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# Newton's and Huygens' Theories of Light

In the seventeenth century, physicists had a significant debate on trying to explain the phenomenon of light. Christiaan Huygens’ theory of light, which states that light is made of waves, was one of the first well-known theories explaining the behaviour of light. Another theory emerged when Newton tried to disprove the wave theory by publishing his corpuscular theory of light,…

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# Newton's and Huygens' Theories of Light

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In the seventeenth century, physicists had a significant debate on trying to explain the phenomenon of light. Christiaan Huygens’ theory of light, which states that light is made of waves, was one of the first well-known theories explaining the behaviour of light. Another theory emerged when Newton tried to disprove the wave theory by publishing his corpuscular theory of light, which states that light is made from small particles.

## What is Newton’s theory of light?

Isaac Newton studied the behaviour of light in his theory of colour. His theory of light was based on his laws of motion, as he thought of light as a straight line motion made out of small particles called corpuscles. Hence Newton’s theory of light is also known as the corpuscular theory. By studying the geometric nature of reflection and refraction of light, Newton concluded that light is composed of coloured particles that combine to appear white.

Reflection is a phenomenon of light that occurs when it impacts a surface that does not fully absorb its radiant energy, resulting in the light bouncing back from the boundary.

Fig. 1 - Reflection of light

Refraction of light is the bending of light when it passes from one medium to another. This changes the velocity of light depending on the density of the mediums.

Fig. 2 - Refraction of light

Check out our explanation, Refraction at a Plane Surface, for more info on refraction.

Newton noticed that reflection and refraction could only be possible if light is made up of particles (or, as he called them, corpuscles). His theory of light states that light continuously emits small particles or corpuscles that seem to change velocity when they pass from one medium to another with different densities. Therefore, the speed of light changes depending on the density of the medium it passes through.

### Wave theory of light: Newton’s prism experiment

Newton conducted an experiment to disprove the wave theory of light and prove that light travels as a flow of particles in a straight line instead of a wave. His experiment explained three main phenomena of light: reflection, refraction, and the rectilinear propagation of light. However, his theory could not explain diffraction, which is a main property of waves.

Newton decided to prove his theory by studying the refraction of light and, more specifically, the colour spectrum that was thought to appear when light passes through glass. He let one beam of sunlight (which is white light) pass through a glass prism. He observed that the light scattered into several colours resembling a rainbow. He named this multicoloured band of light a colour spectrum. Although the colour spectrum was continuous, Newton split the spectrum into seven categories of different colours, namely red, orange, yellow, green, blue, indigo, and violet.

Newton then passed the beam of sunlight through a second prism that was held upside down so that the spectrum passing through the first prism was recomposed into white light (see image1 below).

Fig. 3 - Newton's prism experiment and refraction of light

Newton first thought that the colour spectrum was caused by the glass, but through his experiments, he concluded that every colour has a specific angle of refraction. He observed that (a) all objects appear to be the same colour as the beam of coloured light that illuminates them and (b) that a beam of coloured light will stay the same colour no matter how many times it is reflected or refracted. This led him to conclude that colour is a property of the light that reflects from objects and not a property of the objects themselves.

Newton also proposed the existence of the aether, a suggested medium through which light travels. However, the presence of a so-called aether was disproved in the following centuries.

## What is Huygens’ theory of light?

In the late seventeenth century, Christiaan Huygens proved that his wave theory of light could explain the phenomena of diffraction, interference, and reflection. Huygens’ theory states that each point in a source of light sends a wavefront in all directions in a continuous and homogeneous medium called aether.

Diffraction of light is the bending of light around the edges of an object (waves are bent when they encounter the edges of an object).

Interference is the phenomenon that happens when two waves merge, resulting in a higher, lower, or zero amplitude. When waves merge, they create a higher amplitude wave at the points where the two waves meet, creating an interference pattern of bold and faint shadows.

Huygens thought that diffraction occurs due to the interference of the wavefronts and that light waves differ from mechanical or water waves in the direction of travel. Huygens showed that the edges of the shadows of the interference pattern are not perfectly sharp. As a result, Huygens concluded that light must be a wave and diffracts when it passes through an opening.

Huygens’ theory managed to explain refraction and reflection as well as diffraction and the resulting interference pattern. However, Huygens’ theory was partially disproved in the following centuries. Huygens theory states that light travels through a medium in the form of a wavefront, but Maxwell’s theory proposed that light does not require a medium to propagate and can travel through a vacuum. He also assumed that light is formed from interchanging electric and magnetic fields, which travel as waves at the speed of light.

## What is Huygens’ Principle?

Huygens’ principle states that when a light wave travels in a vacuum or a medium and reaches an opening, its wavefront can be considered as individual points emitting new sources of wavelets that expand in all directions, creating s

Huygens’ principle explains the shape that is observed in a wave, such as water waves. It also explains the phenomenon of wave diffraction around edges. Huygens’ principle is now used to develop optics, for example, mirrors and lenses.

### Thomas Young and the double-slit experiment

As Huygens’ principle was never experimentally validated, Newton’s theory prevailed. Thomas Young decided to conduct an experiment that would help prove Huygens’ theory that light is a wave and not a particle. Young believed that light is indeed a wave and should have properties similar to water waves. He also believed that light waves should interact when they meet (and that this interaction should include the waves merging). This is the behaviour that Young was expecting to observe with his experiment.

