References

Photon life

Some useful information:

diodes: http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/info/comp/passive/diode/diode.htm

Chandra: http://chandra.harvard.edu/index.html

Invisible life references

The Photoelectric Effect: What's the photoelectric effect? (java applet)

Pigment

In biology, pigment is any material resulting in color in plant or animal cells which is the result of selective absorption. Some biological material has so-called structural color, which is the result of selective reflection or iridescence, usually done with multilayer structures. Unlike structural color, pigment color is the same for all viewing angles. Nearly all types of cells, such as skin, eyes, fur and hair contain pigment. Butterfly wings typically contain structural color, although many of them contain pigment as well. Creatures that have deficient pigmentation are called albinos.

Because pigment color is the result of selective absorption, there is no such thing as white pigment. A white object is simply a diffuse reflecting object which does not contain any pigment.

In the coloring of paint, ink, plastic, fabric and other material, a pigment is a dry colorant, usually an insoluble powder. There are both natural and synthetic pigments, both organic and inorganic ones. Pigments work by selectively absorbing some parts of the visible spectrum (see light) whilst reflecting others.

A distinction is usually made between a pigment, which is insoluble, and a dye, which is either a liquid, or is soluble. There is a well-defined dividing line between pigments and dyes: a pigment is not soluble in the vehicle while a dye is. From this follows that a certain colourant can be both a pigment and a dye depending on in which vehicle it is used. In some cases, a pigment will be made by precipi

Refraction

Refraction in geometric optics is the change in direction of a wave due to a change in velocity. It happens when waves travel from a medium with a given refractive index to a medium with another. At the boundary between the media the wave changes direction; its wavelength increases or decreases but frequency remains constant. For example, a light ray will refract as it enters and leaves glass; understanding of this concept led to the invention of the refracting telescope.

In the diagram on the right, ripples travel from the left and pass over a shallower region inclined at an angle to the wavefront. The waves travel more slowly in the shallower water, so the wavelength decreases and the wave bends at the boundary. The dotted line represents the normal to the boundary. The dashed line represents the original direction of the waves. The phenomenon explains why waves on a shoreline never hit the shoreline at an angle. Whichever direction the waves travel in deep water, they always refract towards the normal as they enter the shallower water near the beach.

An example of this is looking into a bowl of water. Air has a refractive index of about 1.0003, and water has a refractive index of about 1.33. If you look at a straight object, such as a pencil, which is placed at a slant, partially in the water, the object appears to bend at the water's surface. This is due to the light rays from the object being bent as they move from the water to the air. This causes water to appear shallower than it really is.

Refraction is also responsible for rainbows and for splitting up of white light into a rainbow-spectrum as it passes through a glass prism. Glass has a higher refractive index than air and the different frequencies of light travel at different speeds (dispersion), causing them to be refracted at different angles. The different frequencies correspond to different colours observed.

The amount that the light bends during refraction is calculated using Snell's law.

Recently some metamaterials have been created which have a negative refractive index.

tating a soluble dye with a metallic salt. The resulting pigment is called a "lake".

Refractive index

The refractive index of a material is the factor by which electromagnetic radiation is slowed down (relative to vacuum) when it travels inside the material. For a non-magnetic material, the square of the refractive index is the material's dielectric constant $ε$ (sometimes expressed as the relative permittivity $ε r$ ) multiplied by the relative permeability, $μ r$ . For a general material it is given by:

$n=\sqrt{\epsilon_r\mu_r}$

The speed of all electromagnetic radiation in vacuum is the same, approximately 3×10⁸ meters per second, and is denoted by c. So if $v$ is the phase velocity of radiation of a specific frequency in a specific material, the refractive index is given by

$n =\frac{c}{v}$

This number is typically bigger than one: the denser the material, the more the light is slowed down. However, at certain frequencies (e.g. near absorption resonances, and for x-rays), $n$ will actually be smaller than one. This does not contradict the theory of relativity, which holds that no information-carrying signal can ever propagate faster than $c$ , because the phase velocity is not the same as the group velocity or the signal velocity.

