Hi, I’m Rachel, the Astronomer here at the Space Centre.

On Earth, our main source of light is the Sun. Solar radiation, or sunlight, provides the energy for photosynthesis – plants create sugars, which when digested, release energy. Some species of animals, like fireflies, can even generate their own light through bioluminescence. In physics, “light” refers to any wavelength of electromagnetic (EM) radiation – from radio waves and microwaves to x-rays and gamma rays. The light that we can see, visible light, is only a portion of the electromagnetic spectrum.
 
But the history of trying to understand light is a long and confusing one; some scientists proposed that light behaved like a shower of particles, with physicist JJ Thomson’s 1897 discovery of the electron providing more evidence of the particle-like nature of light. Other scientists demonstrated that light behaved like waves, much like waves of water in the sea. In short, it looked like light could be a particle or a wave – so which is it? This very question gave rise to the concept of wave-particle duality.
 
In 1938, Einstein wrote, “It seems as though we must sometimes use the one theory and sometimes the other, while at times we may use either… We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do.”

This conundrum was resolved in the mid-1920s with the advent of the correct equations for quantum mechanics. Wave-particle duality is deeply embedded in quantum theory. Without diving into the complex mathematics of the quantum world, we can still paint a rough picture of the breakthroughs from physicists Erwin Schrödinger, Werner Heisenberg, Max Born and others, that paved the way for modern quantum mechanics and later, quantum field theory.
 
But back to this apparent paradox about light or EM radiation behaving like a wave and a particle. Let’s try a short thought experiment (or as Einstein would say, a gedanken-experiment). Say we shine a laser beam on a plate with two parallel slits – as Thomas Young did in 1801 in his famed double-slit experiment – and see what comes out on a screen behind the plate. The wave nature of light causes the light waves passing through the two slits to interfere, producing a fringe pattern of bright and dark bands on the screen.