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Ocean colour: what is light?
Sight as remote sensing Light as photons Light and energy Optical radiation
Sight as remote sensing
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Our ability to detect visible electromagnetic radiation allows us to observe things we are never likely to touch, whether dolphins at sea or mountains on the moon.
We see the world around us because our eyes are sensitive to light - visible electromagnetic radiation.
This radiation bounces off everything around us, and picks up information about the things it has been in contact with.
Our eyes convert the radiation energy into nerve signals that carry this information to the brain. In this way we learn about things the world around us, even if they are far away. So eyes are nature's remote sensing instruments.
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Our sun seen from the SOHO observatory. The sun disk observed in the EIT instrument, which measures in the ultraviolet (left). The sun's corona (right) is measured in the visible part of the spectrum.
Light as photons
Most of the light here on Earth comes from the sun. To get here it travels through roughly 150 million kilometres of empty space in little packets of energy called photons.
A photon is a group of electromagnetic waves with one frequency and wavelength. Photons of visible light have wavelengths between 400 and 700 nanometres, and vibrate at between 430 and 750 trillion (1012) cycles per second.
Sunlight contains millions of photons of many different wavelengths - not all visible. There is also ultraviolet (which can give you sunburn) and infrared (which you feel as heat).
The sun's radiation spectrum. The sun emits most of its light in the visible part of the spectrum. The energy peaks in the blue-green, just the the right wavelengths for photosynthesis by green plants.
Unlike sunlight laser light contains photons of a single frequency and wavelength. They travel in a narrow beam of very intense light.
The energy spectrum of a laser shows just one single, very narrow peak.
These are all part of the electromagnetic spectrum, which extends from very short wavelengths of 10-3 nm (smaller than an atom) to radiowaves with wavelengths of more than a kilometre.
Light and energy
Photons are packets of energy. A beam of sunlight contains large numbers of photons, each with its own energy. The intensity of a beam of light is a measure of the total energy that passes through an area of space in a unit of time.
By plotting the intensity of the light against its wavelength, you get what is known as a radiation spectrum. If you know what to look for, the radiation spectrum contains much information about the source of the light.
Optical radiation
Broadly speaking light is that part of the electromagnetic spectrum known as optical radiation. This includes:
- Visible light (Vis) - the wavelengths we humans can see; that is the colours of the rainbow from violet to red.
- Ultra violet (UV) - shorter wavelengths than visible violet.
- Near infrared (NIR) - longer wavelengths than visible red.
UV and NIR interact with the world in a similar way to visible light, and can be measured by the same type of sensor. By extending our light measurements to include more than just visible light, we often get useful extra information.
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Last update: 28 November 2008 |
Contact: o4s@noc.soton.ac.uk |