  Part I: What is Light?

Introduction:

The choice of lighting is one the most important decisions to make when setting up a reef tank. The light fixtures and related equipment are some of the more expensive pieces of equipment both at initial setup up as well as in their contribution to daily operating costs. In addition to being necessary for the photosynthetic organisms we keep in our aquariums, light also provides the visual element of color. From talking to aquarists and perusing the various reef-related bulletin boards, it has been my experience that lighting and color are often a very misunderstood aspect of aquarium keeping. Given that lighting and color are important in the functional and aesthetic elements of reefkeeping, I feel it is important that hobbyists have a good understanding of light and color. The purpose of this series of articles is to provide beginning and intermediate reef aquarists with a comprehensive understanding of lighting concepts and terminology, and the ability to understand and comprehend lighting related discussions and data. The information will be presented in a series of short columns focusing on a few concepts at a time and build to a comprehensive understanding of light, especially as it relates to reef aquariums.

Light is a form of energy, and to understand light we begin with the electromagnetic spectrum (Figure 1: http://www.lbl.gov/MicroWorlds/.../EMSpec2.html) which is basically a grouping of all electromagnetic radiation arranged according to the amount of energy contained in the radiation. Visible light is a part of this electromagnetic spectrum that creates the sensation of light when it falls on the human eye.

 Figure 1. The electromagnetic spectrum.

The properties of all electromagnetic radiation can be described by three inter-related terms. These are wavelength, frequency and energy. Since light is a part of this spectrum, it too can be described by these terms. Hence, it is important to understand these terms as a first step towards understanding light.

1) Wavelength:

Simplistically, we can think of light traveling as a wave. A typical wave form (e.g., ripples on the surface of water) has crests (or peaks) and troughs (or valleys). The distance between two consecutive peaks (or troughs) is called the wavelength, and is denoted by the Greek letter λ (lambda). Because the wavelength is a measure of distance, it is measured in units of length (meters). Since these wavelengths of visible light can be quite small, they are measured in nanometers (nm) where 1 nm = 1 billionth of a meter (10-9 meters). The wavelength of visible light is between 400-700nm. Incidentally, these also happen to be the majority of wavelengths of light that are relevant to photosynthesis. The combined effect of the complete range of radiation between 400-700nm appears as white light to the human eye. Radiation with a wavelength of 400 nm generates a response in the human eye that makes it perceived as violet, while radiation with a wavelength of 700nm appears red. The different colors of the rainbow (ROYGBV - red, orange, yellow, green, blue and violet) are arranged in descending order of their wavelength. Roughly, we can break down the various colors into wavelength bands as follows:

 Violet - 400 to 440nm Blue - 440 to 490nm Green - 490 to 540nm Yellow - 540 to 590nm Orange - 600 to 650nm Red - 650 to 700nm

Radiation below 400 nm wavelength is called ultraviolet (UV) radiation, and is typically divided into three segments: UV-A (400-315nm), UV-B (315-280nm) and UV-C (280-100nm). UV radiation is not visible to the human eye, but it can have a damaging impact on humans (as well as corals). The UV-A segment, the most common in sunlight, overlaps slightly with the shortest wavelengths in the visible portion of the spectrum. UV-B is effectively the most destructive UV radiation from the sun, because it penetrates the atmosphere and can injure biological tissues. UV-C radiation from the sun would cause even more injury, but it is absorbed by the atmosphere, so it almost never reaches the Earth's surface.

Infrared (IR) radiation has slightly longer wavelengths than visible light. The IR region of the electromagnetic spectrum is also divided into three segments: IR-A (780-1400 nm), IR-B (1400-3000 nm) and IR-C (3000-10600 nm). Infrared radiation is thermal and is felt as heat.

2) Frequency:

The number of waves that pass a given point in space during a specified time interval is the light's frequency; consequently, frequency is a time based unit. Frequency carries the units "per second," but we use a special term for the unit called - Hertz (Hz), where 1 Hz corresponds to 1 wave/second, so 50 Hz would mean 50 waves/second. As seen in the figure above, the wavelength and frequency are related to each other. If we take any two points on the waveform labeled "start" and "end," and count the number of waves in between, we can easily see that we will have more waves if the wavelength is smaller. More waves imply that the frequency will be higher. Thus wavelength and frequency are inversely related: the shorter the wavelength of the wave, the higher the frequency of the wave.

