Competing Models (HSC SSCE Physics): Revision Notes

Competing Models

Historical perspectives on light

For centuries, scholars debated the fundamental nature of light. As early as the 5th century BCE, Greek philosopher Democritus—who proposed the original concept of atoms—argued that light, like everything else, consisted of indivisible particles.

Around 1000 CE, Arab scholar Ibn al-Haytham (also known as Alhazen) made significant contributions through his seven-volume work, the Book of Optics. He disproved Ptolemy's theory that light emanated from our eyes by noting that bright light damages eyes. Ibn al-Haytham studied light's behaviour, including reflection and refraction, but always viewed light as composed of particles.

Newton's corpuscular theory

By the 17th century, two competing models emerged to explain light's behaviour. Isaac Newton championed the corpuscular theory (particle model), whilst Christiaan Huygens developed the undulatory theory (wave model).

In 1704, Newton published Opticks, his comprehensive treatise on light. The work examined refraction, diffraction, and colour mixing. Newton's model proposed that light consisted of tiny particles called corpuscles that possessed mass.

Basic principles of Newton's model

Explaining optical phenomena with particles

Rectilinear propagation and shadows

Newton viewed the straight-line travel of light (rectilinear propagation) as strong evidence for his particle theory. If light particles moved in straight lines, they would create sharp shadows—exactly what he observed. An interesting consequence of this theory is that light particles, having mass, would be subject to gravity and should follow parabolic trajectories in Earth's gravitational field.

Diffraction

Since particles travelling in straight lines cannot bend around corners, Newton explained diffraction (the slight bending of light around edges) as particles colliding with each other at object boundaries.

Reflection

The corpuscular theory successfully explained the law of reflection: the angle of incidence equals the angle of reflection. This matched how a particle, such as a ball bearing, bounces off a smooth surface in an elastic, frictionless collision.

Refraction and Snell's Law

Newton's explanation of refraction required an additional assumption: light particles experience attraction to matter particles. Deep within a transparent material, a light particle would experience equal attraction in all directions, resulting in no net force. However, as a particle approaches the surface of a denser medium at an angle, the short-range attractive forces create a net force perpendicular to the surface (normal to the surface).

This net force causes the particle to accelerate, bending its path in a parabolic arc toward the normal as it enters the denser medium—matching experimental observations.

The component of the light particle's velocity parallel to the boundary remains unchanged by the normal force. Newton showed that the ratio of the sines of the incident angle ($\theta_i$) and refracted angle ($\theta_r$) equals the ratio of light velocities in the two media:

$\frac{\sin \theta_i}{\sin \theta_r} = \frac{v_{\text{water}}}{u_{\text{air}}}$

Partial reflection during refraction

Newton observed that when light refracts, some light also reflects. He explained this through his Theory of Fits: some particles could "fit" between atoms and would refract, whilst others couldn't fit and would reflect instead.

Colours

Newton noted that different colours refract by different amounts—red light bends least, violet most. He explained this by proposing that light particle mass varies with colour. Different masses would have different inertia, so the same force would produce different accelerations as particles approached an optical interface, causing varying amounts of bending.

Polarisation

When you observe a light source through two sheets of polarised material and rotate one, the light intensity varies from maximum to minimum with 90° rotation, then returns to maximum. This demonstrates polarisation. If light particles were spherical, this couldn't occur. Newton therefore proposed that light particles must have sides, though he found it difficult to visualise their exact shape—whether rectangular or plate-like.

The decisive test: Foucault's experiment

In the 19th century, French physicist Léon Foucault used a rotating mirror to measure light's speed. He then applied this technique to test Newton's particle theory, which depended critically on the assumption that light travels faster in water than in air.

Foucault reflected a light beam off a mirror rotating at 800 revolutions per second (driven by a small steam turbine) to a fixed mirror 9 metres away. The light reflected back in 60 nanoseconds, during which time the rotating mirror shifted slightly. This caused the returning beam to deflect slightly below the light source.

Foucault then introduced a 3-metre tube filled with water into the light path between the mirrors, so light passed through it twice. If Newton's theory were correct—if light travelled faster through water—the mirror would rotate less during the journey, and the return beam should deflect closer to the source.

This exemplifies how scientific theories work. As Albert Einstein later observed in 1919:

Huygens' wave model

Robert Hooke initially proposed the wave model of light in 1665, which Christiaan Huygens later refined and developed. Huygens' model explained aspects of light behaviour where Newton's theory struggled, particularly diffraction. However, it appeared weak on rectilinear propagation, which is why Newton rejected it. The scientific community largely ignored Huygens until 1802, when Thomas Young performed his famous double-slit experiment and measured the wavelengths of visible light, vindicating Huygens' ideas.

Huygens' principle

Huygens' principle states that each point on a wave acts as a point source, generating waves in the direction of propagation. The line tangent to these circular wavelets represents the new position of the wavefront a moment later. We don't observe the individual circular wavelets because of the principle of superposition—the wavelets combine to form the overall wavefront.

Wave explanation of reflection

When wavefronts meet a reflecting surface at the angle of incidence, new wavelets form at that point. The associated wavefront leaves the surface at the angle of reflection. Remember that Huygens' principle describes propagation direction.

Wave explanation of refraction

Huygens used his principle to propose that waves slow down when entering an optically denser medium, bending toward the normal and obeying Snell's Law. Importantly, Huygens didn't need to explicitly connect optical density with material density as Newton did.

Diffraction and rectilinear propagation

Diffraction is the bending of light as it passes an object's edge or travels through a gap. The amount of diffraction depends on the relationship between the opening size and the wavelength.

Consider light rays passing through an open doorway into a room—they create sharp shadows. However, sound coming through the same doorway can easily be heard throughout the room. This occurs because sound wavelengths are approximately the same order of magnitude as doorways.