[ ..... Work in Progress ..... ]

Energy

We live in a world of mass (our physical environment) and energy (visible light, heat, radio waves, sound and all its other forms). The human body can detect ultraviolet light (when we tan, or sunburn), visible light (the colors of the rainbow) and infrared (as heat) and sound waves (speech, music, etc.) but not radio waves, x-rays, cosmic rays and the like. All of these are forms of energy which are transmitted through vibrations. The difference between them is how fast the vibrations cycle; their frequency in number of cycles per second.

Frequency

The span of visible, and near-visible, light energy is of interest because vision is so important to our lives and this range of light frequency, from ultraviolet through infrared is called a spectrum. While ultraviolet light energy has a higher frequency than infrared light, all light travels at the same speed in a vacuum (~300,000,000 meters/sec if you're curious). Yes, the speed of light changes when it enters glass but we'll save that thought for later.

[ Aside: Light energy is referenced both as a particle (photon) and a wave (no mass) and both have meaning but in different contexts. For discussions about light colors, wavelengths and spectrometers, we will refer to light as a wave. ]

Wavelength

Since light energy travels at a fixed speed but is also vibrating at some frequency, the distance from one ripple of an energy wave to the next is directly related to the frequency. When you toss a pebble in a pond, you can see the ripples of energy from the pebble and you can see the distance between them which remain roughly the same as they move. For simplicity, if we assume ripples in a pond always travel at the same speed, like the speed of light as a constant, the closer the ripples are together the higher the frequency. This means that we can talk about either the frequency or the wavelength of light energy as they are directly related. [ Mathematically, they have an inverse relationship where C = 1/Lambda where C is the speed of light in a vacuum and Lambda is the wavelength; both have units of measure in meters. ]

Human Vision

Our eyes detect only the visible light spectrum; from the deep reds just above infrared to the deep violets just below ultraviolet. Most of us can detect red from blue (except for those with some color deficiency) and we can detect various shades of colors and some color intensity. However, we are not very adept at measuring color or color intensities. In fact, our brains are part of our visual system and we can easily be fooled into seeing colors which are not really there. Ah, but what if we could! That could be fun and now you can -- indirectly -- using a spectrometer.

Spectrometers

A spectrometer in its simplest form is just an optical device, like a prism, which separates light into separate wavelengths (by frequency) so the amount of light energy at each frequency can be observed -- the light's spectrum. You have probably seen the rainbow colors (visible spectrum) produced by a prism, a sun-catcher or a diamond ring. All of these are primitive spectrometers. What they lack is control over the direction of the incident light and the means to measure and record the energy across the spectrum they display. So, how do they work?

Refraction

Light travels in straight lines and refraction is the change in direction a "ray" of light resulting from the light wave transitioning between different densities at an angle other then 90 degrees. You've seen this effect when you look into a pool of water or look at a spoon in a glass of water; objects appear "bent" or even "disjointed". When light enters glass, from air, the density of glass is much higher than air so the light refracts and the change in angle of the path of light is dependent on the color (frequency or wavelength). [ Technically, refraction is a result of a decrease in the phase velocity of the light when it enters glass which is why a lens can focus light. ] Since white light, like from the sun, contains a wide spectrum of colors, we see a rainbow of colors from a prism or sun-catcher.

Diffraction

Another method of separating light waves into separate wavelengths for observation is diffraction. Instead of bending light by changing the density through which the light travels, the wave nature of light can be exploited. If a "narrow beam of light" (where the light "rays" from the light source are all traveling in parallel in a narrow "beam") is directed at a set of parallel-spaced "lines and spaces" the light waves will "add" or "cancel" with each other depending on the wavelength. This behavior is called diffraction and the "set of lines and spaces" is called a diffraction grating or phase grating. The result is similar to a prism in that they both produce a rainbow spectrum. Diffraction gratings have the advantage of being physically thin and often inexpensive. In fact, the PublicLab spectrometers use an inner layer of a common DVD disk as a diffraction grating because of the narrow spacing of the DVD lines which are normally used to hold data.

