It is common knowledge that hot objects often give off a warm glow and should thus be handled with care, but have you ever wondered how this light comes to be? Essentially, everything around us emits thermal radiation, but only when this wavelength shifts to the visible spectrum (caused by an increase in temperature) does it emit the glowing sign of danger that we are all so familiar with. Specifically, objects only start glowing if their temperature becomes high enough, at around 798K. This article provides a quick overview of the science behind this curious phenomenon.
How Atoms Emit Light According to Quantum Mechanics
You may already know this from physics class or a physics tutor in Singapore, but as a refresher, everything in the known universe consists of several basic particles—photons, electrons, quarks, neutrinos, and so on—governed by quantum mechanics. When these combine and interact with each other, we get atoms.
To learn why heated objects glow, we must first explore how light radiates at the quantum level. Atoms are fundamentally considered positively charged nuclei made of neurons and protons encircled by negatively charged electrons.
Each negatively charged electron has an energy level or exists in its own shell, which is the area that surrounds the nucleus. Now, when an object is heated up an object, its atoms start to vibrate faster, giving it kinetic energy. This energy then causes the ‘excited’ electrons to rise or jump from their initial shell or energy level to a higher one.
However, nature inherently wants everything to be in its lowest energy state possible, so these excited electrons eventually return to their initial energy state after a short period. But since energy can only change from one form to another and never be destroyed, it gets emitted as photons or light.
This is one of the basic mechanisms that causes objects to release light and, if the wavelength of this light reaches the visible part of the electromagnetic spectrum, potentially glow. But most of the time, this will not be the case, and it also doesn’t fully explain how temperature comes into play in the glowing light that we are interested in. So, even though this light emission is a phenomenon of quantum mechanics at its core, there is more to this story, which is further explained by the black body radiation model.
Black Body Radiation and Understanding Why Objects Glow at Extreme Temperatures
First, it is important to note that everything in the known universe gives off thermal radiation (provided its temperature is greater than absolute zero) in all wavelengths of electromagnetic radiation, with a small part of this being the visible light spectrum. As mentioned, we will rarely see a visible glow from the thermal radiation of everyday objects, that is, until they are hot enough. This is where the black body radiation model comes in.
Black body radiation is essentially a statistical model that describes how large objects diffuse light and the role of temperature in this equation. For starters, it assumes that something is a black body, i.e. an object that can only absorb light and never reflect it, has a uniform temperature, and emits thermal radiation that, in this context, is referred to as black body radiation.
The reason why we want to focus on black body radiation is because, in general, thermal radiation is incredibly complicated to understand for several reasons:
- Predicting how all the innumerable atoms in a heated object get excited and release photons from the perspective of quantum mechanics is impractical.
- When an object gets hot, its molecular structures also vibrate, emitting photons or thermal radiation. In other words, an object’s composition can affect the thermal radiation it gives off.
Hence, what’s great about black body radiation is that it is an idealisation, allowing for an accurate way of predicting thermal radiation from an idealised body and serving as an excellent approximation for real-world objects as well. The most crucial result of this model for our needs is that a black body’s entire thermal radiation spectrum is fully determined by its temperature, not its molecular structure or composition.
This temperature determines what the wavelength will be for most of the electromagnetic radiation emitted by the black body, and if it falls anywhere between 380nm-750nm, which is the visible range, it will appear as glowing, visible light.
Conclusion
There are various concepts working behind the scenes that cause certain objects to glow different colours at different temperatures. Lastly, a minimum temperature threshold must be reached before an object can glow visibly at a faint red, known as the Draper point, which is approximately 525°C or 798K.
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