double rainbow

Twitter / @moving_sushi

Put it on a postcard – Cape Town double rainbow dazzles locals

Capetonians wasted no time in capturing the Table Mountain in all its beauty when a double rainbow shone above the city.

double rainbow

Twitter / @moving_sushi

Social media lit up after a double rainbow was noticeably arching over Cape Town. Viewers delightedly took pictures and videos of the meteorological phenomenon whose explanation is as colourful as its display.

Sir Isaac Newton discovered that sunlight falling upon a prism split into various component colours, now known as dispersion.  He classified these components as red, orange, yellow, green, blue, indigo and violet.

How a double rainbow is formed

Combined, the colours appear as white to the eye. After the rain, millions of tiny water droplets remain in the air. The rain droplets serve as a type of light reflector.

When light passes through a transparent material, such as air, into another, such as water, bending occurs. The bending of light is known as refraction.

Blue and violet have shorter wavelengths so they bend the most whereas red and orange have longer wavelengths and are bent the least.

White light enters the droplet at a specific angle, the drop separates the white light into different colours. What colour is refracted is dependant on the angle the sunlight strikes the back of the rain droplet.

Red light exits the droplet at a 42° angle, whereas violet light exits at a 40° angle. The other colours in the spectrum exit the rain droplets at angles between 40° and 42°. This allows for the colourful display of ROYGBIV (red, orange, yellow, green, blue, indigo, violet).

Although only one colour hits the eye (requiring millions of droplets to create a rainbow), at any given point, each raindrop reflects all colours.

While only half is visible due to the horizon, rainbows actually form a circle.

The formation of the double rainbow in Cape Town

In some instances, as was the case in Cape Town, we can observe a secondary rainbow, dimmer and higher than the primary rainbow.

This phenomenon is due to light being reflected twice, instead of just once. As the secondary rainbow is formed by one more reflection than the primary rainbow, it is much fainter and rarer to see.

The colours in the secondary rainbow are in reverse order of the primary rainbow.

Rainbows can form wherever conditions are favourable: over the ocean and waterfalls as well as sprinklers and in mist from a garden hose.

If the rainbow is caused by sunlight, it will always appear in the section of sky directly opposite the sun so make sure you have your back towards the sun when wanting to spot a rainbow after the rain.

According to legend, Newton included indigo because he felt that there should be seven rather than six colours in a rainbow due to his strong religious beliefs.