Soumil Roychowdhury

We have all heard the phrase “red hot” in everyday life. Today, it is often used to describe intense anger or excitement. But the phrase originally had a much more literal meaning. It first appeared in the late 14th century. This was a formative period for the English language: as it transitioned from Middle English toward Early Modern English, written records began to standardise many everyday expressions. References to “red-hot” objects in texts from the late 1300s reflect direct, physical observations from blacksmithing and metalwork.

Over time, the phrase evolved to evoke passion and intensity. The science behind the original observation, however, remains just as fascinating. It also raises an intriguing question: do objects glow in other colours at different temperatures? And if so, is there such a thing as a “hottest” colour?

To begin with, although this effect is most commonly noticed in metals, it applies to all materials. Any object will begin to glow when heated above a certain temperature. We tend to notice it more in metals simply because they can withstand high temperatures without melting or burning as quickly as many other substances.

This phenomenon is known as incandescence and is caused by thermal radiation. At any temperature above absolute zero (about −273°C), the atoms and molecules within a material are in constant, random motion. As they vibrate, charged particles emit electromagnetic radiation. The frequency of this radiation depends on the energy of the particles, which in turn is related to temperature. This emission is what produces the visible “glow,” and its colour changes as the temperature rises.

At room temperature, most objects emit radiation in the infrared part of the spectrum, which is invisible to the human eye. As the temperature increases, the peak of this radiation shifts toward higher frequencies, eventually entering the visible range. That is when the object begins to glow.

At room temperature, most objects emit radiation in the infrared part of the spectrum, which is invisible to the human eye. As the temperature increases, the peak of this radiation shifts toward higher frequencies, eventually entering the visible range. That is when the object begins to glow.

At the lower end of the visible spectrum lies red, which is why heated objects first appear red. As the temperature continues to rise, the colour shifts to orange and then to yellow. At around 1500°C, the object emits light across much of the visible spectrum, producing a white glow.

This is close to the upper limit of what we typically observe on Earth. Under more extreme conditions, such as on the surface of stars, temperatures can exceed 10,000°C. At these temperatures, most of the emitted radiation lies in the ultraviolet range. However, enough visible light is still produced—weighted toward higher frequencies—that the object appears white with a bluish tint. This bluish-white glow represents the hottest colour we can perceive.

In the end, “red hot” carries within it both poetry and physics—a phrase born not of metaphor, but of fire and forge. What begins as a dull, invisible warmth gathers into a deep red glow, then brightens and transforms as heat intensifies, tracing a spectrum that mirrors both nature’s laws and human imagination.