The experience of seeing frequent colored rings, halos, or literal rainbows can be traced to two fundamentally different sources. These visual effects result either from atmospheric physics, where light interacts with moisture outside the body, or from internal physiological conditions affecting the eye’s pathway for light. Understanding the context of when and where you see these colors is important for determining whether the cause is a harmless natural event or a symptom of an underlying health issue.
Environmental and Atmospheric Phenomena
The presence of literal rainbows and light halos in the sky is a display of atmospheric optics, not a problem with your vision. A true rainbow, the arch of color most people picture, occurs when sunlight enters spherical raindrops, refracts, reflects off the back of the droplet, and refracts again as it exits, splitting the white light into its spectral colors. The sun must be behind the observer, with the rain falling in front, for this phenomenon to be visible.
Other colored rings, specifically halos around the sun or moon, are caused by light interacting with hexagonal ice crystals high in the atmosphere, typically found in cirrus clouds. These ice crystals act like tiny prisms, refracting the light at a minimum deviation angle, often resulting in the distinct 22° halo. Environmental factors closer to the ground, such as high humidity, mist, or fog, can also cause light to scatter around artificial sources like streetlights. This effect, known as a corona, is caused by light diffraction around tiny, uniform water droplets or particles in the air.
The Optics of Seeing Light Halos
When the sensation of rings or halos appears around artificial light sources like headlamps or streetlights, the cause often lies in how light is processed inside the eye. The visual perception of a halo is rooted in light diffraction and scattering, which occurs when light waves bend or spread out. For clear vision, the eye’s primary structures—the cornea and the lens—must be transparent and smooth, allowing light to focus cleanly onto the retina.
The appearance of a halo means that light is being scattered as it passes through an imperfect medium within the eye. Any irregularity or cloudiness in the cornea or the lens will cause incoming light rays to be dispersed instead of being directed to a single focal point. This dispersion separates the light into its component wavelengths, allowing the viewer to perceive the spectrum of colors, or the rainbow effect, around a bright source. This optical mechanism acts as the common pathway for several distinct eye conditions.
Underlying Eye Conditions Causing Rainbow Vision
The most concerning cause of seeing rainbow halos is acute angle-closure glaucoma, a medical emergency. This condition involves a sudden spike in the intraocular pressure (IOP). The high pressure quickly causes the cornea to swell with fluid, known as corneal edema. The fluid buildup distorts the cornea’s smooth surface, scattering light and creating colored rings or halos.
A common cause, particularly in older adults, is the gradual development of cataracts. A cataract forms when the eye’s natural lens becomes cloudy due to the clumping of proteins. This clouding disrupts the normal passage of light, causing it to scatter and diffract. This results in the perception of halos or glare, especially during night driving. Unlike glaucoma, the visual disturbance from cataracts progresses slowly over time.
Corneal edema can also occur for reasons unrelated to glaucoma, often presenting as a temporary or fluctuating visual symptom. Overwearing contact lenses, for instance, can lead to a lack of oxygen supply to the cornea, causing it to swell mildly. This temporary swelling alters the corneal shape and can induce halos, which often subside after the lenses are removed. Hereditary conditions, such as Fuchs’ dystrophy, cause a progressive loss of cells that pump fluid out of the cornea, leading to chronic edema that may cause blurred vision and halos, often worse in the morning.
When to Seek Urgent Medical Attention
It is important to differentiate between a benign halo effect and a symptom that signals a serious health threat. If you observe halos or colored rings that are stable, occur mostly at night around bright lights, and lack other symptoms, they may indicate a condition like cataracts or a minor issue with corrective lenses. These situations warrant a non-emergency appointment with an eye care professional for diagnosis and management.
However, if the onset of rainbow halos is sudden and is accompanied by additional symptoms, it requires immediate medical attention. Warning signs that suggest an acute angle-closure glaucoma attack include severe eye pain, a throbbing headache, blurred or rapidly decreasing vision, and feelings of nausea or vomiting. This combination of symptoms indicates a rapid rise in eye pressure that can cause irreversible vision loss. Seek emergency care immediately.