For those interested in physics, this is not an easy question, but after much time and effort, we now have an answer.
From the farthest stars in the sky to the screen in front of you, light is everywhere. Yet, the exact nature of light and how it travels has long puzzled scientists. From Isaac Newton to Albert Einstein, they all questioned whether light is a wave or a particle.
Physicist Riccardo Sapienza from the University of London’s Royal Institution explains that this question has been a topic of discussion for a long time, particularly in the scientific community during the 19th century.
Today, the clear answer is that light is both a particle and a wave. But how did scientists reach this seemingly paradoxical conclusion?
The first step in unraveling this mystery is to scientifically distinguish between waves and particles. According to physicist Riccardo Sapienza, “You describe something as a particle if it can be pinpointed as a single location in space. A wave, on the other hand, cannot be defined as a point in space but requires characteristics such as oscillation frequency and the distance between its peaks and troughs.”
The First Evidence: Light as a Wave
In 1801, Thomas Young provided the first convincing evidence of light’s wave nature through his famous double-slit experiment. Young placed a screen with two small holes in front of a light source and observed the behavior of light after passing through the slits. The light projected onto the wall behind displayed a complex pattern of bright and dark bands, known as interference fringes.
As light waves passed through each slit, they spread out in spherical patterns and interacted, either reinforcing or canceling each other to create the final intensity.
If light were purely a particle, the result would have been two distinct beams of light emerging from the slits. Instead, the observed interference pattern proved that light behaves like a wave.
Light as a Particle: Heinrich Hertz’s Breakthrough
Nearly 80 years later, Heinrich Hertz became the first to demonstrate light’s particle-like behavior. He discovered that ultraviolet light striking a metal surface generates an electric charge, a phenomenon now known as the photoelectric effect. However, this observation puzzled scientists for decades and wasn’t fully understood until much later.
Atoms contain electrons bound at specific energy levels. When light hits these atoms, it provides energy that can eject electrons. Classic physics suggested that brighter light should release electrons faster. Yet, further experiments revealed results that contradicted this traditional understanding.
Einstein’s Nobel-Winning Explanation
The solution to this mystery came from Albert Einstein, who earned the Nobel Prize in Physics in 1921 for his explanation. Instead of absorbing energy continuously from a wave, Einstein proposed that atoms receive energy in discrete packets of light, known as photons. This concept accounted for seemingly strange observations, such as the existence of a threshold frequency below which no electrons were emitted, regardless of light intensity.
Interestingly, the question of whether light acts as a wave or a particle is one physicist Riccardo Sapienza suggests we shouldn’t ask. According to him, light is always both a wave and a particle. “It’s not that light is sometimes a particle and sometimes a wave,” he explains. “It’s always both, but we emphasize one property over the other depending on the experiment we perform.”
In everyday life, we primarily perceive light in its wave-like form, which is also the perspective physicists often find most practical when applying it to real-world problems.
Harnessing Light’s Properties for Innovation
Scientists have discovered that by designing materials with properties similar to light, they can enhance light’s interaction with those materials and better control its waves. For instance, they can create solar energy absorbers that improve light absorption efficiency for energy production. Similarly, advances in material design can lead to more effective MRI sensors, dramatically improving medical imaging technology.
