Operation Module

Understanding the visual process can be deepened through a series of practical activities that allow students to observe both optical and biological phenomena involved in the mechanism of vision. The aim of this section is to apply, in a guided way, the concepts of refraction, focusing, and signal transduction studied in the theoretical part, connecting the physics of optics with the biology of the human eye.

When light passes through media of different optical densities, its trajectory changes direction—a phenomenon known as refraction. The cornea and crystalline lens, which constitute the natural lenses of the eye, act as refracting elements that bend light rays to form a sharp image on the retina. Students can observe this behaviour using a simple experiment: partially immerse a pencil in a glass of water and notice how the object appears “broken” at the interface between air and water. This visual effect directly illustrates how light is deviated in ocular media and introduces the principle of refraction that governs the focusing of the retinal image.

The phenomenon is then explored in a virtual environment through the interactive simulation Geometric Optics, available on the PhET platform – University of Colorado Boulder:
https://phet.colorado.edu/sims/html/geometric-optics/latest/geometric-optics_all.html

Through the simulation, students can vary the distance between object and lens and modify the curvature of the surfaces, observing how the position of the focal point and the image sharpness change. This activity helps students understand how the human crystalline lens changes its curvature to maintain the image in focus on the retina—a process known as accommodation. 

From these observations arise essential questions: what is the relationship between lens shape and focusing distance? Why do excessive or insufficient curvatures lead to conditions such as myopia or hyperopia? Comparing the physical lens in the simulation with the biological crystalline lens clarifies how variations in refractive power affect the position of the focal point and, consequently, visual quality.

The focus then shifts to the retina, the thin sensory membrane at the back of the eye that converts light energy into electrical signals. Students analyse the arrangement of photoreceptors—cones and rods—and their complementary roles: rods are specialized for night vision and perceive changes in brightness, while cones provide colour perception and high acuity in bright light. The high density of cones in the fovea explains central sharpness, while the prevalence of rods in the periphery enhances motion detection.

The class discussion continues by analysing the behaviour of photoreceptors under low-light conditions. Why do colours appear faded at dusk? The answer lies in the higher sensitivity of rods, which enable vision in dim light but cannot discriminate wavelengths responsible for colour. This collective reflection helps students understand how the brain integrates information from different sensory cells to build a coherent visual representation.

Students are then invited to compare the human eye with everyday optical devices such as cameras or smartphones. The crystalline lens behaves like a converging lens, the pupil acts as a diaphragm, the retina functions as a digital sensor, and the brain plays the role of image processor. Through this analogy, vision is reinterpreted as a physical model connecting technology and biology.