Blue is one of the three primary colours in the RYB colour model, as well as in the RGB colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when observing light with a dominant wavelength between approximately 450 and 495 nanometres. Most blues contain a slight mixture of other colours; azure contains some green, while ultramarine contains some violet. The clear daytime sky and the deep sea appear blue because of an optical effect known as Rayleigh scattering. An optical effect called Tyndall effect explains blue eyes. Distant objects appear more blue because of another optical effect called aerial perspective. Blue has been an important colour in art and decoration since ancient times. The semi-precious stone lapis lazuli was used in ancient Egypt for jewellery and ornament and later, in the Renaissance, to make the pigment ultramarine, the most expensive of all pigments.

Read more in the app

Faraway blue star cluster shines in Hubble Space Telescope photo

Outlook for the blue economy

Blue Origin plans to fly all-female crew to suborbital space: report

‘Bolt from the blue’ is a dangerous form of lighting that travels away from clouds for miles, before striking down — Photographs by storm chaser Mike O’Neill capture this phenomenon, for a visual story about a lightning hotspot

SpaceX rocket creates eerie blue spiral in night sky over Hawaii: reports

‘Blue Supermoon’ To An American ‘Ring Of Fire’ Eclipse: 7 Things You Need To Know About The Moon In 2023

An ecofriendly way to convert blue light into UVB

Blue Straggler Stars are Weird

Some details have emerged about Blue Origin’s “Blue Ring” project

The sky isn't just blue — airglow makes it green, yellow and red too

Earth's Sky Isn't Just Blue, And a Close Look at The Darkness of Night Reveals Why

A ‘Blue Moon,’ Two Lunar Eclipses And Four ‘Supermoons:’ Exactly When To See All 13 Full Moons In 2023

Radiant Blue Swirls: Hubble Captures Majestic Barred Spiral Galaxy

Recreated 'Blue Marble' photo of Earth tests powerful new climate model

50 Years Ago, Apollo Crew Gave Humanity a Christmas Gift: Our First Gaze at ‘the Blue Marble’ Still Inspiring us Today

Blue Origin offers new bobbleheads of 1st New Shepard female fliers

SpaceX, Blue Origin Executives Tapped as US Space Council Advisers

A Supercomputer Climate Model is so Accurate it Predicted the Weather Patterns Seen in the Famous 1972 “Blue Marble” Image of Earth

Blue Origin and Dynetics bidding on second Artemis lunar lander

NASA's new moon lander contest heats up with Blue Origin, Northrop Grumman