Human eyes see light in wavelengths from 400 to 700 billionths of a m, or nanometers, nm for short (or 0.4-0.7 millionths of a m or microns) - the range we normally refer to as “visible light”. It’s no coincidence that our eyes evolved to see these wavelengths. There are two important reasons why this part of the spectrum is so important:

1. The light from the Sun reaching through the atmosphere Earth is strongest in this part of the spectrum. The optical Earth Observation range is slightly wider at 400 to 2500 nm, encompassing almost all of the sun’s energy. So the visible part of the electromagnetic spectrum is where all the life happens. Almost all life on Earth is reliant on light in the optical range and the processes of life evolved the way they did to exploit it. (There are very isolated exceptions in the form of “extremophiles” that live on the bottom of the ocean - and survive by metabolising minerals released from so-called ‘black smoker’ vents, but everything else needs light). The Sun’s energy is very strongly peaked in this part of the spectrum – 40% of its energy lies in this tiny sliver, whereas the other 60% is spread over several orders of magnitude of wavelength, from UV to shortwave infrared and beyond. This makes it easier and more efficient to gather energy in these wavelengths.

2. The second contributing reason is that there is a convenient (and totally coincidental) “atmospheric window” in this part of the spectrum. That is, at these wavelengths, the atmosphere allows nearly all the solar radiation to pass through. The atmosphere absorbs the Sun’s light very strongly in other parts of the spectrum – which can be frustrating for gathering some observations from space – ask an astronomer – but very useful for protecting the surface from damaging radiation such as the ozone layer absorbing UV.

The combination of the peak in the Sun’s energy and the atmospheric window means that plants (via chlorophyll pigments) evolved to be sensitive to these parts of the electromagnetic spectrum, as did nearly all animals that have eyes. So the visible is where all the life happens. Almost all the productivity of life on Earth (plants, animals, fossil fuels etc) is therefore a result of these coincidences in this tiny slice of EM spectrum making it the most important part of the spectrum for us to understand. If we use Earth observation to look in this part, first we “understand” what we see because that’s what our eyes and brains are used to dealing with, but secondly the reflected solar signal contains information about the processes controlling life.

It’s useful to bring in other measurements at times, so it’s worth being familiar with some of the other forms of Earth observation being done from space:

Thermal: this is emitted infrared radiation rather than reflected, and is caused by things being warm. Thermal radiation is emitted at much longer wavelengths and lower energy than optical, and is hence harder to measure.

RADAR (radio detection and ranging): this is a so-called ‘active’ form of remote sensing where we generate the signal artificially (as opposed to ‘passive’ where we just measure reflected or emitted radiation). A a radio pulse is sent out and an image built up based on its reflection from the target. This is very useful because it works in all kind of weather and can “see through” clouds. (In optical Earth observation, we can’t measure what is under the clouds because they reflect the Sun’s light – however, data from the clouds themselves can be very useful in studying weather patterns and climate). Active signals require a lot more power, and are more difficult to interpret. Radar doesn’t interact with the photosynthetic apparatus, so doesn’t tell us about how plant life (for example) is operating.

Our blue sky, and the occasional rainbow, teach us that different wavelengths of light correspond to different colours. When all wavelengths of visible light strike the eye at the same time, we perceive white light.

Featured Educators:

• Dr Mathias Disney

Don’t forget you can download the video, transcript and take any quizzes available with the links on the right.