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, which states that light is made from small particles. 

Newton's and Huygens' Theories of Light Newton's and Huygens' Theories of Light

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Table of contents

    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.

    Newton’s and Huygens’ Theories of Light, Reflection of light diagram, StudySmarterFig. 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.

    Newton’s and Huygens’ Theories of Light, Refraction of light diagram, StudySmarterFig. 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).

    Newton’s and Huygens’ Theories of Light, Newton’s prism experiment, StudySmarterFig. 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.

    Newton’s and Huygens’ Theories of Light, Double-slit experiment Huygens’ principle, StudySmarterFig. 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.

    Newton’s and Huygens’ Theories of Light, Double-slit experiment, Huygens’ principle StudySmarterFig. 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

    Frequently Asked Questions about 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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