection, they are scattered in different directions (see Figure). The diffuse reflection is also known as irregular reflection or scattering. The diffuse reflection of light takes place from rough surfaces like that of paper, cardboard, chalk, table, chair, walls and unpolished metal objects. For example, when a parallel beam of light rays falls on the rough surface of a sheet of paper, the light is scattered by making reflected rays in different directions (see Figure).
Thus, a sheet of paper produces diffuse reflection of light. No image is formed in diffuse reflection of light. For example, a rough surface (like that of paper) does not produce an image of the object due to diffuse reflection of light. Actually, the light rays falling on the rough surface of paper are scattered in all directions and hence no image is formed. The diffuse reflection of light from a rough surface can be explained as follows: The particles of a rough surface (like that of paper) are all facing in different directions. Due to this, the angles of incidence for all the parallel rays of light falling on a rough surface are different and hence the angles of reflection for all the rays of light are also different. Since the angles of incidence and the angles of reflection are different, the parallel rays of light falling on a rough surface go in different directions (see Figure). Please note that the diffuse reflection of light is not due to the failure of the laws of reflection. Diffuse reflection is caused by the roughness (or irregularities) in the reflecting surface of an object (like paper or cardboard, etc.). The laws of reflection are valid at each point even on the rough surface of an object.
The surfaces of most of the objects are rough (or uneven) to some extent. So, most of the objects around us cause diffuse reflection of light and scatter the light falling on them in all directions. In fact, we can see these objects only because they scatter light rays falling on them in all directions. For example, a book lying on a table can be seen from all parts of the room due to diffuse reflection of light from its surface. The surface of book, being rough, scatters the incident light in all parts of a room. Hence the book can be seen from all parts of the room. A cinema screen has a rough surface and causes diffuse reflection of light falling on it. The cinema screen receives light from a film projector and scatters it in all directions in the cinema hall so that people sitting anywhere in the hall can see the picture focused on the screen. We will now study the formation of an image by a plane mirror.