The phase velocity is defined as the rate at which the crests of the waveform propagate; that is, the rate at which the phase of the waveform is moving. The group velocity is the rate that the envelope of the waveform is propagating; that is, the rate of variation of the amplitude of the waveform. It is the group velocity that (almost always) represents the rate that information (and energy) may be transmitted by the wave, for example the velocity at which a pulse of light travels down an optical fibre.

Sometimes, a "group velocity refractive index", usually called the group index is defined:

$n_g=\frac{c}{v_g}$ ,

where $v g$ is the group velocity. This value should not be confused with $n$ , which is always defined with respect to the phase velocity.

At the microscale an electromagnetic wave is slowed in a material because the electric field creates a disturbance in the charges of each atom (primarily the electrons) proportional to the permittivity. This oscillation of charges itself causes the radiation of an electromagnetic wave that is slightly out-of-phase with the original. The sum of the two waves creates a wave with the same frequency but shorter wavelength than the original, leading to a slowing in the wave's travel.

If the refractive indices of two materials are known for a given frequency, then one can compute the angle by which radiation of that frequency will be refracted as it moves from the first into the second material from Snell's law.

Recent research has also demonstrated the existence of negative refractive index which can occur if $ε$ and $μ$ are simultaneously negative. Not thought to occur naturally this can be achieved with so called metamaterials and offers the possibility of perfect lenses and other exotic phenomena such as a reversal of Snell's law.

Color

Electromagnetic radiation is a mixture of radiation of different wavelengths and intensities. When this radiation has a wavelength inside the human visibility range (approximately from 380 nm to 740 nm), it is called light. The light's spectrum records each wavelength's intensity. The full spectrum of the incoming radiation from an object determines the visual appearance of that object, including its perceived color. As we will see, there are many more spectra than color sensations; in fact one may formally define a color to be the whole class of spectra which give rise to the same color sensation, although any such definition would vary widely among different species and also somewhat among individuals intraspecifically.

A surface that diffusely reflects all wavelengths equally is perceived as white, while a dull black surface absorbs all wavelengths and does not reflect (for mirror reflection this is different: a proper mirror also reflects all wavelengths equally, but is not perceived as white, while shiny black objects do reflect).

The familiar colors of the rainbow in the spectrum—named from the Latin word for appearance or apparition by Isaac Newton in 1671—contains all those colors that consist of visible light of a single wavelength only, the pure spectral or monochromatic colors.

The frequencies are approximations and given in terahertz (THz). The wavelengths, valid in vacuum, are given in nanometers (nm). A list of other objects of similar size is available.

The colors of the visible light spectrum.

color	wavelength interval	frequency interval
red	~ 625-740 nm	~ 480-405 THz
orange	~ 590-625 nm	~ 510-480 THz
yellow	~ 565-590 nm	~ 530-510 THz
green	~ 500-565 nm	~ 600-530 THz
cyan	~ 485-500 nm	~ 620-600 THz
blue	~ 440-485 nm	~ 680-620 THz
violet	~ 380-440 nm	~ 790-680 THz
Continuous optical spectrum Designed for monitors with gamma 1.5.
Computer "spectrum" The bars below show the relative intensities of the three colors mixed to make the color immediately above.

Color, frequency, and energy of light.

Color	$\lambda \,\!$ /nm	$\nu \,\!$ /10¹⁴ Hz	$\nu_b \,\!$ /10⁴ cm^-1	$E \,\!$ /eV	$E \,\!$ /kJ mol^-1
Infrared	>1000	<3.00	<1.00	<1.24	<120
Red	700	4.28	1.43	1.77	171
Orange	620	4.84	1.61	2.00	193
Yellow	580	5.17	1.72	2.14	206
Green	530	5.66	1.89	2.34	226
Blue	470	6.38	2.13	2.64	254
Violet	420	7.14	2.38	2.95	285
Near ultraviolet	300	10.0	3.33	4.15	400
Far ultraviolet	<200	>15.0	>5.00	>6.20	>598

Light is electromagnetic radiation with a wavelength that is visible to the eye or, in a technical or scientific setting, electromagnetic radiation of any wavelength. The three basic dimensions of light (and of all electromagnetic radiation) are:

intensity (or brilliance or amplitude, perceived by humans as the brightness of the light),
frequency (or wavelength, perceived by humans as the color of the light), and
polarization (or angle of vibration and not perceptible by humans under ordinary circumstances)

Due to wave-particle duality, light simultaneously exhibits properties of both waves and particles. The precise nature of light is one of the key questions of modern physics.

