Table of contents
1 Nature of Light
2 The Speed of Light
3 Optics
4 Color and Wavelengths
5 Measurement of Light
6 Light Sources
7 A Light Wave

Nature of Light

Light often means the portion of the electromagnetic spectrum visible to the human eye, but can also refer to other forms of electromagnetic radiation. The three basic dimensions of light (and of all electromagnetic radiation) are brilliance (or amplitude), color (or frequency), and polarization (or angle of vibration). Due to wave-particle duality, light simultaneously exhibits properties of both waves and particles.

Theories About Light

  • Particle Theory
    • First proposed by Isaac Newton in the seventeenth century
    • Argues that light is made up of tiny particles of matter (corpuscles)
      • <Discounted>
    • Corpuscles are emitted in all directions
      • <Discounted>
    • Supports the fact that light is reflected
    • Argues that light speeds up upon entering a denser medium because gravitational pull is greater
      • <Discounted>
    • Greatly discounted by wave theory

  • Wave (or Ray) Theory
    • First proposed by Christian Huygens in the seventeenth century
    • Argues that light is emitted as a series of waves only
      • <Discounted>
    • Waves are emitted in all directions
    • Waves not affected by gravity, so they slow down upon entering a denser medium
    • Greatly discounted the corpuscular theory
    • Can interfere with each other like sound waves (noted in eighteenth century by Thomas Young)
    • Waves can be polarized
    • Assumes that light needs a medium for transmission like sound
      • <Discounted>

  • Quantum (or Wave-particle duality) Theory
    • Combines previous three theories
    • Pioneered at end of the nineteenth century
    • Max Planck proposed that light waves are made of packets of energy known as quanta or photons in 1900
    • Light behaves as both particles and waves

Visible Light Wavelengths

Visible light is that portion of the spectrum between the wavelengths of about 400 nanometers (abbreviated nm) and 800 nm (in air). Light can also be characterized by its frequency. The frequency and wavelength of light obey the relation

The Speed of Light

(see speed of light)

Speed of Light Formula


where λ is the wavelength, f is the frequency, v is the speed of the light. If the light is travelling in a vacuum, then v = c, thus


where c is the speed of light. We can express v as

where n is a constant (the
refractive index) which is a property of the material through which the light is passing.

Change to the Speed of Light

All light propagates at a finite speed. Even moving observers always measure the same value of c, the speed of light in vacuum, as c = 299,792,458 metres per second; however, when light passes through a transparent substance such as air, water or glass, its speed is reduced, and it suffers refraction. Thus, n=1 in a vacuum and n>1 in matter. It is a violation of the technical terminology of physics to speak of the "velocity of light;" velocity is reserved for a different use.

History of the Speed of Light

The speed of light has been measured many times, by many physicists. The best early measurement is Olaus Roemer's (a Danish physicist), in 1676. He had developed a method for measuring light. He observed and noted the motions of Jupiter and one of its moonss with a telescope. It was possible to time the revolution of the moon because it was eclipsed by Jupiter at regular intervalss. Roemer discovered that the moon revolved around Jupiter once every 42-1/2 hours when Earth was closest to Jupiter. The problem was that when Earth and Jupiter were not as close, the moon's revolution seemed to be more. It was clear that light took longer to reach Earth when it was farther away from Jupiter. The speed of light was calculated by analyzing the distance between the two planets at various times. Roemer reached a speed of 227,000 kilometers per second (approximately 141,050 miles per second).

Albert A. Michelson improved on Roemer's work in 1926. He used rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 186,285 miles/second (299,796 kilometers/second). In daily use, the figures are rounded off to 186,000 mi/sec and 300,000 km/sec.


The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows offers many clues as to the nature of light as well as much enjoyment.

Color and Wavelengths

The different wavelengths are interpreted by the human brain as colors, ranging from red at the longest wavelengths (lowest frequencies) to violet at the shortest wavelengths (highest frequencies). The intervening frequencies are seen as orange, yellow, green, blue, and, conventionally, indigo. The frequencies of the spectrum immediately outside the range the human eye is able to perceive are called ultraviolet (UV) at the high frequency end and infrared (IR) at the low. Though humans cannot see IR, we do perceive it by receptors in the skin as heat. Cameras that can pick up IR and convert it to visible light are called night-vision cameras. UV radiation is not perceived by humans at all except in a very delayed fashion, as overexposure of the skin to UV light causes sunburn, or skin cancer. Some animals, such as bees, can see UV radiation while others, such as pit viper snakes, can see IR using pits in their heads.

Measurement of Light

The following quantities and units are used to measure light.

  • brightness (or temperature)
  • illuminance or illumination (SI unit: lux)
  • luminous flux (SI unit: lumen)
  • luminous intensity (SI unit: candela)

See also: Photometry

Light Sources

A Light Wave

The only characteristic of the wave not seen here (because of need for the passage of time) is

See also: Huygens' principle, Color temperature, Illumination, International Commission on Illumination, Wave-particle duality, Light pollution, photic sneeze reflex

copyright 2004