We see only part of the spectrum
Visible light is the electromagnetic range our eyes can detect. It contains continuous colours from violet to red, and the eyes' photoreceptors respond to this range. UV, infrared, and other bands exist outside it; being invisible does not mean being absent.
NASA's introduction to visible light shows how a prism separates light: different wavelengths bend by different amounts, spreading out colours that were mixed together.
Why is the sky often blue?
As sunlight enters the atmosphere, air molecules scatter it in different directions. Shorter visible wavelengths, such as blue and violet, scatter more readily. Together with sunlight's spectrum and our eyes' colour response, this usually makes the sky appear blue.
At sunrise and sunset, light travels through more atmosphere before reaching us. Some shorter wavelengths scatter away, so light reaching the eyes from the relevant direction may look red or orange. This is described in the US National Weather Service explanation of blue skies and sunsets; clouds, suspended particles, and viewing direction also affect what we see.
How do rainbows reveal colours?
Sunlight refracts as it enters a water droplet, reflects inside, and refracts again as it leaves. Different wavelengths reach the observer from different directions, forming a coloured arc.
A rainbow therefore requires sunlight, water droplets, and a suitable viewing position together. It is not a solid object suspended in one place that everyone sees from the same angle. The US National Weather Service explanation of rainbows also describes the additional internal reflection involved in a double rainbow.
Visible light also relates to daily routines
Light entering the eyes supports vision and provides environmental information to the body clock. This differs from UV reaching the skin. Read How does daylight affect the body clock?.
Enjoy the sky and surroundings without looking directly at the sun. Sky colour or a photograph's brightness cannot replace UV measurements.
Reading and walking during the day
Use suitable natural light for daytime reading and activities. Shade and clothing can coexist with observing your surroundings. Light reaching the eyes and skin UVB concern different questions: good lighting does not establish vitamin D conditions.
With children, start with an object’s colour under different lighting, morning and evening skies, or the viewing position of a rainbow. Describe observations before explaining them; observe surroundings safely without looking at the solar disc.
Photos record observations, but exposure, white balance and screen brightness change their appearance. Note time and weather for comparison. Use UVI for protection and the light-environment guide for routines.