Optical phenomena include all observable events that arise from the interaction of matter and light. Every optical phenomenon aligns with quantum phenomena. Ordinary optical phenomena are predominantly due to the interplay of light from the Moon, Sun, or stars with water bodies, the atmosphere, dust, clouds, and other concentrations of particles. One familiar example is the rainbow when a beam of sunlight is refracted and reflected by water molecules or droplets. Some phenomena, for example, the green ray, are so uncommon that they are sometimes considered to be mythical. Others, like Fata Morganas, are relatively common in favoured locations. Most of the optical phenomena are basically unique aspects of optics or optical stimulations. For example, the colours created by a prism can be put under the umbrella of optical phenomena.
Most of the phenomena that arise from the interaction of light with various substances and processes come under the category of optical phenomena or optically visible events. These include phenomena arising from the optical characteristics of the atmosphere and other natural occurrences. It comprises both natural and human-made processes, which are referred to as optical effects. It also includes entoptic phenomena, which are caused by the properties of our own eyes. Now, let’s explore some of the fascinating optical phenomena that occur in nature.
Aurora is a natural occurrence of light that can be observed in the sky. It happens primarily in the high-latitude areas surrounding the Arctic and Antarctic regions. These lights exhibit a vibrant and ever-changing array of patterns. They include curtains, rays, spirals, and flickers that can cover the entire sky. The cause of auroras can be attributed to disruptions in the magnetosphere that arise from the influence of solar wind.
Significant disruptions occur when there is an increase in the speed of the solar wind, mainly due to coronal holes or coronal mass ejections. These disruptions cause charged particles in the magnetospheric plasma to deviate from their original paths. Subsequently, these particles fall into the upper atmosphere, specifically the exosphere and thermosphere. When charged particles fall into the upper atmosphere, they cause ionisation and excitation of atmospheric constituents. This leads to the emission of light in various patterns and colours. The appearance of the aurora (observed in bands around both polar regions) is also influenced by the degree of acceleration experienced by the falling particles.
In meteorology, an afterglow refers to a series of atmospheric optical events that create a broad, pinkish or whitish arch of sunlight visible in the sky during twilight. This arch comprises two distinct segments: the bright segment and the purple light. The purple light is usually visible during civil to nautical twilight when the Sun is 2-6° below the horizon, while the bright segment persists until the end of nautical twilight. The phenomenon of the afterglow is mainly associated with volcanic eruptions, with its purple light being referred to as the distinct “volcanic purple light”. In such volcanic events, the purple light is created by the scattering of light by fine particles, such as dust, that are suspended in the atmosphere.
Alpenglow (similar to the Belt of Venus) is a phenomenon where the afterglow refers to the golden-red glowing light visible in the sky during sunrise or sunset. More specifically, it is used to describe the final stage of the phenomenon, during which the purple light is reflected. The counterpart of an afterglow is a foreglow, which appears in the sky before sunrise.
A light pillar is an optical event in the atmosphere that produces a vertical beam of light appearing above or below a light source. This phenomenon is caused by the reflection of light from minuscule ice crystals that are either suspended in the atmosphere or present in high-altitude clouds like cirrus or cirrostratus clouds. When the source of light for a light pillar is the Sun, typically when it is close to or below the horizon, it is referred to as a sun pillar or solar pillar. Besides, light pillars can also be created by the Moon or terrestrial light sources like erupting volcanoes and streetlights.
A sun dog, also known as a sundog or mock sun and referred to as parhelion in meteorology, is an atmospheric optical occurrence characterised by a brilliant spot on one or both sides of the Sun. This phenomenon is frequently observed with two sun dogs flanking the Sun within a 22-degree halo. The sun dog is a type of halo created by the bending of sunlight through ice crystals present in the atmosphere. They usually manifest as a duo of faintly tinted light patches located approximately 22 degrees to the left and right of the Sun, positioned at the same height above the horizon. Although visible globally throughout the year, sun dogs may only sometimes be easily observable or vivid. They are most prominent and discernible when the Sun is in proximity to the horizon.
A moonbow, also called a lunar rainbow or moon rainbow, is a rainbow created by moonlight rather than direct sun rays. Other than the variation in the light source, its process is the same as for a solar rainbow: It is created by the refraction of light in many water droplets, such as a waterfall or rain shower, and is always positioned in the opposite section of the sky from the Moon relative to the viewer.
Moonbows are optically thinner than solar rainbows due to the relatively smaller amount of light rays reflected from the Moon’s surface. Because the light beams are usually too faint to stimulate the cone colour receptors in our eyes, it is tough for human eyes to discern colours in a moonbow. Consequently, a moonbow often appears as white. Interestingly, the colours in a moonbow will be visible in long-exposure photos. It is considered that the clarity of the colours depends upon the dimension of the moisture droplets present in the air (the smaller the size, the less clear the colours be).
An earthquake light refers to a bright, atmospheric occurrence that emerges in the vicinity of regions experiencing tectonic stress, seismic movements, or volcanic eruptions. The causes behind these phenomena are still unclear, and there is no widespread agreement regarding their origin. It is not similar to disturbances in electrical grids, such as sparking power lines caused by ground shaking or extreme weather conditions; earthquake lights produce distinct luminous flashes.
There are two distinct types of earthquake lights, which are classified according to their timing. The first one is the preseismic earthquake light, which typically appears several seconds to a few weeks before an earthquake. It is usually visible closer to the epicentre. The second type is the coseismic earthquake light, which may occur near the epicentre as a result of earthquake-induced stress or at considerable distances away from the epicentre during the passage of seismic waves, particularly S waves, due to wave-induced stress.
According to certain models, earthquake lights may be generated by the ionisation of oxygen, which results from the breaking of peroxy bonds in specific types of rocks, such as rhyolite and dolomite, due to the high stress before and during an earthquake. The ions created by this process travel upwards through fissures in the rocks and eventually reach the atmosphere, where they can ionise pockets of air and create plasma that emits light.