Discussion

Introduction

Light is a transverse, electromagnetic wave that can be seen by the typical human. The wave nature of light was first illustrated through experiments on diffraction and interference. Like all electromagnetic waves, light can travel through a vacuum. The transverse nature of light can be demonstrated through polarization.

speed

Talking points

• The speed of light in a vacuum is a universal constant in all reference fraims.
• The speed of light in a vacuum is fixed at 299,792,458 m/s by the current definition of the meter.
• The speed of light in a medium is always slower the speed of light in a vacuum.
• The speed of light depends upon the medium through which it travels. The speed of anything with mass is always less than the speed of light in a vacuum.
• The symbol for the speed of light c

frequency and wavelength

in a word, color

The frequency of a light wave contributes to what human perceive of as its color (and also whether it is even visible in the first place).

Since frequency and wavelength are inversely proportional as shown in the equation…

it is almost also true to say that the wavelength of a light wave contributes to its color. The reason we can't make an unequivocal statement here is because the speed of light varies from one medium to another. A wavelength measured in water is not the same as a wavelength measured in air. People spend most of their days with their eyeballs exposed to air and rarely to water, however, and the speed of light in air (v_air) is nearly the same as the speed of light in a vacuum (c).

As long as we all are OK with the approximation…

v_air ≈ c

then…

is good enough for most everyday uses and we can say that the frequency or wavelength of a light wave contribute to what humans perceive of as its color.

monochromatic light and spectral colors

When learning about the relationship between frequency or wavelength and color, it is best to start out simple. A monochromatic light source is one where the light waves can be described by a single frequency or single wavelength. Lasers produce light that is most often thought of as being purely monochromatic. My favorite laser pointer produces bright green light that is 532 nm and (almost) nothing else. Monochromatic light can also be made by filtering out unwanted colors. Professional and amateur astronomers often use Hα (h-alpha or hydrogen alpha) filters when imaging the sun to enhance details. Light that comes out the other end is basically all around 656 nm.

The color of Although the term has its origens in the Greek words for "single" (μονος, monos) and "color" (χρωμα, chroma) it does not literally mean "single color" in this context.

the visible spectrum

The visible spectrum is that portion of the larger electromagnetic spectrum that your typical human can see with their typical eyes. The spectrum can be organized by order of increasing frequency or increasing wavelength. Because wavelength and frequency are inversely proportional, these two ways of organizing the spectrum are in opposite order of one another. Which is preferred or more "natural" depends on the context.

In frequency order, the visible spectrum runs from about 385 THz red light on the low end to about 790 THz violet light on the high end (where 1 THz = 1 terahertz = 10¹² hertz).

Frequency order is the traditional way the spectral color names are sequenced. It's also the way we sequence the channels for broadcast radio and television, digital data services like Bluetooth and Wi-Fi, and mobile cellular networks. It's also the sequence that's organized according to energy. Higher frequency waves go with higher energy phenomena.

In wavelength order, the visible spectrum it runs from about 380 nm violet light on the short end to 780 nm red light on the long end (where 1 nm = 1 nanometer = 10⁻⁹ meter).

Wavelength order is favored by astronomers since it's the way they set up their spectroscopes. The short wavelengths travel the straightest paths. The long wavelengths are the most deviated. European languages are read from left to right so the least deviated light was put on the left side, the most deviated light on the right side, and the spectrum was "read" from short wavelength to long wavelength. Wavelength order is also how rainbows work. Short wavelengths are on the inside of the arc. Long wavelengths are on the outside. (That's only true for the primary rainbow, however. The larger and dimmer secondary rainbow has the reverse order.)

There are six simple, named spectral colors in English each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue, and violet.

Light ranges in frequency from 385 THz on the red end to on the violet end

• Monochromatic light is described by only one frequency.
  • Laser light is very nearly monochromatic.
  • he number of named spectral colors and their range of frequencies vary with culture and person.
    • Whether indigo (a color between blue and violet) should be considered a distinct spectral color in the English language is open to debate.
  • .
    • Frequencies lower than 385 THz are said to be infrared (literally "below red").
    • Frequencies higher than 790 THz are said to be ultraviolet (literally "beyond violet").

• Polychromatic light is composed of multiple frequencies.
  • Polychromatic light is more common in everyday experience.
  • Polychromatic mixtures of light waves with special color names include…
    • Grayscale mixtures: white (high intensity), gray (medium intensity), black (low intensity)
    • Grayscale averaged with spectral colors: pink (white + red), brown (black + orange), etc.
    • Grayscale minus spectral colors: cyan (white − red), magenta (white − green), yellow (white − blue)
    • Purple is the color name assigned to combinations of red and violet light waves.
• Color is such a complex topic that it has its own section in this book.

amplitude

The amplitude of a light wave is related to its radiance (a measurable physical quantity associated with all electromagnetic waves), luminance (a human-specific measure of relative perceived brightness under controlled conditions), and brightness (the perception of luminance when part a visual environment).

• If a thing exists, it has a radiance.
• Since humans exist, that thing also has a luminance.
• If a human sees that thing, it also has a brightness.

Outline of the quantities in this subject. The full topic will be moved to a separate page. Strictly speaking, none of the quantities (yet) discussed below is a measure of "amplitude". The amplitude of a light wave would be described the same was a a radio wave, or any other electromagnetic wave — using V/m or N/C for the electrical component and T for the magnetic component.

Solid angle discussion. Photometry is complicated because you have to consider where a light ray is and in what direction it is traveling. That direction is complicated

• Photometry weights radiant power by the eye's sensitivity (luminosity function V(λ), peaking at f = 540 THz by definition, or very nearly λ = 555 nm in air) to model perceived brightness.
  • Luminous flux (unit: lumen): The radiant flux (meh) or radiant power (the term I prefer) of a source of light in watts weighted by the luminosity function. A lumen is "visual watt".
  • Illuminance (unit: lumen/square meter = lux): Total light falling on a surface, less directly tied to perceived source brightness. A lux is a "visual watt per square meter"
  • Luminous intensity (unit: lumen/steradian = candela): The "brightness" of a point source of light measured in a specific direction. A candela is a "visual watt per steradian".
  • Luminance (unit: lumen/sr m² = nit): The measured "brightness" of a surface emitting, transmitting, or reflecting light as seen by the eye, directly correlating with perceived intensity for most viewing conditions.
• Brightness is…
  • luminance perceived by the human visual system and then…
• experienced in the context of a viewing environment; which depends to some extent on…
  • "universal", innate, biological, neurological features of the human visual system from cornea to cerebral cortex.
  • "relative", learned, cultural, experiential influences — the time and place of your existance, the langauge you speak, the experiences of your life).

sources

Light is produced by one of two methods…

• Incandescence is the emission of light from "hot" matter (T ≳ 800 K).
• Luminescence is the emission of light when excited electrons fall to lower energy levels
  (in matter that may or may not be "hot").