Shedding light on vision: from ancient Greece to modern optics

Turn off every light in a room and vision disappears. Although everything stays in place and you can still identify objects by touching them, your eyes receive no useful information. This simple fact highlights one of the most profound concepts in the history of science: seeing is not a direct meeting between an observer and an object; something must travel between them to bridge the gap. That something is light. Without this intermediary, our brain receives no visual information about the outside world and cannot form an image.

Fig. 1. A beam of light striking a cup of coffee and part of the table makes them visible to the eyes, whereas other objects in the room remain unseen.

The ancient debate on the role of light

Ancient philosophers recognized light's importance, yet they argued over its role. In ancient Greece, two influential theories of vision emerged: Aristotle's (4th century BC) and Euclid's (3rd century BC). Both theories aimed to explain how an observer and an object communicate without direct contact. However, their approaches differed: Aristotle focused on how the object makes itself visible to the observer, whereas Euclid emphasized the observer's ability to detect the object.

Aristotle understood light as the illuminated state of a medium, such as air or water, because the ideas of light as particles and later as waves were still many centuries away. In his account, an object affects the medium, which then transmits that effect to the observer's sensory organ, the eye. Although this explanation, called the intramission theory, was flawed, the notion that information about an object reaches the observer through an intermediary was remarkably forward-thinking.

Fig. 2. Aristotle's theory of vision: a transparent medium transmits the effect of a coloured object to the eye.

Euclid believed that vision works in reverse: the observer's eye extends toward an object, like a hand reaching out to touch it. According to his theory, the eye emits visual rays that travel in straight lines and make objects visible when they encounter them. Although this explanation, known as the extramission theory, was also incorrect because light enters the eye rather than emerging from it, representing vision through straight rays was a major step forward.

Fig. 3. A smaller nearby object can completely block a larger distant one because both occupy the same visual angle, or visual cone, as seen from the observer's eye.

Fig. 4. The more distant object of the same size appears smaller because it occupies a smaller visual angle than the nearer one.

Euclid's geometric model of vision allowed him to describe what we see in terms of lines and angles. This approach explained why smaller objects can obstruct the view of larger ones (Fig. 3) and why distant objects appear smaller (Fig. 4), laying the groundwork for geometrical optics and the principles of perspective that later influenced art and architecture. Interestingly, the mathematics proved valuable despite the incorrect assumption about ray direction.

Ibn al-Haytham. The debate continued for more than a thousand years until Ibn al-Haytham (965–1040 AD), known in medieval Europe as Alhazen, made a significant breakthrough that brought our understanding of vision closer to its modern form. Rather than relying solely on philosophical arguments or geometrical reasoning, he investigated light through experiment. In his Book of Optics, he argued that vision occurs when external light illuminates an object, reflects off its surface, and then enters the observer's eyes.

In one experiment, he placed several candles outside a darkened chamber, letting their light pass through a small opening and form separate patches on the opposite wall. When he covered one candle, its corresponding patch disappeared, while the others remained visible, demonstrating that light travels in straight lines from the candles. The observer no longer needed to reach out to the world; instead, light from the world was reaching the observer. Light could now be studied as a physical phenomenon in its own right, independently of the act of seeing.

The modern understanding of vision

Today, we understand vision as a process in which light interacts with the external environment,  collects information, and delivers it to our eyes. Unlike Aristotle, we know that light is not a state of the medium through which it passes but a physical phenomenon that can travel through the medium as well as a vacuum. We also know that, contrary to Euclid's intuition, our eyes are not projectors of visual rays that detect objects but receivers of light that conveys information about these objects.

Fig. 5. hh

Light from the Sun, a lamp, or another source shines on an object. Some of this light is absorbed, some may pass through the object, and some is reflected or scattered in various directions. The portion that reaches our eyes (Fig. 5) carries information about the object's shape, color, and position. When light enters the eye, light-sensitive cells in the retina convert this information into electrical signals, which are transmitted to the brain, where our visual perception of the object is formed.

Light is elusive; we can't stop it and place it under a microscope for close inspection, which is why we still have much to learn. The journey from Aristotle to Euclid and al-Haytham illustrates how science evolves, with each thinker contributing something important, even when their explanations were incomplete. Today, we know light exists independently of our eyes and follows physical laws whether anyone is watching. We see the world not because our eyes reach out to it, but because light brings the world to us.

Light is elusive. We can observe its effects and measure its properties with extraordinary precision, yet its nature still challenges our understanding. The journey from Aristotle to Euclid and al-Haytham illustrates how science evolves, with each thinker contributing something important, even when their explanations were incomplete. Today, we know that light exists independently of our eyes and follows physical laws regardless of who is watching. We see the world not because our eyes reach out to it, but because light brings the world to us.