Light and the environment
The wavelengths of light comprising the electromagnetic spectrum are fundamental to all life on Earth. Human eyes are only able to see a very small portion of the electromagnetic spectrum – visible light – which has wavelengths between approximately 400-700nm, but there are key roles played by light of different wavelengths. To fully appreciate light, it is necessary to understand how it interacts with the environment and organisms with which it comes into contact.
Figure 1 Electromagnetic radiation undergoing refraction, reflection, absorption, and scattering
The most important source of light and subsequent energy on Earth is the sun. The sun emits energy that can be absorbed by organisms and molecules where it facilitates countless reactions, including photosynthesis, ozone production, weather phenomena and, in humans, vitamin D production. Light photons follow the properties of electromagnetic radiation, and are able to travel through the vacuum of space at the speed of light (2.997 924 58 ×108 m/s).
The Earth’s atmosphere is able to block many wavelengths of light which can pose a hazard to human health. As light travels through the atmosphere it may also undergo refraction, reflection, absorption, and scattering, all of which impact its velocity, as demonstrated by Figure 1.
Both reflection and refraction are variations of scattering, which can be viewed as the alteration of the path of electromagnetic radiation in any direction. If the radiation passes through an object and its path is altered forward of the object, then refraction has occurred. Reflection describes a process where incoming radiation strikes a surface and bounces off at an equal angle to the angle of incident light. Radiation is said to be absorbed when the energy is transferred to either chemical or thermal energy within the object upon impact.
The ozone layer in the Earth’s atmosphere provides a natural shield to the biologically harmful radiation from the sun, more specifically the sun’s ultraviolet (UV) radiation.
Ultraviolet radiation and the atmosphere
As light enters the Earth’s atmosphere it comes into contact with molecules in the stratosphere, situated 10-50km above sea level. The stratosphere contains the ozone layer which is comprised of ozone, a three-oxygen atom molecule (03) and an important molecule in reducing the amount of harmful UV radiation that passes through our atmosphere.
Figure 2: Production of ozone in the atmosphere
Figure 2 shows the production of ozone molecules, in which oxygen molecules (02) in the upper stratosphere are able to absorb the higher frequency UV light forming two separate oxygen atoms (0 + 0). These atoms can re-join, recreating the oxygen molecule (02), or binding to an oxygen molecule ultimately to form an ozone molecule (03).
The newly formed ozone molecules are good absorbers of UVC, UVB and to a lesser degree UVA radiation, absorbing the radiation and undergoing dissociation from a triatomic ozone molecule (03) to become diatomic molecular oxygen (02) plus a free oxygen atom (0), which can quickly react to reform an ozone molecular (03). Both of the reactions to form ozone molecules and reactions between ozone molecules and UV radiation occur in equilibrium, where neither really dominates.
The amount of UV light that reaches the Earth’s surface varies and is dependent on a number of factors:
Over the past decades it has been observed that the ozone layer is thinning over certain regions of the globe, particularly over Antarctica. This has been attributed to chemicals released by industry, particularly gaseous chlorine and bromine containing substances, e.g. chlorofluorocarbons (CFCs), bromofluorocarbons (BFCs) and hydrochlorofluorocarbons (HCFCs).
Ozone can be broken down by both natural and manmade sources. Atomic chlorine and bromine are two significant contributors, and can be released into the atmosphere by large volcanic eruptions and industry made chemicals, e.g. CFCs and BFCs. CFCs were previously widely used by industry due to their non-toxic and non-flammable properties making them a good substitute for toxic and flammable chemicals e.g. ammonia, for propellants in aerosol cans and refrigerants. Their use during the 20th century grew exponentially (in 1988, over one billion kilograms was estimated to be produced annually worldwide) until the discovery that CFCs had the capacity of thinning the ozone layer. In 1987, world leaders acted by signing the Montreal Protocol, which stipulated that consuming and producing chemicals capable of breaking down ozone be phased out by the year 2000.
CFCs and other similar chemicals are stable and not destroyed in the troposphere, resulting in them slowly rising further towards the stratosphere. These stratospheric molecules are found to be able to break ozone down into an oxygen atom and molecule. This usually occurs at a slow rate, reducing the overall thickness of the ozone layer by about 4% per decade, but in the atmosphere above Antarctica, thinning occurs at an alarming rate. During its winter, Antarctica becomes cold enough (-80°C) for stratospheric clouds (termed Polar stratospheric clouds, PSCs) to form in the ozone layer. On the surface of these clouds, chlorine and bromine are transformed into an active state and when the sun gains intensity in the spring, clouds disappear, freeing chlorine and bromine to rapidly destroy ozone. As temperatures further continue to warm, wind vortexes holding the particles above Antarctica break up, mostly allowing ozone rich air to flow into ‘the hole’. The chlorine and other molecules capable of breaking ozone down can destroy up to a thousand molecules of ozone before being converted into an inactive form, such as hydrochloric acid.
