Huygens proposed the undulatory theory in 1678: light transmitted as waves. Light waves spread in different directions from a light source, and the retina detected their vibrations.

In contrast, Isaac Newton favoured the corpuscular theory: Light as particles. Light rays spread from a light source in a stream of tiny particles or 'corpuscles', shooting through space, and were detected by their impact on the retina.

Light does not appear to bend. Sound can go through crooked pipes, but light is not known to follow crooked bends. However, when light passes through water, the angels of incidence and refraction differ.

• Newton proposed that light's velocity in water was faster than in air. Once they enter water, the corpuscles were acted upon by a force that increased their velocity and changed the direction of the light. But Newton was not convincing as he could not explain the nature of this force.
• Huygens assumed light travelled slower in water and used his undulatory theory in a simple way.

Diffraction was first seen by Francesco Grimaldi in the mid-17 Century. When he shined a light through a small aperture, then passed it through a second aperture onto a screen, the spot of light on the screen was a bit bigger than the second aperture and had coloured fringes. That meant light could be bent.

• Newton claimed that the edges of an aperture interfered with the paths of the corpuscles.
• Huygens undulatory theory explained diffraction better, but Newton's prestige allowed his theory of light to dominate the Age of Enlightenment.

Michael Faraday showed the inter-relationship of electric and magnetic fields of force. James Clerk Maxwell took Faraday's fields and combined them mathematically into a single concept: an electromagnetic wave with transverse electric and magnetic components that would move through the empty space at a speed similar to the measured speed of light.

Different colours of light was found to have different wavelengths and frequencies. In the 1880s, Heinrich Hertz proved these wave's existence experimentally.

• In 1905, Einstein proposed returning to Newton's corpuscular theory in a modern form. Instead of corpuscles, there were now light 'quanta' (or photons.)
• In 1900, Max Planck proposed that the electromagnetic spectra from red-hot items could be explained only if radiation could be emitted or absorbed in small packets or 'quanta.' Einstein added that light itself was quantised.

Physicists today have to accept that light behaves as both a particle and a wave. But its underlying physical nature is still mysterious.