When you enter home in evening, you immediately switch on the lights, which means you switch on electric objects that generate light for us to be able to see everything in dark. The light generated from this objects travels in all directions for us to be able to see in darkness.

Thus we can say that light is a form of energy which helps us see objects. When light falls on an object, some of the light gets reflected. The reflected light comes to our eyes and we are able to see an object.

Luminous Object: An object which produces light is called a luminous object, e.g. bulb, sun etc.

Non-luminous object: An object which does not produce light is called non-luminous object, e.g. moon.

Propagation of Light


We must know how Light travels, it always travels in a straight line.. Let’s try to test this : Take a bent cylindrical tube eg a plumbing pipe. Try to see a source of light; like a bulb or a candle; through the bent pipe. Since  light travels in a straight line, we are unable to see it through a bent tube..

Light Penetrates through objects. Based on the amount of light penetration through the objects or materials we can understand differences in objects. Amount of light penetrating through object reduces from transparent to translucent, while it is zero for opaque objects.

Transparent materials

When light encounters transparent materials, almost all of it passes directly through them.

Materials like clear glass, air, water are transparent.

Translucent materials

When light travels translucent materials, only some portion of the light passes through them. The light does not pass directly through the materials. It changes direction many times and is scattered as it passes through.

Objects on the other side of a translucent object appear fuzzy and unclear.

E.g. Plastics, Frosted Glass

Opaque materials

Most materials are opaque. When light strikes an opaque object none of it passes through. Most of the light is either reflected by the object or absorbed and converted to heat. Materials such as wood, stone, and metals are opaque to visible light.


A shadow is a dark area where light from a light source is blocked by an opaque object. It occupies all of the three-dimensional volume behind an object with light in front of it.

A shadow is made when an object blocks light. The object must be opaque or translucent to make a shadow.

 A transparent object will not make any shadow, as light will pass straight through it.

Formation of Shadow

Have you realized we also have a shadow when we walk around during the day?

Have you noticed that our shadows are longer in the morning and evening and smaller in the afternoon why?

For formation of shadow, a source of light, an object and a screen.

The size of shadow depends on the distance of source of light and on the angle at which the light rays fall on the object.

If the source of light is closer to the object, a larger shadow is formed than when the source of light is far from the object.

If the angle of incident light is smaller, the shadow is longer. On the other hand, if the angle of incident light is bigger, the shadow is smaller.

Structure of a pinhole camera:

Fig: Pinhole camera

A pinhole camera is simple device. One can get an inverted and real image in a pinhole camera.

A pinhole camera is composed of two boxes. One of the boxes snugly fits inside another box and can be slid to and fro. There is a pin-sized hole at the front of the camera and a translucent screen at the back of the camera. The distance between the pin-hole and screen can be adjusted to obtain a clear image of an object.

Mirror and Reflection:

Fig: Reflection of Light

When light falls on a shiny surface, the light rays bounce back. This phenomenon is called reflection of light. Mirror is a surface from which most of the incident light gets reflected.

Difference between Shadow and Image

As the Sun shines on the Earth, a shadow is cast, creating the darkness that we experience at night. As the Earth rotates on its axis, different areas of light and dark are created, which we see as day and night.

A dark patch formed on a surface behind an opaque object placed in the path of light is called a shadow. An image in a mirror can be seen without a screen, whereas it is mandatory to have a screen to form a shadow.

Shadows and reflections are the effects of light. Night is caused by the Earth blocking the light from the Sun - on the half of the Earth in shadow it is night time. Shadows are formed when objects block a source of light.

To see a shadow, there must be a source of light and an object that blocks that light.