Light and Shadow

Light is form of energy and is very important in all aspects of our lives. In the absence of light plants would not able to grow as they require sunlight to prepare their food and in absence of plants, animal and human would have nothing to eat.

We see with our eyes but we cannot see anything when there is no light. We have noticed that in complete darkness no object is visible to us.

Sources of light

Light travels from a light source.

Natural light

Sun is the main source of natural light. Other forms of natural light are other stars and moon. Star provides very less amount of light at night because they are very far from earth compared to sun. Moon, especially full moon provides little light at night but light from the moon is just light reflected from the sun. Some animal also emit their own lights such as glow-worms, fireflies and some fish.

Artificial light

Source of artificial light are burning wood, candles, earthen lamps. Candle are made of wax, oil and wick are used in lamp to burn. Now days we use electric bulb, tube and lamp as source of light.

Luminous bodies are those which emit light of their own such as sun, stars and bulb. Non luminous bodies are those which do not have their own light such as moon, book etc. Non luminous object are visible to us when light from luminous object falls on them.

Properties/Characteristic of light

Light travels in a straight line

Light travels in a straight line in the form of rays. It travels very fast at the speed of 300,000 km in a second! Light does not bend, it can only be blocked. Light continue to travel in straight line until it hits something else.

It is evident from shadow formation that light travels in straight line.

Activity1

Place an opaque object in the path of light. What do you observe?

We see dark area on other side because opaque object has blocked the light from reaching to the other side. The dark area formed is the shadow.

Activity2

Take 4 square card of same size with hole punched in the center of the cards. Place all 4 cards vertically on a table with the help of modeling clay at an equal distance from each other such that holes of all 4 cards are in alignment. Now turn off the light in the room and place flashlight at one end of the row of square cards placed on the table.

What do you observe?

We observe that the holes are in alignment and the light travels in a straight line.

Note: Light covers a large area when the source of light is far off. Similarly sun shines high up in the sky and spreads its light all over.

Light reflects when it hits a surface

When light rays hits on any surface, it bounces back (it reflect back). We are able to see objects because light rays enter our eyes after bouncing off rough surface. A rough surface reflects or bounces off light in all directions whereas smooth and polished surface bounces off light in one direction. Therefore, mirrors are good reflectors.

Transparency of materials

Material that allows light to pass through them in straight lines is called transparent material. Object on other side of transparent material is clearly visible. Example: glass, crystal, clear plastic.

Material that allows some light to pass through them is called translucent material. Object on other side of translucent material is not clearly visible. Example: Tissue paper, frosted glass, colored glass, butter paper, colored plastic.

Material that does not allow any light to pass through them is called opaque material. Example: wood, metal, stone, our body.

Shadow

Shadow is formed when light is not able to pass through an opaque object. When an opaque object come in the path of the light, the light falling on that object cannot reach the other side, therefore, that particular region becomes dark. The rest of the area is lit as there is nothing to stop the light from passing through.

The object must be opaque or translucent to make a shadow. Opaque objects make dark shadows and translucent objects make faint shadows. A transparent object will not make any shadow, as all light will pass through it.

If we bring an object closer to the light source, the shadow becomes bigger as it blocks more of the light rays and if we take an object away from light source, the shadow becomes smaller as it blocks less amount of light.

The Sun casts long shadows in the morning and evening as the Sun is lowest in the sky. The Sun casts the shortest shadows at midday, when the Sun is highest in the sky.

Lateral Inversion

Lateral inversion is the reversal of image when placed against plain mirror. It may also be referred as “mirror image”. This implies that object on the right side appears on the left side in the mirror.