Since all the waves travel at the same speed - the speed of light - the relationship between wavelength and frequency is determined by the following formula:

Wavelength = speed of light / frequency

In the typical notation that you will see in most articles and books:

λ = c/ν

where:

λ = wavelength
ν = frequency
c = speed of light

The speed of light is 299,792,458 meters per second (approximately 3.0 × 108 meters/second). To be precise, what we usually call the "speed of light" is really the speed of light in a vacuum (the absence of matter). In reality, the speed of light typically varies depending on the particular medium that it travels through. Light moves more slowly in glass than in air, and in both cases the speed is less than in a vacuum.

If we look at the colors of the rainbow from blue to red, we can now understand that the blue light (400nm wavelength) will have a higher frequency than red light at 700nm, with the other colors of the rainbow falling in between. In fact, the frequency of blue light will be 57% (400/700 × 100 = 57.14%) higher than the frequency of red light.

3) Energy:

As mentioned earlier, light is a form of energy. According to the quantum theory, all energy is transmitted and absorbed in discrete particles called quanta or photons. Thus, the smallest amount of radiation energy that can exist is one photon.

If one thinks of the photon as a small packet or ball of energy, it is most useful in understanding light, especially for our purpose of reefkeeping. For our purposes, let us take a simplified, unified description that says that light travels as discrete photons along a wave. Visible light is a mixture of many photons with different wavelengths. The photons are reflected and absorbed by various surfaces, and when they reach the eyes, they create the sensation of sight and resultant perceptions of color and brightness. These photons are also directly responsible for photosynthesis in plants and corals. The energy from the photons is used during photosynthesis to convert CO2 into sugar, which is a primary energy source for the photosynthetic endosymbiotic zooxanthellae living within corals.

As discussed earlier, the energy carried by electromagnetic radiation is contained in the photons that travel as a wave. According to quantum theory, the energy in a photon varies with its frequency, according to the equation:

Energy = Plank's constant × Frequency
E = hν = hc/λ
Where h = Plank's constant is 6.626 × 10-34 joules per second

Energy is measured in units called joules.

As the frequency of the radiation increases (wavelength gets shorter), the amount of energy in each photon increases. Now we can begin to understand why the red light gets absorbed quickly in water as a function of depth.

These basic equations provide us with the relationship between wavelength, frequency, energy and photons, and can be used to go back and forth as seen in the following examples.

Example: What is the energy in a single photon of light at 500nm?

E = 6.626 × 10-34 × 3.0 × 108/(500 × 10-9)
E = 0.039756 × 10-17 J

Example: How many photons per joule exist for light at wavelength λ = 500nm?

E = Energy/photon, so to create 1 J of energy we will need N photons.
N × E = 1 joule, hence N = 1/E
N = λ/hc = 25.15 × 1017 photons

As seen above, to produce 1 Joule of energy by light at a wavelength of 500nm requires a very large number of photons. To avoid having to deal with such large numbers, we can measure the number of photons in "moles" where 1 mole = Avagadro's number = 6.02 × 1023. So 25.15 × 1017 photons would correspond to .000004177 moles. Now, this number is too small, so instead we will measure in "micromoles," where 1 micromole (denoted as µmol) is 10-6 mole, giving us 4.177 micromoles of photons.

What about watts? Energy is measured in joules, and the "watt" is the unit used as a measure of power. Power is defined as the rate of flow of energy. By definition, 1 watt = 1 joule/second. So, one watt of power from light at 500nm would need to provide 25.15 × 1017 photons per second or 4.1769 micromoles/sec. The figure below shows the relationship between watts and micromoles of photons to generate 1 watt of power. Summary:

This column has focused on providing the basic terminology required to understand light. Light is a form of energy, and can be simply described as a stream of photons traveling along a wave. Photons are discrete particles of energy. The characteristics of light and the photons are specified by three terms: wavelength, frequency and energy, which are mathematically related. Photons with wavelengths of 400 nm carry more energy than those with larger wavelengths and will appear violet to the human eye, and photons with wavelengths of 700nm carry less energy and will appear red. White light is a mixture of photons in the wavelength range 400-700nm. This range is what the eye can see and is also useful for photosynthesis. The photons carry the energy and the number of photons is measured in units of "micromoles."

The next column will discuss how light sources generate photons, the distribution of the photons in a light source and how this distribution is represented as a spectral plot. 