Heat Radiation

Heat Radiation Definition

There are three ways in which heat transfer can take place: heat conduction, convection, or radiation. In this article, we'll focus on heat radiation. First, let's define what exactly is heat transfer.

Typically, the transfer happens from an object with a higher temperature to that of a lower temperature, which essentially is the second law of thermodynamics. When the temperature of all objects and their environments becomes identical, they're in thermal equilibrium.

Another term for heat radiation is thermal radiation, and all objects at non-zero temperatures emit it. It's a direct consequence of the vibrations and chaotic thermal motion of particles in matter. Whether it's the tight positioning of the atoms in solids or the chaotic arrangement in liquids and gases, the faster the atoms are moving, the more heat radiation will be produced and therefore emitted by the material.

Heat Radiation Properties

Heat radiation is a unique case of heat transfer from the heat source to a body, as it travels via electromagnetic waves. The body can be located near the source or at a far distance, and still, experience the effects of heat radiation. Considering heat radiation doesn't rely on the matter to propagate, it can travel in a vacuum as well. This is precisely how the Sun’s heat radiation spreads in space and is received by us on Earth and all the other bodies in the Solar System.

Electromagnetic waves of different wavelengths have different properties. Infrared radiation is a specific type of thermal radiation, most commonly experienced in our everyday life, just after visible light.

Typically, objects at room temperature will emit infrared radiation. Humans cannot directly observe infrared radiation, so how exactly was it discovered?

At the beginning of the 19^th century, William Herschel conducted a simple experiment where he measured the temperature of the visible light spectrum dispersed from a prism. As expected, the temperature varied depending on the color, with violet color having the smallest rise in temperature, meanwhile red rays produced the most heat. During this experiment, Herschel noticed that the temperature kept rising even when the thermometer was placed beyond the visible rays of red light, discovering the infrared radiation.

Considering it extends just beyond red, the longest wavelength of visible light, it's not visible to us. The infrared radiation emitted by objects at room temperature is not so strong, yet can be seen using special infrared detection devices such as night-vision goggles and infrared cameras known as thermographs.

Fig. 1 - Night vision goggles are widely used in the military, where the goggles enhance the small amount of infrared radiation reflected by objects.

As the temperature of a body reaches around a couple of hundred degrees Celsius, the radiation becomes noticeable from a distance. For example, we can feel the heat radiating from an oven that has been turned on for a longer period of time, just by standing next to it. Finally, as the temperature reaches roughly \(800\, \mathrm{K}\) all solid and liquid heat sources will start glowing, as the visible light starts to appear alongside the infrared radiation.

Heat Radiation Equation

As we already established, all bodies that have a non-zero temperature will radiate heat. The color of an object determines how much thermal radiation will be emitted, absorbed, and reflected. For example, if we compare three stars - emitting yellow, red, and blue light respectively, the blue star will be hotter than the yellow star, and the red star will be cooler than both of them. A hypothetical object which absorbs all radiant energy directed at it has been introduced in physics as a blackbody.

This concept does approximately explain the characteristics of stars, for instance, so it's widely used to describe their behavior. Graphically, this can be shown using the blackbody radiation curve as the one displayed in Figure 1, where the intensity of the emitted thermal radiation depends only on the temperature of the object.

This curve provides us with a lot of information and is governed by two separate laws of physics. Wien's displacement law states that depending on the temperature of a black body, it will have a different peak wavelength. As illustrated by the figure above, lower temperatures correspond to larger peak wavelengths, as they are inversely related:

$$ \lambda_\text{peak} \propto \frac{1}{T}. $$

The second law that describes this curve is the Stefan-Boltzmann law. It states that the total radiant heat power emitted from a unit area by the body is proportional to its temperature to the fourth power. Mathematically, that can be expressed as follows:

$$ P \propto T^4.$$

At this stage in your studies, knowing these laws is not essential, just understanding the overall implications of the blackbody radiation curve is sufficient.

Heat Radiation Examples

There are countless examples of various types of heat radiation surrounding us in everyday life.

Microwave oven

Thermal radiation is used to quickly warm up food in a microwave oven. The electromagnetic waves produced by the oven are absorbed by the water molecules inside the food, making them vibrate, therefore heating the food up. Although these electromagnetic waves could potentially cause harm to human tissue, modern microwaves are designed so that no leaks can occur. One of the more visible ways to prevent unwanted radiation is placing a metal mesh or repetitive dot pattern on the microwave. They are spaced in such a way that the spacing between each metal section is smaller than the wavelength of the microwaves, to reflect all of them inside the oven.

Infrared Radiation

Some examples of infrared radiation were already covered in the previous sections. An example image of the thermal radiation detected using a thermograph is visible in Figure 3 below.

Fig. 3 - The heat radiated by a dog and captured using an infrared camera.

The brighter colors, such as yellow and red, indicate regions that emit more heat, while the darker colors of violet and blue correspond to cooler temperatures.

Turns out, even our cellphone cameras are capable of picking up some infrared radiation. It's mostly a manufacturing glitch, as seeing infrared radiation isn't the desired effect when taking regular pictures. So, usually, filters are applied to the lens ensuring only visible light is captured. However, one way to see some of the infrared rays missed by the filter is by pointing the camera toward a remotely controlled TV and turning it on. By doing that, we'd observe some random flashes of infrared light, as the remote uses infrared radiation to control the TV from a distance.

Cosmic Microwave Background Radiation

The ability to detect thermal radiation is widely used in cosmology. Cosmic microwave background radiation, pictured in Figure 4, was first detected in 1964. It's the faint residue of the first light that traveled through our universe. It is considered to be the remnants of the Big Bang and is the furthest light humans have ever observed using telescopes.

Fig. - 4 The cosmic microwave background radiation uniformly spread throughout the universe.

Ultraviolet radiation

Ultraviolet (UV) radiation takes up roughly \(10\%\) of the thermal radiation emitted by the sun. It's very useful to humans in small doses, as that's how vitamin D is produced in our skin. However, prolonged exposure to UV light can cause sunburns and leads to an increased risk of getting skin cancer.

Another important example we briefly touched upon at the beginning of this article is the overall heat radiation circulating between the Sun and the Earth. This is especially relevant when discussing effects such as greenhouse gas emissions and global warming.

Heat Radiation Diagram

Let's look at the different types of heat radiation present in the Sun-Earth system, as displayed in Figure 5.

The Sun emits thermal radiation of all different kinds. However, the majority of it is made up of visible, ultraviolet, and infrared light. Roughly \(70\%\) of the heat radiation is absorbed by the atmosphere and Earth's surface and is the primary energy used for all the processes occurring on the planet, while the remaining \(30\%\) is reflected into space. Considering the Earth is a body with a non-zero temperature, it also emits thermal radiation, though a much smaller amount than that of the Sun. It mainly emits infrared radiation, as the Earth is around room temperature.

All of these heat flows result in what we know as the greenhouse effect. The temperature of the Earth is controlled and kept constant through these energy exchanges. Substances present in the Earth's atmosphere, such as carbon dioxide and water, absorb the emitted infrared radiation and redirect it either back towards the Earth or into outer space. As the CO2 and methane emissions due to human activity (e.g. burning of fossil fuels) have increased over the last century, heat gets trapped near the Earth's surface and leads to global warming.