Study Notes on Color Science: Light

TMD 113: Color Science: LIGHT

Overview of Color Science

  • Key Questions Addressed:

    • What is light?

    • How is light produced (theory and practice)?

    • How can we describe the quality of light?

    • What are standard light sources for consistent color viewing?

What is Light?

  • Fundamental Characteristics:

    • Behavior of light.

    • Production of color by light.

    • Philosophical inquiry: Is color possible without light?

    • Hypothetical scenario: "If a tree falls in the forest and no one is there to hear it, does it still make a sound?"

    • Inquiry into a red rose's color in a dark room.

Historical Review of Light Understanding

  • Purpose: To showcase how scientific knowledge has developed regarding light, resembling a mini physics course through the ages.

Early Experiments to Measure Light Speed

Galileo's Experiment (c. 1600)

  • Objective: Measure the speed of light.

    • Method: Used two mountains, mountain #1 and mountain #2, with shutters to time light travel.

  • Conclusion: Experiment failed as light is too fast for this method.

    • Notably, Galileo invented the telescope during this period.

Roemer’s Conclusions (1676)

  • Observation: Studied Jupiter's moons.

    • Noted discrepancies in timings due to Earth’s varying distance to Jupiter:

    • Moons appeared early or late depending on the Earth’s position.

  • Conclusion: Discrepancies were due to the time it takes for light to travel different distances.

    • Calculated the speed of light, albeit about 20% off from modern measurements.

Fizeau’s Experiment (~1850)

  • Method: Reflected light through a rapidly rotating toothed wheel and measured time for light to return after hitting a distant mirror.

  • Results: Utilized the known distance and timing to calculate light speed.

  • Accepted Speed of Light: 299,792,458 meters per second (rounded to 300,000,000 m/s).

    • Approximately 186,000 miles per second.

    • Contextual reference: Light takes 0.00005 seconds for a 10-mile journey.

Measurement Standards for Light Speed

  • Definition of Measurements:

    • Meters (standardized wavelengths for precision).

    • Seconds (defined by atomic transitions, atomic clocks).

  • Current Definition: Meters defined as the distance light travels in 1/299,792,458 seconds.

Philosophical and Physical Theories of Light

Early Theories by Greek Philosophers

  • Light as a “stream of particles” (Democritus) vs. “waves” (Aristotle).

Newton's Corpuscular Theory (1672)

  • Introduced the idea of light as a stream of particles (corpuscles).

  • Recognized differences in behavior between light, sound, and water waves.

  • Notable contributions:

    • Split white light into spectrum and noted seven colors in the rainbow.

Huygens' Wave Theory

  • Proposed light travels through an unseen medium, an “all-pervading medium.”

Young's Two-Slit Experiment (1800)

  • Demonstrated light as a wave through interference patterns.

    • Conducted using light passing through two slits.

    • Observed constructive and destructive interference.

Maxwell and Electromagnetic Theory (1860s)

  • Combined electricity and magnetism, leading to the electromagnetic (e/m) wave theory, predicting light as an electromagnetic wave traveling at 3 x 10^8 m/s.

  • Notable implications in color vision and photography.

Quantum Theory and Light's Nature

Planck's Introduction of Energy Quantization

  • Proposed that electromagnetic radiation energy is frequency-dependent (quantized).

  • Basis for the development of Quantum Theory in modern physics.

The Photoelectric Effect

  • Demonstrated light behaving like particles (photons).

  • Evident that light frequency above a threshold is required for the ejection of electrons.

  • Einstein's contribution earned him a Nobel Prize, emphasizing the particle nature of light.

Particle-Wave Duality

  • Established that:

    • Low energy (e.g., radio waves) exhibits wave characteristics.

    • High-energy (e.g., electrons, gamma rays) exhibits particle characteristics.

    • Intermediate forms (UV, visible, IR) exhibit dual characteristics relevant for practical applications.

Measuring Light Characteristics

Wavelength, Frequency, and Speed

  • Light as a wave includes:

    1. Wavelength (λ): Distance between crests.

    • Measured in nanometers (nm), micrometers (μm), etc.

    1. Frequency (ν): Number of crests passing per second, measured in Hertz (Hz).

    2. Speed (c): Defined as constant, the relationship between speed, wavelength, and frequency:

    • c = ν imes λ

  • Given c = 3 imes 10^8 m/s, relationships mean as frequency increases, wavelength decreases, and vice versa.

Energy of Light

  • Energy (E) per photon is described by:

    • E = ħ
      u

    • where ħ (Planck's constant) = 6.63 imes 10^{-34} J imes s; relationship shows as frequency increases, energy also increases, while increasing wavelength leads to a decrease in energy.

The Electromagnetic Spectrum

  • Definition: A range of all wavelengths and frequencies, only a small portion is visible ( visible light)

  • Light spectrum includes:

    • Visible Light (400-700 nm): Colors transition from violet (400 nm) to red (700 nm).

    • Other categories:

    • Ultra violet, infrared, radio waves, etc., with varying wavelengths and energy levels.

Polarization of Light

  • Concept: Light waves can vibrate in various orientations, becoming polarized through reflection, passing through filters, or scattering.

  • Effects of Polarization: Mode of selective transmission impacts visual clarity, such as in polarized sunglasses which reduce glare.

Sources and Quality of Light

Types of Light Sources

  1. Incandescence: Emission from heated objects.

  2. Fluorescence: Light produced by specific atom excitation.

  3. Electroluminescence: Light from generating an electric current with specific materials such as LEDs.

Incandescent Light Details

  • As a body gets hotter:

    • Energy radiated increases (Stefan’s Law).

    • Peak wavelength decreases (Wien’s Law).

  • Continuous spectrum is generated, revealing color temperature can be categorized.

Fluorescent Light

  • Relies on energy sources exciting specific atoms, light emitted upon de-excitation.

  • Different phosphors in lamps produce varying light qualities (e.g. sodium lamps emit yellow light).

LEDs (Light-Emitting Diodes)

  • Energy-efficient sources producing light through electron-hole recombination in semiconductors; allowing for flexibility and broad spectrum emission enabling different colors and efficient light production.

Characterizing Light Quality

Methods to Measure Quality

  1. Color Temperature: Standardized temperature relating to coloration emitted by a light source.

    • Typical standards include D65 (normal daylight) and D75 (cool daylight).

  2. Spectral Power Distribution (SPD): Detailed representation of emitted wavelengths and their relative intensities.

  3. Color Rendering Index (CRI): Measurement of color accuracy compared to a reference light source such as daylight or tungsten bulbs.

Calibration of Standard Lights

  • CIE (Commission Internationale d’Eclairage) defined standard illuminants for consistency in color measurement, producing reliable baselines for survey and application of color.

  • Instruments for Light Measurement: Radiometry for electromagnetic energy and photometry for human-visible light measurement, essential in color science.

Summary of Light’s Nature and Sources

  • Light Characteristics:

    • Light behaves both as a particle and as a wave.

    • Speed of light is approximately 3 imes 10^8 m/s.

    • Various sources of light (incandescent, fluorescent, LEDs) have distinct characteristics.

  • Measurement Standards and Applications Provided by CIE:

    • Standard illuminants for accurate reproduction and study of colors.

Conclusion

  • Understanding light is essential for accurate color perception and applications in art, science, and technology. Continuously evolving methods for producing and measuring light are integral to advancements in color science.