Newton's Discovery of the Refraction of Light

Episode Eleven – Achievements of Newton



Newton is best known for the legendary book Philosophiæ Naturalis Principia Mathematica (Latin for Mathematical Principles of Natural Philosophy). Here he sets down the sum total of his considerable discoveries in mechanics.

This is a rigorous presentation of his well-known laws of mechanics and his theory of gravitation.

His law of universal gravitation states that everything in the universe attracts everything else in the universe with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

He used his theory of gravitation to prove Kepler’s laws of planetary motion, account for the tides, the trajectories of comets, the precession of the equinoxes and more.

He demonstrated that the motion of objects on Earth and celestial bodies in space could be accounted for by the same set of principles. This finally made sense of the heliocentric model of the universe and explained Kepler’s astronomical observations. Thus dismissing any serious doubt of the heliocentric model.

We have not talked a lot about Kepler. But Newton’s work help put his astronomical observations on firm theoretical ground.

This was a crucial moment in physics! The importance of providing a single set of principles which explained the behaviour of both Earthly and celestial bodies can hardly be overstated.

Before Newton, it was customary to view the Heavens as a strange place very different from the Earth and not subject to the same kinds of laws. But Newton helped change this by showing that in fact the heavens and the Earth obeyed fundamentally the same equations.

This greatly demystified the heavens and helped to make the point that physic was a truly universal science which could explain everything, even the heavens.

But what about his other laws of motion? Chances are that if you have a science education, you have encountered these laws, even if you can’t remember them offhand. They are very simple laws which can be very easily understood and used by school children.

That is part of their beauty. They are not the sort of complicated, difficult to work with equations that take considerable mathematical expertise or computational tools to deal with. Which is more than can be said for several other equations in modern physics.

But, surely the worth of a theory is not in how elegantly simple to work with its mathematics is. Several aspects of the mathematics of quantum theory is much more difficult to work with. But that does not make it any less true or important. Regardless of what we think of the interpretations of the mathematics of quantum theory.

If you are dealing with something that moves, at least something that does not move at an appreciable fraction of the speed of light, then Newton’s Laws of Motion are bound to be relevant. You can use them to understand the behaviour and trajectory of the objects in question.

In fact, the motion of virtually everything we deal with in normal life can be understood in terms of Newton’s Laws of Motion. Without them, the motion and behaviour of a great many objects in our world could not be properly understood.

They serve as the basis for much of physics, certainly much of the branch of physics known as mechanics. And have been important in several other branches of physics.

Until they were superseded by Einstein’s Relativity, his law of gravitation were used to understand the motions and behaviour of much of the celestial objects in our universe. Without an understanding of gravity as provided by Newton or Einstein’s equations, the behaviour of most of the objects in space cannot be understood.

But, the importance of this goes far beyond providing an understanding of the motion of Earthly and celestial objects. Newton was one of the first to show that many aspects of nature can be easily understood by the application of simple physical principles.

Not only that, he showed that the behaviour of these physical objects could be calculated and predicted using simple equations.

The three Laws.
1) an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force
2) Force equals mass times acceleration.
3) If you apply force to an object, it applies an equal but opposite force in response.

This made an immense case for the power of science, physics in particular, for understanding the universe and predicting its behaviour.

The work of Newton showed just how much of the real world could be understood and predicted using simple equations. But, not only this, it showed the immense power of induction.

This was key to Newtons immense success. He was a master of grasping the commonalities between seemingly disparate things such as rocks on Earth and comets in space and identifying general principles which applied to both kinds of entities.

We will talk further about induction in a later episode. But for now note that a key reason for Newton’s success at explaining so much of nature with a few simple principles, is his application of induction.

Another important aspect of Newton’s laws of motion is that his theory of calculus was crucial in helping to derive them. In fact, you can, as Newton did, use simple calculus to derive various of his laws of motion from previously established ones.

This illustrates the vital role mathematics plays in physics. Mathematics is the science of quantifying relationships between things. Physics of course deals with many such relationships.

What Newton did was start with something he knew to be true, such as his Second Law of Motion, and perform mathematical operations so that a consequence of this is that F equals mass times acceleration.

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.

One major advantage of a reflecting telescope is that it is free from the severe chromatic aberration of refracting telescopes.

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