Contents
Physics Topics cover a broad range of concepts that are essential to understanding the natural world.
Explanation and Facts of Radiation
When we switch on an electric bulb, it becomes hot and gives out heat and light. Now, if we keep our hand a short distance below the glowing bulb, we can feel its heat on the hand (see Figure). This means that the hot, glowing bulb is transferring some of its heat to our hand held below it. The hot electric bulb cannot transfer its heat to our hand by conduction because air is a poor conductor of heat. The hot bulb also cannot transfer its heat to our hand by convection because the convection currents of air always carry heat in the upward direction (but our hand is held below the bulb). The hot electric bulb transfers its heat to our hand held below it by the process called ‘radiation’. Please note that even if there is no air in the
room, the hot electric bulb can transfer its heat to our hand by radiation. In fact, all the hot objects (whether solids, liquids or gases) can transfer heat by radiation. This happens as follows.
Every hot object emits (gives out) invisible heat rays in all directions. These heat rays carry heat energy. When these heat rays fall on a cold object, the cold object receives heat energy and gets heated. In this way, heat energy is transferred from hot object to cold object by means of heat rays. This method of transfer of heat energy by heat rays is called radiation. Heat can be transferred from a hot body to a cold body by radiation even if there is no material medium (like solid, liquid or gas) between them.
In other words, heat can be transferred by radiation even through vacuum (or empty space). We can now define radiation as follows: Radiation is the transfer of heat energy from a hot body to a cold body by means of heat rays, without any material medium between them. The best example of radiation is the transfer of heat energy of the sun to the earth. When we come out in the sunshine, we feel hot. This means that the heat from the sun is being transferred to us which makes us feel hot. We will now describe how heat from the sun reaches us on the earth.
How Does Heat From the Sun Reach on the Earth
The sun is very far away from the earth and there is mainly empty space (vacuum) between the sun and the earth (see Figure). Even then heat from the sun reaches the earth. Now, heat from the sun cannot reach the earth by conduction or convection because both these processes require a material medium (like solid, liquid or gas) to transfer heat, they cannot take place in empty space (or vacuum). Heat from the sun reaches on the earth by the process of radiation. This happens as follows :
The sun is an extremely hot object. The sun emits heat radiations (or heat rays) in all directions. These heat radiations travel through vacuum between the sun and the earth at a very high speed and reach us on the earth. When the sun’s radiations fall on the earth and its objects, they receive heat energy and hence get heated. Thus, the sun’s heat reaches the earth by the process of radiation. The invisible heat rays which transfer heat by radiation are called infra-red rays.
We have just said that the transfer of heat from a hot object to a cold object by the process of radiation does not require any medium. It can take place whether a medium is present or not. Now, though the heat from the sun reaches us mostly through empty space or vacuum (having no air, etc.) but in our day to day life we have many situations where heat is transferred by radiation through a medium called air. In all the following examples, heat is transferred by radiation through air.
- When we stand next to a burning fire, we can feel the heat of the fire transferred from the fire to our face by the process of radiation.
- When we sit in front of a room-heater, we get heat directly by the process of radiation.
- A hot utensil (say, filled with hot milk) kept away from the flame cools down by transferring its heat to the surroundings by radiation.
- Depending on the temperature of surroundings, our body too gives heat to the surroundings or receives heat from the surroundings by radiation.
Absorbers of Heat Radiations
The amount of heat which an object can absorb by radiation depends on the colour of the object. The objects having dark colours absorb more heat radiations than the objects having light colours. We will now describe an activity to show that objects having dark colour are better absorbers of heat than the objects having light colour on their surfaces. In this activity, we will take a black painted tin can as an example of dark coloured object and a white painted tin can as an example of light coloured object. Let us describe the activity now.
Activity 6
We take a black painted tin can and a white tin can of the same size and place them on two wooden blocks separately (as shown in Figure). Pour equal amounts of water in both the tin cans and fix thermometers in them with the help of rubber corks. The initial temperatures of water in both the tin cans are noted. Their initial temperatures will be exactly equal.
We place both the tin cans in bright sunshine for one hour. The heat radiations of sun will fall equally on both the tin cans. After one hour, we note down the temperatures of water in both the tin cans again. We will find that water in black tin can is at a higher temperature than in white tin can (see Figure). Since the temperature of water in black tin can is higher, it shows that the black coloured tin can has absorbed more heat of the sun.
And the lower temperature of water in the white tin can shows that the white coloured tin can absorbs less heat from the sun. From this activity we conclude that a black object absorbs more heat radiations than a white object. Since a black object is a good absorber of heat radiations, it also means that a black object is a bad reflector of heat radiations. On the other hand, since a white (or silvery) object is a poor absorber of heat radiations, it means that a white (or silvery) object is a good reflector of heat radiations.
Emitters of Heat Radiations
The amount of heat which a hot object can emit (give out) by radiation also depends on the colour of the object. The hot objects having dark colours emit more heat radiations than the hot objects having light colours. We will now describe an activity to show that the objects having dark colours are better emitters of heat than objects having light colour on their surfaces.
Activity 9
We take a black tin can and a white tin can of the same size. These tin cans are placed on wooden
blocks separately and kept in a shady place inside a room (see Figure). Both the tin cans are filled with the same hot water. We note the initial temperatures of hot water in both the tin cans. Their initial temperatures are exactly equal.
Allow both the tin cans to stand for ten minutes. After ten minutes, note the temperatures of water in both the tin cans again. We will find that the water in black tin can is at a lower temperature than the water in white tin can (see Figure). This means that the black coloured tin can has lost heat faster than the white coloured tin can. Thus, a black object is a better emitter of heat. On the other hand, the higher temperature of water in white tin can shows that the white coloured tin can has lost heat slowly. Thus, a white object is a poor emitter of heat. In other words, a white object loses heat slowly.
From the above two activities we conclude that black objects absorb heat better and also emit heat better than white objects. In general, we can say that dark coloured objects absorb heat better and also emit heat better than light coloured objects. A yet another way of saying this is that dark coloured objects are poor reflectors of heat but light coloured objects are good reflectors of heat.
Importance of Colour of Objects in Everyday Life
We will now describe some everyday situations where colour of the object plays an important role. We use dark coloured objects where we want to absorb more heat and light coloured objects where less heat is desired. This point will become more clear from the following examples.
1. Colour of Clothes. People prefer to wear white clothes (or light coloured clothes) in the hot summer days because white clothes (or light coloured clothes) absorb less heat from the sun. In fact, white clothes (or light coloured clothes) reflect most of the sun’s heat rays which fall on them and keep us cool and comfortable in hot weather. For example, in summer a white shirt reflects the sun’s heat rays and makes us stay cooler. People prefer to wear dark coloured clothes in the cold winter days. This is because dark coloured clothes absorb more heat rays from the sun and keep us warm in winter season. For example, a black shirt worn in winter absorbs the sun’s heat rays more efficiently and keeps us warm.
2. Colour of Houses. The houses in hot, sunny countries (like ours) are usually painted white or with light colours from outside. This is because a house painted white or with light colours absorbs very little sun’s heat rays, it reflects most of the sun’s heat rays. This keeps the house cool in the hot days of summer.
3. Solar Cooker (and Solar Water Heater). The box of solar cooker (and solar water heater) is painted black from inside. This is because a black surface is a very good absorber of heat and it will absorb maximum heat radiations coming from the sun.