Let there be Photons
Visible light is part of the electromagnetic spectrum which ranges in wavelength from short wavelength gamma rays to long wavelength radio waves. It is produced in stars such as the sun when hydrogen turns into helium during nuclear fusion. Visible light is the portion of the spectrum with which life interacts most obviously. It is a strange fact about light, that it can be considered to be both waves and particles. The particles called photons, carry energy which can be morphed into other forms of energy. This is an effect which is essential to life. The most fundamental process that uses light energy is photosynthesis. Every green plant uses light energy to make its food and every animal ultimately depends on plants for food. The green pigment in plants, called chlorophyll, contains magnesium and when a photon arrives at the magnesium it excites its electrons which capture energy by moving to higher orbitals. The energy then drives the biochemical processes that make glucose. Although there are bacteria that can start food chains using chemical energy at deep sea heat plumes the vast majority of life depends on light.
Organisms also use light to sense the environment around them. Complex eyes have evolved but even single celled organisms may be sensitive to light. Vertebrate eyes have light sensitive cells in a layer called the retina. When a photon arrives at the cell it triggers a nerve impulse which is transmitted to the brain along the optic nerve. The nature of the retina is such that the photon may pass through without producing a response and so nocturnal animals which require extra sensitivity have a reflective layer called the tapetum which sends the photon back through the retina and gives it a double chance of eliciting a response. This is why the eyes of nocturnal animals such as deer shine in car headlights. We are diurnal and don’t have a tapetum.
Light helps to regulate rhythmic behaviours such as the circadian rhythm, migration and hibernation. It is the shortening or lengthening of day length that stimulates migratory birds such as the swallow to embark on their journeys. Our daily rhythm of sleeping and waking is regulated by light penetrating a structure beneath the brain called the pineal body. This is thought to have evolved from a third eye such as is still found in New Zealand’s living fossil the Tuatara. In the Tuatara the third eye is a light sensitive area between the two image forming eyes. It does not have the capability of forming an image but probably regulates rhythms of behaviour in a manner similar to our pineal.
Organisms can produce their own light. There are bioluminescent algae and deep sea fish which make light by mixing chemicals. The fish may use the light to communicate and to lure prey. Fireflies and glow worms make use of the light that they generate to attract mates.
Plants grow toward the light so that they can capture the energy of sunlight. They have hormones called auxins and gibberellins that allow them to direct their growth.
Light in the form of ultraviolet light, generates vitamin D in our skin and is vital in preventing rickets and other diseases. On reflection (refraction, diffraction and interference), this is only the tip of the optical iceberg, but indubitably, we can say “Many photons make light work”.