Thermal Radiation

So, as discussed on other articles, heat transfer can occur through contact, like in convection and conduction. Conduction literally discusses how heat transfers between two objects in contact, while convection discusses how heat transfers through direct heating in air.

However, probably our most prominent source of heat on Earth is our Sun, which is by no means direct. After all, we're about 150 million km from the Sun(on average) so clearly, the Sun isn't in direct contact with any of our skins. Similarly, if you've gone camping before, you'll notice you don't need to touch(and shouldn't) a campfire to feel its heat.

If that's the case, how is the Sun providing us with so much heat? It does so through electromagnetic radiation. Just for reference, below is the electromagnetic spectrum:

When the frequency of the EM(electromagnetic) wave decreases(wavelength increases), the energy of the wave increases, so keep that in mind.

Every object emits and absorbs EM radiation, but how effectively they do so is dependent on primarily their color. This is because the amount of heat radiated is dependent on temperature, where higher temperatures radiate more heat. Since temperature is measured by energy, and the energy of an EM wave is dependent on wavelength, through:

where E is the energy in joules, h is the Planck constant, which is 6.63 * 10^-34 J s, and c is the speed of light, which is 3 * 10⁸ m/s.

This means that the temperature absorbed by an object is dependent on that object's color. This is why black objects absorb lots of heat while white objects don't. An example of this is shown below:

The black surface on the right melted the ice far more than the white surface on the left. This is why it's often not an amazing idea to where black clothing in the summer, for you don't want to be taking in a lot more heat than you could if you just wore a different color.

The equation for the rate at which heat transfers through EM radiation is given by the Stefan-Boltzmann Law of Radiation:

So, let's break this equation down.

The left side is just the rate at which heat transfers, in terms of the change in heat Q and the change in time t.

On the right, lower-case sigma denotes a constant known as the Stefan-Boltzmann constant. This constant is equal to 5.67 * 10⁸ J/(s m² K⁴).

e is a quantity known as the emissivity of the object. It's between 0 and 1, where 0 means it can't emit or absorb any EM radiation and 1 means its a perfect emitter and absorber of EM radiation, otherwise known as a blackbody(because black objects emit and absorb EM waves so well).

A is the surface area of the object. It makes since that heat transfer increases with surface area because if there's more surface area for an object to radiate heat from, more heat can be emitted.

T is the temperature of the object.

The net rate of heat transfer by radiation between an object and its environment is given by:

where T2 is the temperature of the surrounding environment and T1 is the object's temperature.