Light and photosynthesis

Making tomato plants happy

Green plants invoke a process called photosynthesis whereby they absorb light energy and use the absorbed energy to synthesize compounds called carboydrates, using carbon dioxide and water as the raw materials for the synthesis.

Photosynthesis equation

The process is illustrated in words (top line of the diagram), and chemically in the two following lines.

The middle reaction is an inaccurate (unbalanced) chemical equation since the total number of atoms in the synthesis is not the same on both sides of the equation. The bottom line remedies this problem a balanced chemical equation.

Photosynthesis is actually a two stage process:

  1. A light reaction in which the energy from absorbed sunlight is used to split water molecules into hydrogen and oxygen.
  2. A dark reaction whereby hydrogen from the light reaction combines with carbon dioxide to form carbohydrates.

Light: Not all light is the same. Just ask a plant!

When sunlight is passed through a prism it is refracted into a spectrum of colours.

This effect was studied in detail by Sir Isaac Newton. He reported to the Royal Society in 1675 the results of research into his corpuscular (particle) theory of light.

Solar light

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If we make a graph showing the intensity of the light in the spectrum (on the vertical axis) for all of the various colours it looks similar to the graph shown to the left. This is called a "continuous spectrum" because there is radiation in every region of the spectrum.

If one were to heat a substance to a temperature of 6000K, the light emitted would have a spectrum identical to that of the Sun. The Sun emits thermal radiation.

Solar light

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Incandescent light

The light from an ordinary household bulb has a continuous spectrum.

It arises from the heating of a tungsten filament which is electrically heated to about 1200K. This is called an incandescent light source and it emits thermal radiation - light as a result of its temperature.

Lamp light

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The spectrum of an incandescent bulb is shown in the diagram to the left. Because of the relatively low temperature (compared to the Sun for example), incandescent light bulbs emit much of their energy in the invisible infrared part of the spectrum as heat.

Incandescent light bulbs are a notoriously inefficient source of light... but they are very simple and cheap to produce.

Lamp light

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Fluorescent light

Home lighting

Interior Fluorescent lights are much more efficient at producing light. They are ideal for home use since they produce much more visible light (i.e. electrical watts to watts of visible light) than incandescent bulbs.

Notice in the image to the left that the spectrum has shifted towards the blue end of the spectrum (making the light appear "whiter") and that the infrared emission is much less than the incandescent bulb show above.

Home lighting

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Also notice that the spectrum has quite a different shape than the continuous spectrum produced by the light emitted from hot objects.

The actual shape of the spectrum is a result of the chemical formulation of the various minerals used to coat the inside of the tube. When the chemical coating is illuminated by ultraviolet light from within the tube, it causes the coating to glow (fluoresce) producing a bright whitish coloured light.

This a non-thermal source of light.

Home lighting

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Plant lighting

By adjusting the chemical composition of the tubes coating, it is possible to create fluorescent bulbs that emit light with specialized spectral characteristics.

The image to the right shows one such application.

Plant lighting

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Here the tube is designed to emit as much light as possible in the part of the spectrum that it best suited to the need of plants. Chlorophyll absorbs light most strongly in two bands, one at the red end of the spectrum and the other near the blue end of the spectrum. The red and blue spikes in the spectrum to the left are centred on the absorption bands of chlorophyll.

Plant lighting

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The figure to the left shows the spectra of sunlight before and after its journey to a green leaf (shown in highly magnified cross-section).

When sunlight falls on a green leaf some of the sunlight is absorbed by chlorophyll.

Green plants use the absorbed energy in a process called photosynthesis; a process whereby green plants store solar energy in the form of compounds called carbohydrates.

If we compare the spectrum of sunlight incident on a green leaf with the spectrum of light reflected from a green leaf, we see that a portion of the red and blue regions of the solar spectrum has been absorbed.

Compare the two spectra at the top of the image to the left. Note the "notches" in the red and blue parts of the reflected spectrum. These are the parts of the spectrum "preferred" by photosynthetic plants.

Leaf cross-section

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The primary absorption bands for chlorophyll are shown on the graph to the left. The shaded bands indicate the part of the spectrum which is best suited for photosynthesis to occur in green plants.

Note that the fluorescent "grow lights" are much better suited for growing plants (using artificial illumination) than incandescent lights or standard indoor fluorescent lights. Incandescent lights waste most of their energy as heat and emit almost no light in the blue part of the spectrum. Indoor fluorescent lights are more efficient but waste a lot of light in parts of the spectrum which are largely reflected by green plants.

If tomato plants are to be grown on Mars under conditions of artificial light, the spectrum of the light will have to be well chosen to ensure maximum plant growth and to ensure the energy is not wasted generating light which is of little or no use to photosynthetic plants.

Absorption band

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Units for Measuring the Brightness of Light

The units used to measure light intensity are varied, and somewhat obscure.

Physical SI Units: (W/m2)

In the Tomatosphere Project we have used watts per square metre as a measure of optical brightness incident on a surface. It translates directly into the rate at which energy is transfered, since one watt is simply one joule per second.

The solar irradiance above the Earth's atmosphere is measured by instruments on spacecraft which are calibrated to determine the amount of incident light (in W/m2) in the wavelength range from about 400 to 1100 nanometres, spanning a major portion of the solar spectrum. This quantity can then be used to calculate (using the inverse-square law of optics) the values of the solar irradiance throughout the solar system, for example at the upper atmosphere of Mars.

Comparing the measured solar irradiance on the Earth's surface (in W/m2), using instruments similar to those in spacecraft), with space based measurements, it is possible to make accurate determinations of the energy absorption properties of the Earth's atmosphere.

Photosynthetic Photon Flux Density: (mol/m2/s)

For many studies of the photosynthetic process it is helpful to measure the intensity of light in the wavelength range most important to plants, namely 400 to 700 nm (nanometre) range. In this instance scientists use a unit micromole per square metre per second (umol/m2/s). This quantity is the number of photons which would fall on (or pass through) one square metre each second, one mole being 6.02 x 1023 (Avogadro's number).

The "Einstein" (symbol E) has been adopted to mean a mole per square metre per second; for example, 400 umol/m2/s could also be written 400 uE. (symbol u meaning "micro").

Photometry: candela (cd)

The candela (cd) is defined as the luminous intensity, in a perpendicular direction, of a surface of 1/600 000 m2 of a black body (a perfect radiator) at the freezing temperature of platinum under a pressure of 101.325kPa. This is approximately 1/683 W.

Derived units that are related to the candela are;

The mid-day sun

Since each method integrates energy over different spectral regions it is not possible to convert directly from one unit of measure to another when comparing the brightness of different luminous objects.

However, when the same object, at the same temperature, is being viewed at a common distance (for example at the distance of Mars), conversions from one unit to another scale linearly. For example, the sun viewed from a distance where the brightness is reduced by 50% of the Earth's value, would be reduced 50% in all methods and units of intensity measurement.

On the Earth's surface the noontime brightness of the Sun (for purposes of comparison):

1060 W/m2 ~ 2000 uE ~ 106 000 lx

The values above are approximate, and assume a clear (cloud and haze free) and dry (low relative humidity) sky condition, with the Sun directly overhead.