- Latitude: Near the equator where the sun is directly overhead, the distance from the sun to the ground is the shortest, and so UV has to pass through the least amount of atmosphere. Therefore, on the equator, the UV intensity is the highest on Earth. Likewise, on the poles, the sun appears low in the sky, meaning the light has to pass though more radiation-absorbing atmosphere to reach the surface.
- Elevation: At higher altitudes there is less atmosphere to absorb UV, therefore individuals are exposed to a greater amount of UV radiation.
- Proximity to an industrial area: Photochemical smog produced in industrial processes contains artificially produced ozone. The ozone produced can absorb more UV radiation, but also poses significant health risks if it is low lying.
- Weather conditions: Cloud cover can reduce the UV levels on the surface, but often incompletely.
- Reflection: Some surfaces are able to reflect UV radiation, meaning there is a greater chance of getting sun burnt, even in shady places. Snow, sand, grass and water are all good reflecting surfaces.
- Time of day/year: During the middle of the day the sun is at its highest position in the sky, resulting in the least distance for radiation to travel, and more exposure to UV light. The summer months also provide the most intense UV radiation, as the sun’s angle (zenith angle) is reduced.
- Ozone layer: The depth of the ozone layer plays a vital role in reducing exposure to UV light. The ozone layer has been found to be thinning in the past few decades, presumably due to the release of ozone degrading chemicals by industry.
Vitamin D synthesis
While the harsher effects of UV radiation have been described, vitamin D synthesis in humans requires the exposure of skin to UVB radiation (280-320nm). Following this exposure, a series of reactions produces vitamin D3 (cholecalciferol), which is then transferred into the bloodstream with the aid of a carrier protein.
Vitamin D is largely responsible for regulating calcium and phosphate levels in the blood and therefore is crucial for skeletal health; it plays a vital role in bone development, maintenance and repair. Vitamin D presents a controversial issue in modern day medicine since the main source (over 90%) comes through dermal contact with solar radiation. Given the known effects of extended skin exposure to UV rays, such as skin cancer and photoaging, many people try to gain an adequate intake through dietary changes or supplements. Despite these advancements, there is a troubling trend towards an increase in vitamin D deficiency, perhaps due to public health campaigns on sun protection and an increase in the awareness of the harmful effects of UV radiation.
The heat supplied by sunlight has resulted in the Earth becoming a suitable environment for us to survive.
Once radiating energy is passed through the atmosphere, the photons collide with matter, transferring energy. This causes heat, wind, rain, clouds and other weather phenomena. Wind is a result of the varying temperatures of the air above land and water. The hot air rises, causing a flow of cooler air to occupy its previous place, and eventually cool air, high in the atmosphere, drops down to sea level, completing the cycle. When ocean water is heated, water particles evaporate into the air, where they culminate to form clouds. When these clouds are pushed over land by winds they experience lower temperatures, causing condensation into droplets and precipitation.
Light and the human eye
Human eyes are only able to view the visible light portion of the electromagnetic spectrum, approximately 400-700nm. The different portions of our eyes are shown below in Figure 3
Figure 3: The human eye
Light enters the eye via the cornea, the clear, dome-shaped surface covering the front of the eye, and the cornea bends or refracts incoming light. The iris, the coloured portion of the eye, regulates the size of the pupil; in environments where light is more intense, the pupil constricts, while in conditions of lower light the pupil is dilated.
Behind the pupil is the lens which further focuses light onto the retina. The retina is a thin, delicate, photosensitive tissue that contains the special “photoreceptor” cells called rods and cones. There are three types of cones, each responding to different wavelengths and responsible for daylight vision, while rods are responsible for night time vision. Once stimulated by light, the rods and cones are able to convert light into electrical signals, which then travel from the retina, through the optic nerve to the brain. Our brains are able to collect and process the visual information, allowing us to see.
Figure 4: The process of photosynthesis
Light is also a necessary requirement for photosynthesis, a process which humans, animals and plants all depend on for survival. Organisms that carry out this procedure are called photo-autotrophs, and after carrying out a reaction between water and carbon dioxide in the presence of light, produce oxygen and glucose (sugar) as shown in Figure 4.
Photo-autotrophs require light exposure in order to survive and feed themselves. Examples of photo-autotrophs includes plants, algae and certain types of bacteria, with plants being the most common photo-autotroph found on land.
Figure 5: Diagram of a chloroplast
Photosynthesis takes place in plants in organelles called chloroplasts, with sunlight energy specifically captured by chlorophyll, a pigment found in chloroplasts. Chlorophyll is of biological relevance due to its chemical structure, and subsequent absorption spectra; it absorbs light most strongly within blue and red light regions in the electromagnetic spectrum while being a poor absorber of the green region, hence reflecting the green colour of chlorophyll. Also present in chloroplasts and involved in photosynthesis are grana, which are stacks of thylakoid discs, and the stroma, which is a dense fluid surrounding the grana (see Figure 5).