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Light

All energy on our planet is emitted from the Sun in the form of radiation waves. At a speed of 300,000,000 meters/second, nothing travels faster than electromagnetic waves as they can beam from New York to Los Angeles in 0.016 seconds. It only takes a quick 8 minutes to make the 150 million kilometer journey from the Sun to the Earth.

All electromagnetic waves travel at the same speed, but they can have different wavelengths and frequencies.

© 2010 FLVS 

The  wavelength  of the wave is the distance between two crests, or peaks, of the wave.

The  frequency  is the number of waves in a certain period of time.

The frequency of the electromagnetic wave determines the amount of energy it contains, the shorter frequency waves packing more punch.

The  electromagnetic spectrum  is the range of electromagnetic waves when they are place in order of increasing frequencies.

White light is the combination of all the colors in the visible light spectrum. When separated by a prism, the different frequencies of energy create different colors. (Remember Roy G. Biv, the Rainbow Man of Many Colors?)

© 2010 FLVS, Prism image: Jupiterimages Corporation 

A color’s  hue  is determined by the light’s wavelength while its  brightness  is determined by the amplitude, or crest height, of the wave.

Eye Structures

Image © 2010 Jupiterimages Corporation

The structures of human eyes are specially tuned to  transduce  a specific range of visible electromagnetic light. Our eyes absorb reflections of light from objects in the environment and turn that energy into messages for the brain and nervous system to interpret.

Look carefully at these simulations of human eye structures and the process of vision. The visualizations will help you comprehend the vision process. Record notes as you view the simulations and the web sites, since these notes will be beneficial in reviewing concepts for discussion-based assessments, quizzes, and exams.

The Science of Seeing simulations below describe how light passes from the outside world into messages our brain can interpret. Be sure to note the pathway of light as it passes through the visual structures.

Visual Processing

What happens to the sensory information once it reaches the visual cortex of the brain?

Much like a computer, the brain integrates information from many visual characteristics into our singular perception by means of  parallel processing . Colors, movements, line form, and depth are compared with previous memories to synthesize our visual experiences.

Feature detector  neurons occur throughout the visual cortex. These cells fire when the stimulus contains specific shapes, angles, or movements. For example, one feature detector neuron may only fire when exposed to vertical lines in the visual field. If the line is moved 30 degrees clockwise, that neuron no longer fires, while a different neuron responsible for lines in that orientation fires.

Perception of Color

Young-Helmholtz Tri-Chromatic Theory

Image: Public Domain, via Wikimedia Commons

Thomas Young and Hermann von Helmholtz observed the additive properties of color mixing. Rather than turning into a darker color like mixed paint, red, blue and green light combine to make white light. Any shade of color can be achieved by mixing these three colors in different proportions. The  Young-Helmholtz Tri-Chromatic theory  of color vision proposes that humans have three types of receptors, one representing each of these primary colors.

This theory explains many aspects of color vision including cases of  color blindness . Most people with color blindness are  dichromats , meaning they only have two types of color receptors. There are three types of dichromats including people who cannot sense red, green, or blue, supporting the Tri-Chromatic Theory.

Opponent Process Theory

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Tri-Chromatic Theory cannot explain all color vision phenomenon.

German physiologist Ewald Hering noted that individuals who are blind to red and green can still see yellow. This would not be possible if the Tri-Chromatic Theory were valid.

Likewise, when we stare at a vivid color and then turn our gaze to a white background, an image of the opposing color persists. Stare at the image of the flag above for about 60 seconds then look at a plain white wall to experience the effect in action.

Hering proposed the  opponent process theory  that color perception is due to opposite responses of three pairs of colors; red vs. green, blue vs. yellow, and black vs. white.

Modern vision research seems to validate both theories. George Wald won the Nobel Prize in 1967 when he discovered that the eye has three types of cones, which supports the Tri-Chromatic Theory.

Scientists have also discovered cells in the visual cortex that are stimulated by green and inhibited by red.

It seems our perception of color has different stages of processing.