Digital Spectrometers

Digital cameras, including computer webcams, contain a silicon imaging sensor chip which converts light energy to electrical voltages and then to digital data which can be recorded and analyzed by computer. By passing light through an inner layer taken from a DVD disk, the light can be separated into its spectrum and a digital camera, like a webcam, can convert that spectrum into computer data. This is what the PublicLab Spectrometer devices are designed to do. They let you indirectly "see" the light spectrum from ultraviolet through infrared and measure the energy at each wavelength -- something more than what your eyes are able to do. However, there is one more required element; light from the source must be directed in a narrow, parallel "beam" at the diffraction grating.

The Slit

The PublicLab spectrometer, like many spectrometers, is contained within a black enclosure with light only entering through a narrow slit. The slit is a simple and inexpensive method to simulate collimated light; light traveling in parallel lines from a source. Light from the sun, filtered through the leaves of a tree, produces reasonably parallel "collimated" light because the sun appears quite small in the sky. If a light source for the spectrometer is some distance away and must pass through a very narrow slit, then that light will also be reasonably "collimated". However, it is not perfect, so the lens of the webcam is adjusted to focus on the slit. Keeping the slit very narrow (~0.010 inches) improves the resolution (detail) of the webcam output spectrum but there is also less light -- it is a trade-off, but narrow slits are generally beneficial.

[....Editing to be continued..... -Dave ]

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Refraction

This slowing down is accompanied by a bending, called refraction, and is why a surface of water appears to shear drinking straws or other objects passing through them. This bending is also proportional to the wavelength: shorter wavelength light refracts greater than longer wavelength light passing through the same material. When the surfaces of the refracting material are parallel, such as an acrylic cube, the wavelengths refract again at the opposite surface and are once again combined at the same angle. But when the surfaces are not parallel, such as an acrylic prism (or a rotated cube), the light is dispersed: longer wavelengths travelling at shorter angles than shorter wavelengths. This is the purpose of the plastic CD piece in our spectrometer: to disperse the incoming light into its component wavelengths.

This equal dispersion of light is what we perceive as a rainbow: violet for the shortest wavelengths and red for the longest wavelengths. This means that 'white' light must be an even combination of the entire spectrum: a multitude of frequencies of light all travelling close together.

Photons can have a higher energy (and therefore higher frequency and shorter wavelength) than violet light, called ultraviolet, and an energy lower (lower frequency and wider wavelength) than red light, called infra-red. In fact, radio waves are also photons, just on the far 'red' end of the spectrum, with significantly wider wavelengths and therefore lower frequency and lower energy than visible light.

Photons exhibit characteristics of a wave but also those of a particle: called wave-particle duality. They will 'bend' and transmit through substances like sound waves, but they can also be absorbed and reflected like individual particles. It's important that we can bend light to disperse it into component frequencies and absorb light (as particles) to collect and measure it in a sensor.

http://en.wikipedia.org/wiki/Photon

Electromagnetic Spectrum?

The Electromagnetic Spectrum is a linear diagram of the various wavelengths of photons. The spectrum is broken up and named into smaller sections. We will be focusing on the piece of the spectrum corresponding to visible photons: the visible spectrum.

http://en.wikipedia.org/wiki/Electromagnetic_spectrum

What creates the light (photons) in the first place?

Nuclear reactions, such as in stars (our sun being the closest)

Electrical excitation of noble gasses, such as in sodium lamps

LASERs

Thermal radiation of high temperature materials, such as a lightbulb's filament or a 'red-hot' piece of metal.

If light is usually released at a small number of specific frequencies, why do we usually get entire spectrums of light, instead of specific frequencies like those created by a LASER?

Technical References

What are the two different types of spectroscopy, and how does the usage of the spectroscope differ?

http://en.wikipedia.org/wiki/Fluorescence_spectroscopy

http://en.wikipedia.org/wiki/Absorption_spectroscopy

Rayleigh scattering in the sun and atmosphere.

http://en.wikipedia.org/wiki/Rayleigh_scattering

http://en.wikipedia.org/wiki/Optics

http://en.wikipedia.org/wiki/Lens_(optics))

http://en.wikipedia.org/wiki/Refraction

http://en.wikipedia.org/wiki/Dispersion_(optics))

http://en.wikipedia.org/wiki/Fraunhofer_lines

http://en.wikipedia.org/wiki/Chromism

http://en.wikipedia.org/wiki/Chromaticity_diagram

http://en.wikipedia.org/wiki/Image_sensor

http://en.wikipedia.org/wiki/Bayer_filter

http://en.wikipedia.org/wiki/Exposure_(photography)