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.

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

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.

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.

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 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.

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.

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