This is one of a series of diagrams exploring colour perception through visual illusions.
Visual illusions reveal anomalies in the way humans interpret the world.
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About the diagram
Looking steadily at strong colours confuses the light-sensitive cells in our eyes. The cells don’t recover immediately when we look away.
Stare at the white square in the diagram without blinking for at least 15 seconds. Now close your eyes. Can you name the colours in the after-image?
Some key terms
Optical illusions and other visual anomalies are caused by the way the human visual system processes information.
Physical illusions result from the limitations and assumptions of the human visual system when interpreting the external world. Examples include:
- The Sun and Moon appear larger near the horizon as a result of the brain’s interpretation of distance cues.
- Rainbows are composed of a continuous range of wavelengths across the visible spectrum but appear to be formed from a series of bands of colour.
Physiological illusions are often connected with the different attributes of visual perception and occur when visual stimuli are beyond our brain’s processing ability.
Physiological illusions arise due to the way that the human eye and visual system process information from the outside world, such as lighting, contrast, and colour. Examples include:
- After-images occur when the eye’s photoreceptor cells become fatigued due to overstimulation, resulting in an image appearing after the stimulus is removed.
- Moiré patterns occur when two similar patterns with slightly different frequencies overlap, creating a new pattern that appears to move or vibrate.
Cognitive illusions result from the brain’s inability to correctly interpret visual information, leading to uncertainties or errors in perception. Examples include:
- Ambiguous illusions are images that can be read in more than one way, depending on contextual cues and the viewer’s past experiences. They often cause a perceptual “switch” between alternative interpretations.
- Geometrical illusions occur when the brain uses contextual cues and assumptions to interpret visual stimuli, leading to distortions in size, length, position, or curvature.
- Paradox illusions occur when visual stimuli contain conflicting information that cannot be resolved by the brain, leading to a perceptual paradox.
- Fictions are created when the brain fills in missing visual information based on contextual cues and past experiences, leading to the perception of additional content that is not actually present.
- The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
- As light travels through the air it is invisible to our eyes.
- Human beings don’t see wavelengths of light, but they do see the spectral colours that correspond with each wavelength and colours produced when different wavelengths are combined.
- The visible spectrum includes all the spectral colours between red and violet and each is produced by a single wavelength.
- The visible spectrum is often divided into named colours, though any division of this kind is somewhat arbitrary.
- Traditional colours referred to in English include red, orange, yellow, green, blue, and violet.
- When sunlight is dispersed by rain and forms a rainbow, an observer often distinguishes red, orange, yellow, green, blue and violet bands of colour.
- Although an atmospheric rainbow contains electromagnetic waves with all possible wavelengths between red and violet, our eyes encounter difficulties in distinguishing between colours within specific regions of this spectrum. For example, all wavelengths between 520 to 570 nanometers may appear to be exactly the same green to most observers.
- Visible light is a form of electromagnetic radiation.
- Other forms of electromagnetic radiation include radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays.
- Visible light is perceived by a human observer as all the spectral colours between red and violet plus all other colours that result from combining wavelengths together in different proportions.
- A spectral colour is produced by a single wavelength of light.
- The complete range of colours that can be perceived by a human observer is called the visible spectrum.
- The range of wavelengths that produce visible light is a very small part of the electromagnetic spectrum.
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