Contents
Physics Topics can be both theoretical and experimental, with scientists using a range of tools and techniques to understand the phenomena they investigate.
What are the Similarities and Differences Between Radiant Heat and Light?
Heat can be transmitted from one place to another by radiation, even without the presence of any material medium. There is no material medium above the atmosphere, between the sun and the earth, but still heat from the sun reaches the earth. When we sit near an oven or a fire-place, we feel warm. In this case heat does not reach us by conduction. There is air between the oven and us and we know that air is an insulator. Also the air above the oven gets heated and rises upwards while cold air around the oven moves towards it. So heat does not reach us by convection either. Clearly there is another process by which the heat reaches us. This process is called radiation.
Radiation: The process by which heat is transmitted from one region to another in the form of electromagnetic waves even in the absence of a material medium, or without heating the material medium (if present) is called radiation.
Radiant heat: In the process of radiation, heat energy is transmitted in all directions, from the source, in the form of waves. This heat wave is called thermal radiation or radiant heat.
Generally all heated bodies spread heat in the form of radiant heat in all directions. During transmission, when the radiated heat gets absorbed by a body, the kinetic energy of its molecules increase. This means that the temperature of the body increases.
Similarities between radiant heat and light:
- Radiant heat is an electromagnetic wave like light and can travel in vacuum.
- Radiant heat, like light, travels in a straight line. That is why an umbrella saves our body from the heat of the sun.
- Radiant heat and light have the same speed. In vacuum, this speed is 3 × 1010 cm ᐧ s-1.
- Radiant heat exhibits optical properties like reflection, refraction, interference, diffraction and polarisation and affects photographic plates.
- Heat radiation obeys the inverse square law, i.e., intensity at a point at a distance d from the source is inversely proportional to d2, which is similar to light.
- From a source, both radiant heat and light propagate as waves.
- Radiant heat does not heat a medium while passing through it; but it heats bodies that obstruct its flow.
Differences between radiant heat and light:
Radiant heat | Light |
1. Produces the sensation of warmth. | 1. Produces the sensation of sight. |
2. Possesses longer wavelength. | 2. Has comparatively shorter wavelength. |
Nature of Thermal Radiation
We have already seen that radiant heat and light have a lot of similarities. They are of the same nature and they belong to the same family of waves. These waves are known as electro-magnetic waves. Only the wavelength of radiant heat is greater than those of visible light. In the spectrum of light, red light has the highest wavelength. As the wavelength of radiant heat more than that of red light too, radiant heat is known as infrared waves. Radio waves, ultraviolet waves. X-rays, gamma rays too are different kinds of electromagnetic waves. All of them have the same nature; the difference lies only in wavelengths.
The table provided below lists the wavelength bands of elec-tromagnetic waves with their names and ranges, in order of decreasing wavelength.
Radiation | Range of wavelength |
Radio wave | 104 m to 10-2 m |
Infrared waves (Radiant Heat) | 10-2m to 7.5 × 10-10m |
Visible light (Red to violet) | 7.5 × 10-7m (red) to 4 × 10-7m (violet)t |
Ultraviolet rays | 10-7 m to 10-8 m |
X-rays | 10-8 m to 10-10 m |
γ (Gamma rays) | 10-10 m to 10-12 m |
All the electromagnetic waves mentioned above are called radiation although we have seen earlier that radiation generally refers to a specific process. In the context, it should be mentioned that the term radiation is used to mean emission too.
As infrared waves give us the sensation of heat when they fall on our bodies., they are known as thermal radiation.
Every body, at any temperature above absolute zero (T = 0 K), emits and absorbs radiation. It has been observed experimentally that lower the temperature of a body, higher is the wavelength of the radiation emitted by it and vice versa. This is why when a body is heated at first it emits infrared radiation. As the temperature rises, the emitted radiation falls in the visible light region. The body goes from being ‘red-hot’ to ‘white-hot’ as its temperature increases.
Prevost’s Theory of Heat Exchanges
Any hot substance radiates heat to as well as absorbs radiant heat from its surroundings at all times. Hence, there always exists a radiant heat exchange between a body and its sur-roundings. Therefore, the rise or fall of temperature of a body depends on the exchange of heat between the body and its surroundings. This is called Prevost’s theory of heat exchange. According to this theory,
- when the rate of absorption of heat from the surroundings is greater than the rate of radiation of the body, the temperature of the body rises up.
- when the rate of absorption of heat from the surroundings is lower than the rate of radiation of the body, the temperature of the body decreases.
- when the rate of radiation equals the rate of absorption, the temperature of the body remains constant and the body remains at thermal equilibrium with its surroundings.
For example, when we stand near a fire-place, we feel hot. The radiant heat absorbed by our body from the fireplace is more than the heat that our body radiates; hence the feeling. Again, when we stand near a big chunk of ice we feel cold because the amount of radiant heat absorbed by our body from the ice is less than the heat that our body radiates.