Lecture 27: Color

Introduction to Color and Visible Light

  • Visible light waves are defined by their different frequencies, which appear to the human eye as different colors of light.
  • Selective interaction with light waves based on their frequency causes objects to take on different colors. These interactions include:
    • Reflection
    • Transmission
    • Absorption
    • Scattering
  • The visible light section of the electromagnetic spectrum encompasses light waves ranging from red to violet.
  • The acronym ROYGBIV is used to represent the major colors of the visible light spectrum:
    • Red
    • Orange
    • Yellow
    • Green
    • Blue
    • Indigo
    • Violet
  • Visible light is accurately described as a continuous spectrum rather than discrete quantities.
  • Wavelength is the distinguishing physical characteristic of each color:
    • Red light: Long wavelengths.
    • Green light: Medium wavelengths (located in the middle of the spectrum).
    • Violet light: Shortest wavelengths of visible light.

Human Perception and Biology

  • Human eyes contain specialized photoreceptor cells called cone cells that allow for the perception of light and color.
  • There are three distinct types of cone cells, each sensitive to a different range of wavelengths:
    • Short wavelength cones
    • Medium wavelength cones
    • Long wavelength cones
  • Perceived color is the result of light waves of varying wavelengths and intensities interacting with these three types of cones.
  • Color-blindness can be caused by physical or inherited issues with the functionality or presence of these cone cells.

Interaction of Light with Matter: Reflection, Absorption, and Transmission

  • Observation of objects is not the observation of the physical object itself, but the observation of the light that interacts with it and reaches the eyes.
  • Reflection:
    • Occurs when light waves bounce off the surface of an object.
    • Objects reflect the colors that we see and absorb the colors that we do not see.
    • Example: A blue object reflects blue light into our eyes and absorbs red and green light waves.
    • Example: A red object reflects red light and absorbs blue and green light.
  • Transmission:
    • Occurs when light waves pass through an object.
    • Example: A blue transparent object transmits blue light while absorbing green and red light.
    • Example: A yellow transparent object transmits red and green light while absorbing blue light.
  • Light Sources:
    • Some objects are seen via the direct transmission of light from the source itself.
    • Includes lightbulbs, LEDs, traffic lights, and street lights.
    • A red lightbulb transmits red light waves; a green lightbulb transmits green light waves.

Color Addition (Additive Color Model)

  • Color addition is the process of using light to create secondary colors by mixing primary colors.
  • The three primary colors of light are Red, Green, and Blue (RGB).
  • Mixing various intensities of these three primary colors allows for the generation of all other possible colors of light.
  • Addition is associated with light seen via transmission.
  • Secondary Colors of Light:
    • Red + Blue = Magenta
    • Red + Green = Yellow
    • Blue + Green = Cyan
  • Combining Primaries:
    • Mixing all three primary colors (Red, Green, and Blue) at equal intensities results in white light.
    • The color black is the perception of the absence of all light.
  • Technology Applications:
    • Electronic device screens (monitors, phones) use the additive color model.
    • LED Displays: Consist of tiny pixels made of red, green, and blue LEDs.
    • LCD Displays: Consist of red, green, and blue filters.

Color Subtraction (Subtractive Color Model)

  • While color addition pertains to transmission, color subtraction pertains to the light reflected off a surface.
  • Subtractive color models are primarily used in the print industry, such as magazine, books, and posters.
  • Color subtraction refers to the colors that are absorbed by pigments, inks, or materials.
  • Secondary colors subtract (absorb) specific primary colors:
    • Cyan absorbs (subtracts) Red.
    • Magenta absorbs (subtracts) Green.
    • Yellow absorbs (subtracts) Blue.
  • Mixing in the Subtractive Model:
    • Cyan + Yellow = Green (Red and Blue are subtracted).
    • Cyan + Magenta = Blue (Red and Green are subtracted).
    • Magenta + Yellow = Red (Blue and Green are subtracted).
  • Black in the Subtractive Model:
    • Mixing Cyan, Magenta, and Yellow (CMY) results in black.
    • Printers identify this model as CMYK, which stands for Cyan, Magenta, Yellow, and Black (K represents the separate black ink used to avoid excessive use of CMY inks).

Scattering

  • Scattering is a physical process where the direction of travel for light is changed without affecting the wavelength of the light.
  • Earth's Atmosphere and the Blue Sky:
    • The atmosphere is composed of various gas particles.
    • Short wavelength light (blue and purple) is scattered more efficiently by these small atmospheric particles than longer wavelengths.
    • Though both blue and purple are scattered, human eyes are more sensitive to blue, causing us to perceive the sky as blue.
  • Cloud Appearance:
    • Cloud particles are larger than atmospheric gas particles.
    • These larger particles scatter all wavelengths (long and short) with equal efficiency.
    • This uniform scattering makes clouds appear white. On overcast days, the sky appears a muted white, known as gray.
  • Sunrise and Sunset:
    • At the horizon, the sun sits at a shallow angle, forcing light to travel through a much greater volume of the atmosphere to reach the observer.
    • In this process, so much blue light is scattered away throughout the long path that only the longer wavelengths (reds and oranges) remain, resulting in a red sky.
  • Demonstration of Scattering:
    • Setup: A fish tank filled with water and a small amount of milk (to simulate atmospheric molecules) with a flashlight representing the sun.
    • Result: When white light is directed into the mixture, short wavelengths (blue) scatter immediately near the source, while long wavelengths (orange/red) continue to pass through the tank.

Extraterrestrial Sky Characteristics

  • Colors of skies on other celestial bodies depend entirely on the presence and composition of an atmosphere.
  • The Moon:
    • Has effectively no atmosphere to scatter light.
    • Consequently, there is no "sky"; the sun appears as a distinct orb in space, and the surrounding space remains black.
    • This is documented in photos from moon landings.
  • Mars:
    • The specific atmospheric composition of Mars results in a brownish-pink sky color.
    • Color photos from Mars landers verify this unique appearance.