Photon

In physics, the photon (from Greek φοτος, meaning light) is a quantum of the electromagnetic field, for instance light. Photons were originally called "energy quanta".

The photon is one of the elementary particles. Its interactions with electrons and atomic nuclei account for a great many of the features of matter, such as the existence and stability of atoms, molecules, and solids. These interactions are studied in quantum electrodynamics (QED), which is the oldest part of the Standard Model of particle physics.

In some respects a photon acts as a particle, for instance when registered by the light sensitive device in a camera. In other respects, a photon acts like a wave, as when passing the camera optics. According to the so-called wave-particle duality in quantum physics, it is natural for the photon to display either aspect of its nature, according to the circumstances. Normally, light is formed from a large number of photons, with the intensity or brightness related to the number of them. At low intensity, it requires very sensitive instruments, used in astronomy, for instance, to detect the individual photons.

Transparency

In optics, transparency is the property of being transparent, or allowing light to pass. The opposite property is opacity. Though transparency usually refers to visible light in common usage, it can actually refer to any type of radiation. For example, flesh is transparent to X-rays, while bone is not, allowing the use of medical X-ray machines.

Examples of transparent materials are air and some other gases, liquids such as water, most glasses, and plastics such as Perspex. Where the degree of transparency varies according to the wavelength of the light, the image seen through the material is tinted. This may for instance be due to certain metallic oxide molecules in glass, or larger colored particles, as in a thin smoke. If many such particles are present the material may become opaque, as in a thick smoke.

Transparent materials can be seen through; that is, they allow clear images to pass. Translucent materials allow light to pass through them only diffusely, and hence cannot be clearly seen through. Examples of translucent materials are frosted glass, paper, and some kinds of amber.

There are transparent glass walls that can be made opaque by the press of a button, a technology known as electrochromics.

Wavelength

The wavelength is the distance between repeating units of a wave pattern. It is commonly designated by the Greek letter lambda (λ).

In a sine wave, the wavelength is the distance between peaks:

The x axis represents distance, and I would be some varying quantity (for instance air pressure for a sound wave or strength of the electric or magnetic field for light), at a given point in time as a function of x.

Wavelength λ has an inverse relationship to frequency f, the number of peaks to pass a point in a given time. The wavelength is equal to the speed of the wave type divided by the frequency of the wave. When dealing with electromagnetic radiation in a vacuum, this speed is the speed of light c, for signals (waves) in air, this is the speed of sound in air. The relationship is given by:

$\lambda = \frac{c}{f}$

where:

λ = wavelength of a sound wave or electromagnetic wave

c = speed of light in vacuum = 299,792.458 km/s ~ 300,000 km/s = 300,000,000 m/s or

c = speed of sound in air = 343 m/s at 20 °C (68 °F)

f = frequency of the wave

For radio waves this relationship is approximated with the formula: wavelength (in metres) = 300 / frequency (in megahertz).

When light waves (and other electromagnetic waves) enter a medium, their wavelength is reduced by a factor equal to the refractive index n of the medium but the frequency of the wave is unchanged. The wavelength of the wave in the medium, λ' is given by:

$\lambda^\prime = \frac{\lambda_0}{n}$

where:

λ₀ is the vacuum wavelength of the wave

Wavelengths of electromagnetic radiation, no matter what medium they are travelling through, are usually quoted in terms of the vacuum wavelength, although this is not always explicitly stated.

Louis de Broglie discovered that all particles with momentum have a wavelength associated with their quantum mechanical wavefunction, called the de Broglie wavelength.