About Atmospheric Optics
This simulation models four distinct optical phenomena that occur in Earth's atmosphere: Rayleigh scattering (which explains why the sky is blue and sunsets are red), rainbow formation through refraction and internal reflection inside spherical water droplets, 22-degree ice-crystal halos produced by hexagonal ice prisms in cirrus clouds, and a ray-tracing view of the path light takes through a single rain droplet. Each scene is grounded in classical wave optics and geometric ray theory, letting you explore how wavelength-dependent refraction and scattering shape the colours we see every day.
Atmospheric optics has guided navigation, agriculture, and weather forecasting for millennia, and today underpins remote sensing, climate modelling, and the design of optical instruments from cameras to satellite sensors.
Frequently Asked Questions
Why is the sky blue?
Sunlight contains all visible wavelengths, but when it passes through air, nitrogen and oxygen molecules scatter shorter (blue) wavelengths far more strongly than longer (red) ones. Rayleigh scattering intensity is proportional to 1/wavelength^4, so blue light at 450 nm is scattered roughly five times more than red light at 700 nm. When you look anywhere except directly at the sun, the blue scattered light dominates what reaches your eyes.
How do I use the simulation?
Select a scene from the dropdown: "Sky" shows Rayleigh scattering changing with the Sun elevation slider; "Rainbow" shows primary and secondary bows with the antisolar geometry; "Ice-Crystal Halo" displays the 22-degree and faint 46-degree rings; and "Inside a Rain Droplet" traces how individual wavelengths travel through a spherical droplet. Use the Sun elevation slider to move the sun from the horizon to overhead, and in the droplet scene use the wavelength slider to see how the refractive index varies by colour.
At what angle does a primary rainbow appear?
The primary rainbow arc appears at approximately 42 degrees from the antisolar point (the point directly opposite the sun relative to your eye). This angle arises from the geometry of one internal reflection inside a spherical water droplet: the minimum deviation angle for red light is about 42.5 degrees, while violet is about 40.6 degrees, spreading the colours across roughly 2 degrees of arc. The secondary rainbow sits near 51 degrees with colours reversed.
What is the physics behind the 22-degree ice halo?
Hexagonal ice crystals in cirrus clouds act as 60-degree prisms. When light enters one face and exits a non-adjacent face at the minimum deviation angle, the deflection is approximately 22 degrees regardless of wavelength (to first order). Because crystals are randomly oriented, the scattered light forms a complete ring around the sun. The minimum-deviation condition concentrates many ray paths near 22 degrees, creating a bright ring with a red inner edge (shorter path through the prism) and a white-to-blue outer edge. A fainter 46-degree halo forms via a 90-degree prism path through the top and side faces.
Where do real rainbows appear in everyday life?
Rainbows are visible whenever the sun is behind you and rain, mist, or spray is in front of you. Garden hose spray, waterfalls, and fog in morning sunlight all produce rainbow arcs. Because the antisolar point must be at the centre of the arc, the sun must be below 42 degrees elevation for the primary bow to appear above the horizon; near sunrise and sunset, when the sun is very low, you can see nearly a full semicircle. From an aircraft or high vantage point, a full circular rainbow is possible.
Is it true that the sky should be violet, not blue, since violet scatters even more than blue?
This is a common misconception. Violet light (around 380-420 nm) does scatter more than blue (around 450 nm) according to the 1/lambda^4 law. However, several factors shift the perceived colour to blue: the sun emits less violet than blue in the visible spectrum, our eyes have far lower sensitivity to violet (the short-wavelength S-cones are not very numerous), and much violet is absorbed in the upper atmosphere by ozone. The combined effect makes the scattered sky appear blue rather than violet to human observers.
Who first explained Rayleigh scattering and when?
Lord Rayleigh (John William Strutt) published his explanation of sky colour in 1871, showing that the intensity of scattered light from small particles (much smaller than the wavelength of light) scales as the fourth power of frequency (equivalently, the inverse fourth power of wavelength). He built on earlier work by Tyndall, who in the 1860s demonstrated that very fine particles scatter blue light preferentially. The detailed quantum-mechanical treatment came later, but Rayleigh's classical formula remains accurate for atmospheric scattering by air molecules.
What other phenomena are related to atmospheric optics?
Related phenomena include the green flash (refraction and dispersion at the horizon at sunset), glory (backscattering from cloud droplets, visible from aircraft around the aircraft's shadow), the Belt of Venus (pinkish band opposite the setting sun caused by backscattered reddened light), circumzenithal and circumhorizontal arcs (other ice-crystal optical effects), and Mie scattering (the white appearance of clouds and fog, where droplets are much larger than the wavelength and scatter all colours nearly equally). The Rayleigh Scattering simulator on this site lets you explore the blue-sky mechanism in more depth.
How is atmospheric optics used in engineering and technology?
Remote sensing satellites use atmospheric scattering models to correct raw images for the haze introduced by the atmosphere, a process called atmospheric correction. LIDAR instruments (Light Detection and Ranging) exploit Rayleigh and Mie backscattering to measure aerosol concentration, wind speed, and cloud height from the ground or from orbit. Sunset-reddening calibrates dust and aerosol optical depth in field measurements. Anti-reflective coatings on lenses are designed by understanding how light refracts at boundaries, the same physics as droplet ray tracing. Meteorological optics also guides the design of solar concentrators and greenhouse glazing.
What are current research frontiers in atmospheric optics?
Researchers are using atmospheric optics to improve climate models by better quantifying aerosol radiative forcing — the degree to which particles scatter or absorb sunlight and thereby cool or warm the planet. Polarimetric remote sensing (measuring the polarisation state of scattered light) can distinguish mineral dust from sea salt from smoke, improving air-quality monitoring. Adaptive optics systems for large telescopes correct in real time for atmospheric turbulence by measuring wavefront distortion and applying corrections with deformable mirrors. There is also active research into how changing cloud microphysics (due to warming-driven shifts in ice crystal habits) will alter halo frequencies and global albedo.