Newton's Discovery of the Refraction of Light

Episode Eleven – Newton’s Achievements


Now, this itself does not prove that force equals mass times acceleration. But it suggests that this might be the case. You can form a hypothesis that it might be so, one you can test to show that indeed, force does equal mass times acceleration.

So, mathematics helps to identify relationships, which can help you formulate hypotheses to test and thus help develop theories in physics. Not just physics, but almost anything else.

This is how Newton worked and Principia masterfully shows how mathematics can be used to tease out the implications of your theories.

It also once more shows the vital role of calculus. Without which, Newton would not have been able to discover many of the Laws of Motion.


Newton observed that a prism refracts different colors of light at different angles. Which led him to conclude that color is a property intrinsic to light. Something which was then and is still sometimes hotly debated.

He then investigated the refraction of light and demonstrated that the multi-colored spectrum of light produced by shining light through a prism could be recomposed into white light by using a lens and another prism.

This showed that white light was in fact all the colors mixed together and that the prism merely served to separate them.

He was thus the first to understand the rainbow as the result of light being separated into different colors. Rain drops work rather like a prism. White light enters raindrops and the rain drops act much like a prism, separating out different colors and thus producing the range of colors seen in a rainbow.

He then proceeded to show that color is the result of objects interacting with colored light rather than objects generating the color themselves. Now we know that each color from the visible spectrum of light has a specific wavelength. And that the color of an object is determined by the wavelengths of light that gets reflected or absorbed when interacting with the object.

Newton’s work helped set the stage for this understanding and our understanding of how color and vision work.


Optometrists still study this to get a good understanding of how light and vision works.

His findings led him to conclude that colour is a property of the light, not a property of the objects themselves.

Before this it was customary to assume that color is an intrinsic property of objects. As if an apple is red because of some inherent property of redness in the apple. Or that the sky is blue because it has a inherent blueness property.

Newton’s experiments show that this is not the case and that colour is a result of the interaction of entities with light.

Red apples are red because their nature is such that when light interacts with it, the wavelengths of light that get reflected or transmitted are the wavelengths from the red spectrum of visible light. The sky is blue because of the way light interacts with the particles in the sky. Not because the apple has the property of “being red” or because the sky has the property of “being blue”.

Redness is not “in the apple”, nor is blueness “in the sky”. Colors are not properties of the object. They are a result of the way the object interacts with light.

All these discoveries about light, served as the basis for our understanding of how light interacts with objects and what color is. As well as what light is, at least for a time. But what did he think light is?

He argued that light is composed of particles or “corpuscles” which were refracted by accelerating into a denser medium . This was dominant for about 100 years, but was eventually superseded by a wave theory of light.

However, light currently occupies a weird limbo state. It sometimes seems to behave like a particle and sometimes like a wave. It cannot be both, so there must be some explanation which would explain this apparent contradiction.

Newton’s particle theory of light may yet prove to have some truth to it.

There is of course more to the concept of colour since we have not discussed the role of sense perceptions yet. But let’s not steal the light from Newton, we will cover this in another blog post in the future.

For instance, if redness is not in the apple, is it in the light? While we can separate white light into different colors using a prism, does this mean that those different colors are intrinsic properties of light?

reflecting telescope

The first reflecting telescope, as built by Newton himself.

Reflecting Telescopes

Newton built the first practical reflecting telescope, also known as the Newtonian telescope or the Newtonian reflector.

Although it was not the first telescope, it was the first practical telescope to work on the principles of reflection of light. Previous telescopes, such as those Galileo built in 1609, were refracting telescopes.

Refracting telescopes work by using lenses bending light rays and causing them to converge at a focal point, thus producing a magnified image.

However, reflecting telescopes work differently. They use a combination of curved mirrors to reflect light and form a magnified image.

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