During the seventeenth century, physicists had a big argument about understanding light. Christiaan Huygens came up with a theory that light is made of waves, which was one of the first well-known explanations of light's behavior. Another theory was introduced by Newton, who wanted to challenge the wave theory. He published his corpuscular theory of light, which suggested that light is made up of tiny particles.

What is Newton’s theory of light?

Isaac Newton was interested in light, and he explored the topic in his theory of color. He believed that light moved in a straight line, and that it was made up of tiny particles called corpuscles. Therefore, Newton's theory of light is also called the corpuscular theory. When he studied how light is reflected and refracted, Newton concluded that light is made up of colored particles that combine to look white.

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If you want to learn more about refraction, check out our explanation called "Refraction at a Plane Surface." Newton observed that light could only be reflected and refracted if it was made up of particles, which he called corpuscles. His theory suggested that light released tiny corpuscles that appeared to change speed when they moved from one medium to another of different densities. As a result, the speed of light varies depending on the density of the medium it passes through.

Wave theory of light: Newton’s prism experiment

Isaac Newton aimed to disprove the wave theory of light and support his theory that light travelled in a straight line as a flow of particles. He conducted an experiment to explain three key behaviours of light which were reflection, refraction, and rectilinear propagation of light. However, his corpuscular theory could not explain diffraction, which is a typical wave property.

Newton's experiment focused on the refraction of light and the colour spectrum that occurs when white light passes through glass. He allowed a beam of sunlight to pass through a glass prism, and he observed that the light scattered into several colours that resembled a rainbow. Newton named this multicoloured band of light a colour spectrum, and he split it into seven distinct colours:, 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 white. Newton initially believed that the colour spectrum was caused by the glass. However, his experiments showed each colour had a specific angle of refraction. He also observed that all objects appeared to be the same colour as the beam of coloured light that illuminated them. Additionally, a beam of coloured light would remain the same colour regardless of how many times it was reflected or refracted. This led him to conclude that colour was a property of the light that reflected from objects and not a property of the objects themselves.

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

What is Huygens’ theory of light?

iaan Huygens' wave theory of light was proven in the late seventeenth century and could explain the phenomena of diffraction, interference, and reflection. According to Huygens' theory, each point in a source of light sends a wavefront in all directions in a continuous and homogeneous medium called aether.

Diffraction of light happens when waves bend around 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 believed that diffraction occurs because of the interference of the wavefronts and that light waves differed from mechanical or water waves in their direction of travel. He showed that the edges of the shadows of the interference pattern were not perfectly sharp, leading him to conclude that light must be a wave and diffracts when it passes through an opening.

Huygens' theory was able 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 stated that light travelled 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 a new wavefront that is tangent to all of the wavelets . This phenomenon is known as wave diffraction, which is the bending of a wave around the edges of an opening or an obstacle . Huygens’ principle is used to explain the behavior of waves, including water waves, sound waves, and light waves. It is also used to explain the laws of reflection and refraction, as well as to understand the movements of waves around objects. In addition, Huygens’ principle is used to develop optics, such as mirrors and lenses.

Thomas Young and the double-slit experiment

Huygens’ principle was never experimentally validated, and Newton’s theory that light was made up of particles prevailed. However, in the early 19th century, Thomas Young conducted an experiment that helped prove Huygens’ theory that light is a wave and not a particle. Young believed that light should have properties similar to water waves, and he believed that light waves should interact when they meet, including merging.

Young's experiment involved placing a light source behind a single 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. When two crests of a wave met, they combined to form a wave with a larger amplitude. When a crest met a trough, the waves cancelled each other out and formed a flat wave or a wave with less amplitude at that point. Young observed this behaviour in his experiment and concluded that light was indeed a wave with properties similar to other types of waves, such as water waves.

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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.

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Affect on the wave-particle duality of light

In the 20th century, Einstein's theory of relativity 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, which 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.

In summary, Isaac Newton’s theory of light states that light is a straight-line motion made out of small particles called corpuscles, while Huygens’ theory of light states that light is made out of waves. Both theories were proven to be partially valid, as light was later proven to have both wave and particle properties. These light theories eventually led to the development of the quantum mechanics dual theory of light.

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Newton’s and Huygens’ Theories of Light

What did Isaac Newton discover about light?

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.

What are the two theories of light?

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.

What is Huygens’ theory of light?

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.

Who gave the corpuscular theory of light?

Isaac Newton gave the corpuscular theory of light.

Why is Huygens’ principle important?

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.

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