5.5- The color of light is determined by its wavelength

Question 1

Trichromatic theory

Answer 1

• According to the trichromatic theory, color vision results from activity in three types of cones that are sensitive to different wavelengths.
• One type of cone is most sensitive to short wavelengths (blue-violet light), another type is most sensitive to medium wavelengths (yellow-green light), and the third type is most sensitive to long wavelengths
• The three types of cones in the retina are therefore called ‘S’, ‘M’, and ‘L’ cones because they respond maximally to short, medium, and long wavelengths
• For example, yellow light looks yellow because it stimulates the L and M cones about equally and hardly stimulates the S cones

Question 2

Color blindness

Answer 2

• There are two main types of color blindness, determined by the relative activity among the three types of cone receptors
• People may be missing the photopigment sensitive to either medium or long wavelengths, resulting in red-green color blindness
• Alternatively, they may be missing the short-wavelength photopigment, resulting in blue-yellow color blindness

Question 3

Opponent-process theory

Answer 3

• According to this theory, red and green are opponent colors, as are blue and yellow
• When we stare at a red image for some time, we see a green afterimage when we look away, and vice versa
• The receptors for red become fatigued when you stare at red. The green receptors are not fatigued and therefore, the afterimage appears green
• While the trichromatic theory characterizes the properties of different types of cones, the opponent-process theory describes the second stage in visual processing
• This stage occurs in the ganglion cells - the cells that make up the optic nerve, which carries information to the brain
• Different combinations of cones converge on the ganglion cells of the retina. One type of ganglion cell receives excitatory input from L cones (that see red), but is inhibited by M cones (that see green).
• Cells of this type create the perception that red and green are opponents
• These different types of ganglion cells, working in opposing pairing, create the perception that red and green are opponents

Question 4

Hue, saturation, and lightness

Answer 4

• Color is categorized into three dimensions: hue, saturation, and lightness
• Hue consists of the distinctive characteristics that place a particular color in the spectrum - the color’s greenness or orangeness, for example
• These characteristics depend primarily on the light’s dominant wavelength when it reaches the eye
• Saturation is the purity of the color. Saturation varies according to the mixture of wavelengths in a stimulus. Basic colors of the spectrum (e.g. blue, green, red) have only one wavelength, whereas pastels (pink, baby blue) have a mixture of wavelengths, so they are less pure
• Lightness is the color’s perceived intensity. This characteristic is determined primarily by the total amount of light reaching the eye. How light something seems also depends on the background