Colourless rainbows: The stunning science behind them 

No matter who you are, or how old you are, you must admit that rainbows are a thing of beauty and joy forever. After a storm passes by, we see them as symbols of better times, isn’t it? For us, these amazing natural spectacles are a representation of fortune, inspiration, hope, and fulfilled wishes. There are two different kinds of rainbows that you can witness: Colourful rainbows and colourless ones, often known as fogbows. While colourful rainbow is more common, a fogbow is a rare event. While we will find out more about these colourless rainbows, let’s first find out how these colourful natural wonders are formed.  

Formation of rainbows 
We all know that a rainbow is an event that occurs because of light being reflected, refracted, and dispersed due to water droplets. When sunlight shines on a water droplet, some of it enters the raindrop and is refracted. The light then strikes the water droplet's back at a 42° angle, splitting it into its individual wavelengths which we see as colours. When light exits the droplet, it creates a rainbow, a beautiful colourful arc. So, you may have spotted rainbows near a fog, sea spray, or around waterfalls. 

A wheel of colours  
Ever noticed why a rainbow’s colours always go in the same order as red, orange, yellow, green, blue, indigo, and violet? Well, let’s decode the stunning science behind the mystery of colours. Sunlight is made up of many different wavelengths of light, which is what creates colours. Now, some of the wavelengths get refracted more than others when they enter a water droplet. Red light (longest wavelength) has the maximum speed and is the least refracted, while violet light (shortest wavelength) has the minimum speed and is most refracted. When sunlight hits water droplets, it is refracted in a cone-like manner, with the lowest wavelength at the centre and the highest wavelength at the edges. As a result, the colour of the rainbow always starts with the least refracted colour (red) and gradually changes to the most refracted colour (violet). This order never changes because we always see the corresponding colours according to their wavelength, from longest to shortest. 

Rainbows seem like a multicolour arc, but they are actually part of a full circle. The centre of the rainbow circle can occasionally be seen by the viewers in an aircraft. However, from the ground, only the light reflected by the raindrops over the horizon is visible to viewers, thus we see only half the circle, an arc. A rainbow is an optical illusion. The appearance of a rainbow depends on your position and the direction of the sun. You see a rainbow only when the sun is behind you and there is light rain or water droplets in the sky in front of us.  

The recipe of white rainbows 
Colourless rainbows are sometimes called fogbows, white rainbows, cloud bows, or ghost rainbows. You will be surprised to know that a fogbow can also appear in a moon-lit foggy light. As a result of the low intensity of moonlight, a fogbow appears more white and less vivid, creating a ‘moonbow’.  

Fogbows are rare and mostly appear in foggy conditions. When sunlight and moisture combine, the light-refracting properties of the tiny water droplets suspended in the air create fogbows. Similar to colourful rainbows, fogbows always appear in the opposite direction of the sun. They arise when sunlight shines on fog, which is made up of much smaller water droplets than rain. The smaller size of the droplets prevents the light from spending enough time inside the droplet, or to refract sufficiently to separate cleanly into the various colours that form light. They cannot refract much light but do allow light to diffract off the tiny droplets and combine to form a ring of white haze as the colours mix. Hence, fogbows have only weak colours or are almost colourless.  

Spotting a fogbow 
You might spot fogbows in the morning or evening because the sun needs to be at a low angle (around 30-40 degrees high) to the atmospheric fog. When the sun shines brightly and the fog is thin, there are maximum chances of spotting a fogbow; it appears as the sun breaks through the fog behind you. Fogbows can be seen over the oceans or down a mountain slope. If you are really lucky, you might see a full circle rather than an arch if you happen to be in the centre of a fog or cloud, such as on an aeroplane! So, the next time you are enjoying a foggy weather, look out for a fogbow!