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The Properties of Light

Light is electromagnetic radiation which can be thought of as either particles or waves, because they are both. Classically, light was thought of in terms of waves. Quantum physics redefined light as particles called photons. Both theories work in most situations, but it is easier to think of light as waves in common applications, while more complex theoretical applications require the perspective of light as photon particles.

  • What is Light: Play-Hookey offers an online mini course on light
  • What is Light: ThinkQuest offers an exhaustive online resource about light
  • Mass and Photons: Cornell Center for Materials Research - “Do Photons have Mass?”
  • What are Photons: The International Society for Optical Engineering offers an explanation of Photons
  • What Wavelength Goes With a Color? A detailed definition of the color/light spectrum
  • Electromagnetic Spectrum: Charts to explain the spectrum
  • Electromagnetic Energy: Energy from the sun
  • Quantum Theory and Wave/Particle Duality: How light can be both wave and particle?
  • Wave-Particle Duality: Light as both wave and particle
  • Triple-Slit Photon Test: Test results support quantum theory
  • Atomic Emission Spectra: The origin of spectral lines
  • Einstein on the Photoelectric Effect: A summary of Einstein’s opinion
  • Primer on Quantum Theory: Short articles on all elements of quantum theory many on light
  • Production of Light: Discussed continuous spectrum and blackbody spectrum
  • What is a Blackbody: Blackbody emitters and blackbody radiation

According to the classical interpretation light travels in waves. The three important properties of light waves are amplitude, which determines brightness, wavelength, which determines the color of the light, and polarization. Polarization involves the angle in which light vibrates either up and down or side to side.

Quantum theory states that brightness is determined by the number of photons, and the color of light is the energy each photon contains. X, Y, Z and T are used to describe polarization. Quantum theory necessitates the idea that light consists of tiny weightless particles called photons. The two theories have become married into a system in which we acknowledge that light is both a wave and a particle. As tiny seashells are driven by the powerful ocean waves, so photons travel in the pattern of invisible waves, but they are actually particles. 

The human eye is only sensitive to a narrow region of the electromagnetic spectrum. Visible light corresponds to 400 to 700 nanometers (nm) these create the colors violet through red. White light is a combination of all the visible colors. Color is determined by wavelength and measured in nanometers. Violet begins at 400nm, and the rainbow extends through to red at 700 where it again disappears from our sight. Outside this visible spectrum are ultraviolet rays, x-rays and gamma rays, which have significantly shorter waves and infrared, microwaves, and radio waves, which have significantly longer waves.

When light interacts with matter it reflects, refracts, diffracts, disperses and/or absorbs. When light passes through different transparent substances it refracts or bends. Different colors refract at different angles which results in a rainbow effect when white light passes through a prism. This effect is called dispersion. When light strikes a solid white or light colored surface most of the light reflects or bounces off the surface making the area seem brighter. Darker colors and black absorb more light and can make rooms painted black seem dark.

Light intensity refers to brightness, and can be measured in many different units of measure, including candela, foot candle, radiance flux (measured by watt or joule) and luminous flux (measured in lumens.) A single foot candle is equal to the amount of light a single candle flame puts off at exactly one foot. A candela is sometimes called a candle, but is not to be confused with foot candle. A candela is most easily understood by explaining that a hundred watt incandescent light bulb emits about 120 candela.

The Electromagnetic spectrum is also measured in relation to wavelength, frequency and energy which are all related. Light travels at a consistent speed of 299792.458 km per second or 186,000 miles per second. Frequency is therefore determined by length, the shorter the wavelength the more waves are sent per second. The shorter the wavelength, the higher the frequency is, and the higher the frequency is, the greater the energy. The relationship between a photon’s frequency and its energy can be expressed as E = h nu.

Wave propagation refers to the way waves travel in oscillations similar to vibrations. It is a way to distinguish between longitudinal and transverse waves. It is helpful to visualize the way in which tiny photons might move along these waves, in tight patterns. The speed at which a resultant wave packet travels is called the group velocity. Group velocity is determined from the gradient of the dispersion relation. The direction of group velocity is called the propagation direction. It can also be considered the direction of time-average energy flow.

The Doppler Effect is an interesting phenomenon observed in electromagnetic waves when the source of the waves is moving. Sometimes when the object emitting waves, moves towards the observer, the waves moving seem closer together than normal, and thus frequency appears to increase. This is called a blue shift. When the object is moving away from the observer, the frequency seems lower, and this is called a red shift.

 The study of the behaviors and physical properties of light is often called optics. In biology optics often describes the function of the eye, because vision is merely the eye’s interpretation of the actions of visible light. Most light however is invisible. Humans can only visually discern light within the spectrum between 400 and 700, which is a very small fragment of the entire electromagnetic spectrum.

  • Measuring Light and Light Intensity: Discusses the units of measurement for light
  • Intensity: Light intensity as it relates to distance angle, and solar energy
  • Units and Measures: Conversion table for different light measurements
  • Units of Measure: List of units of measure for light
  • Spectroscopy: Using the light spectrum to identify elements
  • Spectroscopy Basics: An overview of light as energy, wave and particle
  • How the Eye Sees Color: An easy explanation with pictures by Color Matters
  • Reflection of Light: Description of aspects of reflection with links to Java tutorials
  • Solar Physics Glossary: Explains terms used to describe sunlight
  • Solar Cell Output and Light Intensity: An experiment to determine output and intensity
  • Light and Matter: An open source textbook
  • The Interaction of Light and Matter: Molecular interactions between light and matter
  • Cal Tech Light and Matter: A scholarly discussion of light and matter
  • Illustrating Reflection Refraction & Diffraction: An elementary explanation, and an experiment 
  • Light Reflection Refraction Diffraction: Tutor Vista has a list of articles explaining all aspects of light reflection, refraction and diffraction
  • Dispersion: Using a prism to create dispersion of white light into a rainbow of color
  • Prism Defined: An elementary explanation of the geometry of a prism
  • Light, Prism and the Rainbow: Light-Science explains rainbows and prisms 
  • Polarization: Many diagrams, and a detailed explanation with examples
  • Polarization and Electromagnetic Waves: Java display of polarization

A great deal of technology is based on the consistent actions of the entire electromagnetic spectrum. Man relies on this spectrum to cook food, treat illness and transmit information. Our biological reliance on visible light is profound. Electromagnetic waves are vital to daily life in so many ways, and as technology broadens we will become even more reliant on this free gift of our universe, which is constant and renewable. 

  • Light Propagation Direction: Physics of light propagation direction and geometric optics
  • Propagation Direction: Wave propagation as it relates to ultrasonic testing
  • Frequency/Wavelength/Energy: Explains the relationship between frequency, wavelength and energy
  • Conversion from Frequency to Wavelength: Conversion calculator and explanation
  • Energy of Light: Energy and momentum in Lorentz transformations
  • Phase, Group and Signal Velocity: Describes the relationship between phase, group and velocity
  • Phase: Equations related to phase velocity
  • Speed:  Speed of light information
  • Photons, Waves, Particles, Optics: Properties of photon particles in context
  • Photons: What are photons?
  • Particles: Light Behaving a Particle
  • Optics: Harvard’s natural sciences glossary for optics
  • Subatomic Particles: What is a particle?
  • Doppler Effect: What is the Doppler Effect?
  • Doppler Effect Equation for Light: Many Doppler Effect equations, including red-shift
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