Imagine stepping outside your house on a chilly winter day. You're bundled up in your warmest coat,
thick gloves, and a wool hat. As you make your way towards your destination, you start to feel warm,
and energy is released. Some of that energy is released into the atmosphere as heat.
The principle of blackbody radiation is based on this concept.
Now, let’s break down the term black body radiation to understand the idea a bit better.
A black body is a theoretical or imaginary object that perfectly absorbs all incoming
electromagnetic radiation, and also emits radiation, like heat and visible light,
based on its temperature. A black body is considered theoretical because no object can be
a perfect absorber of electromagnetic radiation. Think of it this way: when you heat an iron rod,
it becomes extremely hot, and its color changes to dull red, followed by bright red,
orange and then yellow. This is a good example of how black body radiation works.
Imagine throwing a stone in a lake. It produces ripples that spread out in all
directions. Electromagnetic radiation is somewhat similar, only, instead of
being made of water, these waves transport energy. These waves are of different sizes,
or in more technical terms, you’d say that these waves have different wavelengths.
Some wavelengths are short, while others are long, but each wavelength corresponds to a different
form of energy. For example, the light we see around us is also a type of electromagnetic wave,
with wavelengths in the range of 380 to 700 nanometers. The human eye is able to see
electromagnetic radiation that falls within this range, and therefore it’s called visible light.
Similarly, there is also electromagnetic radiation that we cannot see, like radio
waves with large wavelengths, which enable us to use smartphones and WiFi.
Another term you will often hear in correlation with black body radiation
is the black body radiation curve. This is a simple graph that depicts the intensity
of the emitted radiation from a black body on the vertical axis and the wavelengths
of that radiation on the horizontal axis. Since the temperature of the black body is
directly related to the intensity of its emitted radiation, you will notice that as the black
body’s temperature changes, the shape of the black body curve changes as well. At low temperatures,
most of the emitted radiation is in the form of long wavelengths (for example, infrared), i.e.,
the radiation that we cannot see with our naked eye. On the other hand, at high temperatures,
the graph peaks at shorter wavelengths, which corresponds to colors that we are able to see.
The process is comparable to how the color of an iron rod changes with the application of heat,
as mentioned earlier, resulting in an increase in temperature.
Now, what celestial body comes to mind when you think of very high temperatures
and high amounts of radiation? The sun, of course! Although it’s
not a perfect blackbody, its surface is at a temperature of around 5500 degrees Celsius,
and a portion of the radiation it emits is in the range of visible light.
Since black body radiation depends on temperature, this concept also helps us
understand the temperature, size, and age of celestial objects, like stars and planets.
By analyzing the radiation of a star, including its color and intensity, we can learn a lot
about the physical properties of that star. To sum this all up, black body radiation is
the principle that all objects emit energy in the form of heat, depending on their temperature. The
concept of a black body helps us understand how this emission happens,
as well as the types of energy being emitted.
This energy—the electromagnetic radiation—spans across multiple forms, including visible light,
giving us the ability to observe and investigate the universe around us.