Sight is remote sensing

We see the world around us because our eyes are sensitive to light - visible electromagnetic radiation. Because of this we can see things that are far away - even distant stars.

Radiation from the sun 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 the world around us, even if they are far away. You could say that eyes are nature's remote sensing instruments.

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).

These are all part of the electromagnetic spectrum, which extends from very short wavelengths of 10^-3 nm (smaller than an atom) to low energy radiowaves with wavelengths of more than a kilometre.

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.

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.

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.

Our sun seen from the SOHO observatory.
Left: The sun disk observed in the EIT instrument, which measures in the ultraviolet.
Right: The sun's corona is measured in the visible part of the spectrum. (Click on the images for a larger version and more information.)

The sun's radiation spectrum. The sun emits most of its light in the visible part of the spectrum, peaking 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.