What are blackbodies, and why are stars near-perfect examples of this physics concept?
Dive into the world of blackbodies, and discover how their unique properties led to groundbreaking discoveries in quantum physics.
Uncover the mysteries of blackbodies and how quantum physics reshaped our world, driving technological advancements. Illustration: Shutterstock Have you ever wondered why apples are red? This is because the fruit’s surface reflects only red light while absorbing all of the other colours. So if an object absorbs all light and reflects none, what will be its colour?
What is a blackbody?
A blackbody takes in all of the light that hits it. You might think that since a blackbody does not reflect any light, it should appear black. But that is not true. In fact, blackbodies emit light and can be any colour.
This is because reflection and emission are slightly different processes. While reflection happens when radiation – for example, visible light – bounces off a surface without being absorbed, emission occurs when something releases its own energy in the form of radiation.
In 1893, German physicist Wilhelm Wien discovered that blackbodies at different temperatures emit energy at different wavelengths, resulting in a range of colours.
Keep in mind that a blackbody is an idealised object, meaning it is the perfect version of this concept in physics. Even though a perfect blackbody does not exist in nature, there are some objects that come close to it.
Stars like the sun are considered near-perfect blackbodies. They absorb almost all wavelengths of electromagnetic radiation without reflection. The light we see emitted from the sun is a result of its temperature. Our sun emits most of its radiation at the yellow part of the spectrum, while cooler stars will emit most of their radiation in the red part.
In fact, even the human body can show blackbody behaviour, though it is far from perfect since we do not absorb most electromagnetic waves. Still, our bodies radiate infrared light, which cannot be seen with the naked eye. On the other hand, as iron is heated, its colour changes from red to orange then yellow (see graph).
Why do blackbodies behave like this?
For many years, scientists were confused about blackbodies. According to classical physics, a blackbody should release an unlimited amount of energy in the ultraviolet range. But this did not make sense with what was observed in experiments.
In 1900, a German physicist, Max Planck solved the puzzle by proposing that energy is not radiated continuously, but comes in discrete parts, now known as quanta.
He explained that the energy of the light quanta – which are called photons – is determined by the formula (E=nh). In this formula, “h” is a constant, “” is the light’s frequency, and “n” is an integer (1, 2, 3 and so on). The speed of light is equal to the product of its frequency and wavelength.
Planck’s equation showed that the energy of a photon is directly related to its frequency. As temperature increases, higher energy photons are more likely to be radiated at higher frequencies.
How quantum physics impacts us
Planck’s breakthrough laid the groundwork for quantum physics – the science that describes how matter behaves at the smallest levels. Quantum physics has led to many technological advancements.
For example, our smartphones and computers rely on semiconductors and transistors, which were made using quantum science. This theory also led to the development of lasers, which are used in medicine, telecommunications and manufacturing.
Quantum physics has allowed us to understand how atoms and fundamental particles behave. This helps us in studying chemistry, making new materials and understanding how stars work and how the universe began.
Celebrating 100 years since quantum mechanics was first developed, 2025 is the International Year of Quantum Science and Technology. This is an opportunity to explore how these theories have changed the way we interact with the world around us.
Young Post has partnered with Hong Kong Science Museum and Hong Kong Space Museum to encourage your pursuit of science. Every month, the museums answer questions about the world around us, the cosmos and beyond.