Radiation and perception: what is light ?

© Pixabay/Pexels

In the meanders of the infinitely large and the infinitely small, a physical phenomenon plays with the boundaries of our understanding: light. Throughout this article, we will reveal its many facets, from its propagation in the cosmic void to its intimate interaction with matter. Both a wave and a particle, light shapes our perception of the world, allows life to exist on Earth and continues to be the subject of study for numerous scientific research projects.

Light, a wave of energy in motion

Light is above all an electromagnetic wave, a disturbance that propagates through space like ripples on the surface of a pond. But unlike water waves, light does not need a material medium to travel; it undulates at a speed of 299,792,458 m/s through a vacuum.

This wave nature ise manifests itself in every moment of our daily lives. When you admire the shimmering colors of a soap bubble, you are actually observing the interference of light waves reflected by the thin layers of soap. Similarly, the rainbow that adorns the sky after rain is nothing other than the decomposition of white light into a range of colors, each hue corresponding to a specific wavelength.

Visible light is actually only a tiny part of a vast electromagnetic spectrumwhich encompasses a large number of radiation continua, of which visible light constitutes only a narrow fraction, approximately between 380 and 780 nanometers in wavelength. This spectrum extends from low-frequency radio waves, with wavelengths up to several kilometers, to high-energy gamma rays, whose wavelengths can be smaller than the size of an atomic nucleus.

Electromagnetic spectrum showing the different wavelengths of radiation, ranging from gamma waves to radio waves © Lena Lighting

The quantum ballet of photons

However, light can also behave like a stream of particles. These particles of light, called photons, are the messengers of light energy. Think of them as tiny packets of energy, traveling in straight lines through space and time. This corpuscular nature of light is revealed in phenomena such as the photoelectric effect. When light strikes certain materials, it can knock electrons off them. This is the very principle at work in solar panels, transforming the light of the Sun into a flow of electricity.

The wave-particle duality (very well explained in the video below, with French subtitles if necessary) of light is a fundamental characteristic that has revolutionized our understanding of the universe. It behaves both as a wave, which propagates and can interfere with itself, and as a flow of particles, photons, which carry energy. Discovered at the beginning of the 20th century, this astonishing property paved the way for a scientific revolution, laying the first foundations of quantum mechanics.

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This duality is not just a theoretical abstraction. It is at the heart of many modern technologies, from lasers to electron microscopes, illustrating how a deep understanding of the nature of light can lead to major technical advances.

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When light becomes vision

Our perception of light is comparable to a true biological alchemy. When a ray of light enters our eye, it strikes our retina. The retina, which lines the back of the eye, is a mosaic of photoreceptor cells (sensitive to light) including cones and rods, the main players in vision. The cones capture shades of red, green and blue and allow us to see during the day. The rods, for their part, allow us to distinguish our environment in near darkness.

These cells then transform the light energy into electrical signals, which our brain immediately decodes. It is in the depths of our visual cortex that our perception of shapes, colors and movements is born. Vision is therefore not just a simple photograph of the world, but rather a complex and subjective interpretation of luminous reality.

Light is a very complex phenomenon and still remains to this day a subject of inexhaustible study: quantum optics, materials physics, quantum computing or cryptography, etc. Its investigation continues to provide us with valuable insights into the nature of reality, from the quantum scale to the cosmological scale. The more we deepen our mastery of luminous phenomena, the more we refine our most fundamental theories; which allows us to develop transformative technologies. From a purely symbolic point of view, it perfectly embodies the link between the fundamental theories of physics and our everyday experience of the world.

• Light is both an electromagnetic wave and a stream of particles (photons), illustrating the wave-particle duality.
• It is part of a vast electromagnetic spectrum, of which only a small portion is visible to the human eye.
• Our perception of light involves a complex process from the retina to the brain, transforming light energy into visual experience.

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