Why things turn red when they're hot: A lesson on light, colour, and matter

Have you ever noticed that when you heat something, it takes on a reddish hue? Until the 1800s, some of the world's greatest scientists couldn't figure out why this was happening. They reasoned that if all objects behaved similarly, there should be a universal theory underlying this (such as gravitation or the law of motion). However, no one seemed to be able to figure it out.

But, as years passed, more scientists began to study light and heat and they began to understand how colour relates to heat. In this article, we'll shed light on what these scientists discovered all those years ago.

Now, in order to understand why objects turn red when they are hot, we must first learn about the following three topics:

  1. Light
  2. Colour
  3. Matter

1. Light

What is light?

Light is a form of energy that travels in waves. If you've ever been to the beach, you've probably seen the up and down movement of the ocean's waves. Invisible light travels in a similar way.

And, just as ocean waves contain water, light waves contain electric and magnetic fields. That is why light waves are referred to as "electromagnetic radiation" by scientists. You can learn more about waves by clicking on this link.

Now going back to our beach example, if you decided to dive into these waves of water, wouldn't you keep bobbing up and down between two sets of waves? When a wave comes, you go up, then down until another wave lifts you back up. A "Wavelength" is the amount of time that passes between these two waves. Light has a wavelength as well.

What's more...Light has different wavelengths, just like waves in the ocean (on good days, the waves are smaller, and on bad days, the waves are bigger).

2. Colour

Colours are nothing more than different wavelengths of light striking your eye. Each colour has a unique wavelength, as seen in the image below:

We can see from this image that the wavelength of red is much longer than that of violet.

3. Matter

Metals gain energy when they are heated. And they start to release this extra energy in form of waves.

The hotter the matter, the longer the wavelength. And if we look at our wavelength colour chart, we notice that red is the colour associated with longer wavelengths. This is the reason why as the matter gets hotter, we start seeing red around it. 

You can read more about the transfer of heat energy and its effects here.



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