Color and Vision

COLOR AND VISION

The Electromagnetic and Visible Spectra

• electromagnetic waves are waves that are capable of traveling through a vacuum
• mechanical waves require a medium in order to transport energy, electromagnetic waves are capable of transporting energy through the vacuum of outer space
• electromagnetic waves are produced by a vibrating electric change and consist of both an electric and a magnetic component
• electromagnetic spectrum: continuous range of frequencies. The entire range of the spectrum is often broken into specific regions

Visible Light Spectrum

• our eyes are sensitive to only a very narrow band of electromagnetic waves, limit human eyes can see: visible light spectrum
• ROYGBIV: narrow band of visible light - each individual wavelength within the spectrum of visible light wavelengths is representative of a particular colour
• Isaac Newton discovered: light shining through a prism will be separated into its different wavelengths and will show various colors of visible light. Separation of colors: dispersion - each color is characteristic of a distinct wavelength and different wavelengths of light waves will bend varying amounts upon passage through a prism

Visible Light and the Eye’s Response

• our eyes are sensitive to a very narrow band of frequencies within the large range of frequencies of the electromagnetic spectrum: visible light spectrum
• specific wavelengths within the spectrum correspond to a specific color based upon how humans typically perceive light of that wavelength
• the long wavelength end of the spectrum corresponds to light that is perceived by humans to be red and the short wavelength end of the spectrum corresponds to light that is perceived to be violet. Other colors within the spectrum include orange, yellow, green and blue
• the picture depicts the approximate range of wavelengths that are associated with the various perceived colors within the spectrum

Color Cones

• color can be thought of as a psychological and physiological response to light waves of a specific frequency or set of frequencies impinging upon the eye
• light that enters the eye through the pupil ultimately strikes the inside surface of the eye known as the: retina - lined with a variety of light sensing cells known as rods and cones The rods on the retina are sensitive to the intensity of light, they can’t distinguish between lights of different wavelengths
• Three kinds of cones: red, green and blue. Since the red cone is sensitive to a range of wavelengths, it is not only activated by wavelengths of red light, but also by wavelengths of orange light, yellow light and even green light
• green cone is most sensitive to wavelengths of light associated with the color green but can be activated by wavelengths of light associated with the colors yellow and blue

• the cone sensitivity curve shown above helps us to better understand our response to the light that is incident upon the retina

• light is simply a wave with a specific wavelength or a mixture of wavelengths; it has no color in or of itself
• an object that is emitting or reflecting light to our eye appears to have a specific color as the result of the eye brain response to the wavelength. Technically there is no such thing as yellow light - light with a wavelength of about 590 nm together appears yellow

Light Absorption, Reflection and Transmission

• visible light waves consist of a continuous range of wavelengths or frequencies
• when a light wave with a single frequency strikes an object, several thing can happen
• light wave could be absorbed by the object: energy is converted to heat
• light wave could be reflected by the object 
• light wave can be transmitted by the object
• atoms and molecules contain electrons: the electrons and their attached strings have a tendency to vibrate at specific frequencies
• electrons of atoms have a natural frequency which they tend to vibrate
• if a light wave of a given frequency strikes a material with electrons having the same vibrational frequencies, then those electrons will absorb the energy of the light wave and transform it into vibrational motion. During vibration, the electrons interact with neighboring atoms in such a manner as to convert its vibrational energy into thermal energy. As a result the light wave with that given frequency is absorbed by the object, never again to be released in the form of light
• reflection and transmission of light wave happen because the frequencies of the light do not match the natural frequencies of vibration of the object]
• if the object is transparent - transmitted: if the object is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object
• if the object is opaque - reflected: vibrations of the electrons are not passed from atom to atom through the bulk of the material. The electrons of atoms on the material’s surface vibrate for short periods of time and then reemit the energy as a reflected wave
• the color of the objects that we see are largely due to the wave those objects interact with light and ultimately reflect or transmit it to our eyes
• the color of the object is the light that shines upon it and reflected/transmitted to our eyes

Color and Vision

• when you look at an object and perceive a distinct color, you are not necessarily seeing a single frequency of light

example: looking into a purple shirt - there are several frequencies of light striking your eye but the brain interprets the eye and decodes as being the color purple

• white is not a color at all, but rather the presence of all the frequencies of visible light
• primary colors of light: three colors of light that produce white light when combined with specific intensity. ex. red, green and blue
• secondary colors of light: they are produced by the addition of equal intensities of two primary colors of light .ex. yellow, magenta and cyan

• complementary colors of light: any two colors of light that when mixed together in equal intensities produce white are said to be complementary colors of each other. ex. complementary color of light is cyan light 

Color Subtraction

• materials that have been permeated by specific pigments will selectively absorb specific frequencies of light in order to produce a desired appearance
• process of color subtraction: if white light is shining on a shirt, then red, green and blue light is shining on the shirt. If the shirt absorbs blue light, then only red and green light will be reflected from the shirt. So while red, green and blue light shine upon the shirt, only red and green light will reflect from it. Red and green light striking your eyes always gives the appearance of yellow
• the ultimate color appearance of an object is determined by beginning with a single color or mixture of colors and identifying which color or colors of light are subtracted from the original set

Complementary colors and color subtraction

• a pigment that absorbs a single frequency is known as a pure pigment
• rule: pigments absorb light. Pure pigments absorb a single frequency or color of light. The color of light absorbed by a pigment is merely the complementary color of that pigment

Filters and color subtraction

• opaque materials selectively absorb one or more frequencies of light and reflect what is not absorbed
• transparent materials selectively absorb one or more frequencies of light and transmit what is not absorbed
• both materials are permeated by pigments that contain atoms that are capable of absorbing light with a single frequency or even a range of frequencies

Primary Colors of Paint

• thee primary colors of paint used by artists/ color printer/ film developer are cyan, magenta and yellow
• each primary color of paint absorbs one primary color of light
• the color absorbed by a primary color of paint is the complementary color of that paint