When two crests of a wave meet, they combine to form a wave that has a larger amplitude. When a crest meets a trough, the waves cancel each other out and form a flat wave or a wave with less amplitude at that point.

Young placed a light source behind a slit to allow a beam of light to enter through a single point. The light then spread out and entered a second screen with two slits with a screen in the front. Two light sources were required to obtain interference, and Young observed that light spreads out by diffraction at the double-slit. He also observed an interference pattern formed on the screen with bright and dark shadows.

Fig. 4 - Double-slit experiment

From this experiment, Young realised that light is a form of a transverse wave because the light behaved exactly as expected (there was an interference pattern with bright and dark shadows indicating the points where two crests and troughs met respectively). If light were indeed a particle, as Newton predicted in his theory of light, then the light beam passing through the slit would have been replicated on the viewing screen, as shown below.

Fig. 5 - Diagram showing the experiment where light is replicated on the viewing screen behind two openings

## Affect on the wave-particle duality of light

A century later, Einstein proved that light can also exist as individual particles of energy, similar to what Newton predicted. As a result, both Newton’s and Huygens’ theories were somewhat correct as light behaves as both a wave and a particle. Young’s experiment was also recreated in several variations, including using electrons instead of light beams. This showed that when detectors were used at the slits, the particles seemed to behave as waves.

Many physicists tried to explain the dual behaviour of light, which seems to behave as particles at times and as waves at other times. These experiments and theories led to the development of the modern quantum mechanics theory, which states that light behaves as a particle and a wave, but we can’t observe both properties at the same time.

## Newton’s and Huygens’ Theories of Light - Key takeaways

• Isaac Newton’s theory of light states that light is a straight line motion made out of small particles called corpuscles.

• Huygens’ theory of light states that light is made out of waves.

• Huygens’ theory was partially proven by Thomas Young using the double slits experiment.

• Both theories were proven to be valid as light was later proven to have both wave and particle properties.

• These light theories eventually led to the quantum mechanics dual theory of light.

Images

A Simple Experimental Setup to Clearly Show that Light Does Not Recombine After Passing Through Two Prisms. https://aapt.scitation.org/doi/10.1119/1.5018680

Isaac Newton discovered that light is made up of coloured particles that combine to appear white. His theory of light was based on his laws of motion, as he thought of light as a straight line motion made out of small particles called corpuscles.

The two theories of light are Newton’s theory of light, which states that light is made out of particles he called corpuscles, and Huygens’ theory of light, which states that light is a wave.

Huygens’ theory of light states that each point in a source of light sends a wavefront in all directions in a continuous and homogeneous medium called aether.

Isaac Newton gave the corpuscular theory of light.

Huygens’ principle is important as it states that light is a wave, which led to the discovery of the wave-particle duality behaviour theory of light.

## Newton's and Huygens' Theories of Light Quiz - Teste dein Wissen

Question

What are the two theories of light?

The two theories of light are Newton’s corpuscular theory and Huygens’ wave theory.

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Question

What is Newton’s corpuscular theory of light?

Newton’s corpuscular theory of light states that light is moving in a straight line made out of small particles.

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Question

What is the colour spectrum?

The colour spectrum refers to the different light colours that make up white light.

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Question

What colours does the colour spectrum include?

The colour spectrum includes the following colours: red, orange, yellow, green, blue, indigo, and violet.

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Question

How did Newton prove his experiment?

Newton proved his experiment by leading white light into a prism, which caused the light to scatter. He then positioned a second prism upside down, and this caused the scattered light to recompose into white light.

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Question

What did Newton’s prism experiment show?

Newton’s prism experiment showed that the colour spectrum is not caused by the glass, but that every colour has a specific angle of refraction.

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Question

What is Huygens’ principle?

Huygens’ principle states that light is made out of points emitting wavelets in all directions causing light to behave as spherical waves.

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Question

Which of the following did Newton’s light theory manage to explain?

Colour spectrum.

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Question

What did Newton’s theory fail to explain?

Newton’s theory failed to explain interference, diffraction, and polarisation.

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Question

Was Huygens’ theory proven?

Yes, Huygens’ theory was proven by Thomas Young, who conducted the double-slit experiment.

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Question

How does the double-slit experiment work?

In the double-slit experiment, a light beam is passed through an opening causing it to diffract. It is then passed through a second screen with two openings causing an interference pattern between the two diffracted waves.

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Question

Which of the two theories of light was proven to be correct?

Both theories were proven to be valid. Light behaves as a particle and a wave.

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Question

Which of the following phenomena was not explained by Huygens’ principle? Choose the correct answer.

Colour spectrum.

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Question

Which principle explained refraction, interference, and diffraction?

Huygens’ principle explained refraction, interference, and diffraction.

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Question

Is light a particle as stated in Newton’s theory or a wave as stated in Huygens’ theory?

Quantum mechanics tells us that light can behave simultaneously as a particle or a wave.

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Question

What is the quantum mechanics theory of light?

The quantum mechanics theory of light is that light can be thought of as a wave and a particle simultaneously.

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