ICSE Class 7 Physics Chapter 06 Reflection of Light

Reflection of Light

When light travelling through a medium (like air) strikes a surface, for example, a wall or a floor, some of it bounces back or travels back into the medium. We call this the reflection of light. All surfaces around us reflect light. That is how we see things. We see things when the light reflected by them enters our eyes.

Regular and Diffuse Reflection

Though all surfaces reflect light, some do it better than others. Smooth, polished surfaces are better reflectors than rough, dull surfaces. And really smooth surfaces, like that of a mirror, reflect light so well that they let us see clear images of things near them.

Why is it that smooth surfaces create images, while rough surfaces do not? When a parallel beam of light from an object falls on a smooth surface, it remains parallel after reflection. When this reflected beam enters our eyes, we see a clear image of the object. However, when a beam of light from an object falls on a rough surface, the bumps and pits on the surface make the reflected rays travel in different directions. Hence, most of them enter our eyes from slightly different directions, and we see a hazy image or no image at all. Reflection from a smooth surface is called regular reflection, while reflection from an uneven surface is called diffuse reflection.

Teacher's Note

You can observe regular reflection in a mirror and diffuse reflection on paper, helping students understand why mirrors show clear images while textured surfaces do not.

Plane Mirrors

The reflection of light from a perfectly smooth surface, like that of a mirror, follows certain laws. Before we discuss these laws, we must become familiar with the terms and diagrams used in relation to reflection.

Terms Related to Reflection

A ray of light is a narrow stream of light. It represents the path along which light travels, and is shown by a straight line with a small arrow. The arrowhead indicates the direction in which light travels.

A mirror is shown by a straight line with hatching on one side. Mirrors have a layer of aluminium or silver at the back, called silvering. It is this layer which is really responsible for reflection. The hatching in a drawing of a mirror represents this layer.

A ray of light travelling towards a mirror is called the incident ray. A ray of light travelling away from a mirror after reflection is called the reflected ray. In Figure 6.3, IP is the incident ray and PR is the reflected ray. P is the point of incidence of the ray IP.

A line drawn perpendicular to the reflecting surface is called the normal to the surface. PN is the normal in the figure.

The angle between the incident ray and the normal is called the angle of incidence, while the angle between the reflected ray and the normal is called the angle of reflection. In the figure, i is the angle of incidence and r is the angle of reflection.

Laws of Reflection

We are now in a position to state the laws of reflection.

1. The incident ray, the normal to the reflecting surface (at the point of incidence) and the reflected ray lie in the same plane.

2. The angle of incidence is equal to the angle of reflection. In Figure 6.3, this would mean \(\angle i = \angle r\).

You can verify the laws of reflection quite easily. All you will need is a mirror, a sheet of paper and a source of rays. You can use a cardboard box to make a source of rays. Cut out one side of the box and cut a window on the opposite side. Make a slit (to get one ray) on a piece of stiff paper and fix it over the window. Shine a torch behind the slit to get a ray.

Now draw a straight line (XY in Figure 6.4) on the sheet of paper. Stand a mirror on the paper with its silvered surface on the line. Let a ray of light from the box fall on the mirror. You will see its reflection. Trace the incident and reflected rays and remove the mirror from the paper. Draw a normal to XY at O, where the incident and reflected rays meet. Measure the angles of incidence and reflection with a protractor. Change the angle of incidence and repeat the activity a couple of times. You will see that \(\angle i = \angle r\) each time.

The incident ray, the reflected ray and the normal lie on the same plane, i.e., the plane of the paper. If you place the sheet of paper at the edge of a table and fold it over the edge near O, you will not see the reflected ray on the paper. This is because the folded part of the paper is in a different plane.

Image Formed by a Plane Mirror

You may have noticed that when you hold an object in front of a mirror, an image seems to form behind the mirror. What really happens is that light from the object is reflected by the mirror and enters your eyes. The reflected rays seem to come from an image behind the mirror. However, if you place a screen at the position where the rays seem to be coming from, you will not see an image. Such an image that cannot be formed on a screen is called a virtual image. Images formed by cameras (for example, the pinhole camera you made) on the other hand, are real images. An image that can be formed on a screen is called a real image.

It is possible to locate the position of an image formed by a plane mirror by tracing rays or making a ray diagram. Suppose the object in front of the mirror is very small, like a point. Consider two rays OC and OD from the object incident on the mirror at C and D. They will get reflected along CE and DF. While drawing the reflected rays remember that the angles of incidence and reflection must be equal. If you extend the rays CE and DF backwards, they will meet at the point I. This is the point where these rays seem to be coming from. In other words, this is the image of the object O. You will find that ON = IN. This is always true for an image formed by a plane mirror. We say that for reflection at a plane surface, the image is as far behind the surface as the object is in front of it. If you wish, you can do the activity related to this at the end of the chapter.

Teacher's Note

Students can verify plane mirror properties using household mirrors and objects, seeing firsthand how images appear behind the mirror at equal distances.

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