E21 Electromagnetic Channel

Topic

Electromagnetic radiation constitutes one of the sensory channels richest in information from the environment, because it covers an enormous range of frequencies, from radio waves to gamma rays. Human eyes use only a small region of this spectrum: visible light. Other species expand this range and obtain information inaccessible to humans. Insects and birds can detect ultraviolet, while other animals have sensitivity toward infrared regions associated with the detection of thermal sources. Thus, the spectral range to which each sensory system responds directly determines what aspects of the environment it can perceive.

The energy of each photon depends on its frequency and determines whether it can produce a photochemical reaction in a photoreceptor. In the human eye, opsins absorb photons belonging to the visible range and trigger the visual response. Outside this range, the interaction changes: ultraviolet photons have higher energy and can produce cellular damage instead of controlled photochemical activation, while photons corresponding to thermal infrared have too low an energy to activate those same visual mechanisms. Therefore, the fact that there is electromagnetic radiation in the environment does not necessarily mean that it can be detected by the eye: its energy must be within the range to which its photosensitive molecules respond.

Planck and Wien's laws relate the temperature of a body to the spectrum of radiation it emits. The Sun has its maximum emission in a region close to the range where human vision is most sensitive, while the body of a warm-blooded animal mainly emits much longer wavelength radiation. A sensor adapted to this last region can detect body heat that the human eye does not perceive. Furthermore, there is an atmospheric window that transmits with particular ease a band of radiation coinciding with an important part of the thermal emission of warm-blooded organisms. Biological or artificial sensors tuned to this band can therefore detect thermal information directly.

The propagation of radiation through different media is conditioned by its absorption. The Beer-Lambert law describes how intensity decreases when passing through biological tissues, water or the atmosphere itself. This absorption depends on the wavelength. Water is relatively low in the blue-green region of the visible spectrum, so these wavelengths penetrate more deeply than other colors. This contributes to the visual systems of aquatic organisms being adapted to the spectral region that remains available at greater depth.

The human eye also has selective absorption. Their structures strongly absorb ultraviolet radiation, reducing the amount that reaches the retina and protecting it from damage associated with higher energy photons, while much more efficiently transmitting the visible range necessary for vision. In this way, both the molecular properties of the photoreceptors and the optical characteristics of the tissues in front of them jointly determine how much of the environmental radiation can be converted into visual information.

Even within the range that the eye traverses and activates the photoreceptors there is a physical limit to the amount of detail that can be obtained. The pupil functions as a circular aperture and, therefore, the light that passes through it experiences diffraction. According to the diffraction limit, two points that are too close together produce patterns that can no longer be distinguished independently. The minimum resolvable angle depends on the wavelength and the diameter of the aperture: decreasing the size of the pupil increases the effect of diffraction and worsens the angular resolution.

According to the text, this physical limit remarkably coincides with the separation between the cones of the human fovea. Visual resolution thus results from the adjustment between the optics of the eye and the spatial density of its receptors: having receptors much closer together would provide little additional information if diffraction itself already prevented us from distinguishing smaller details.

Taken together, vision emerges from a chain of physical constraints. The available spectrum determines what radiation exists; The energy of the photons determines which ones can activate the photoreceptors; Planck and Wien relate temperature to the radiation emitted; Beer-Lambert determines which wavelengths pass through each medium, and diffraction ultimately fixes the spatial detail that can be resolved. Each species thus accesses only a particular part of the electromagnetic information present in its environment, determined jointly by the physics of radiation and by the characteristics of its own sensory system.

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