The world viewed in near IR light.
The world would not look the same if we were able to see near IR light instead of visible light.
visible light reflected by the tree
and photographed with normal film
 near IR light reflected by the tree
and photographed using near IR film


The picture at left was taken using normal film, film that is sensitive to visible light.  The picture at right used near infrared film.  In both pictures we are looking at sunlight that strikes the tree or the ground and is reflected toward the camera where it can be photographed (i.e. these aren't photographs of visible light being emitted by the tree or the ground). 

Some things change, some things stay the same.  The tree and the grass at left are green and relatively dark (they reflect green light but absorb the other colors of visible light).  The tree and grass at right are white, almost like they were covered with snow.  The tree and grass are very good reflectors of near infrared light.  Clouds are also good reflectors of near IR light, they appear white in both images.  Here are many more images taken with infrared film.



Photographs of the ground taken from an air plane using ordinary film at left (responds to visible light) and near infrared film at right.  Notice how much clearer the river is in the picture at right.  The IR photograph is able to "see through" the  haze.  The haze is light being scattered predominantly by air molecules.  You may remember from the 1S1P topic on scattering that air molecules scatter shorter wavelengths in much greater amounts that longer wavelengths.  Near IR light is not scattered nearly as much as visible light.

You wouldn't have seen the tree or the river if the photos above had been taken at night.  That is because they are photographs of reflected sunlight. 




This is a picture of the far IR light that is emitted by a house (source of this image).  You'd see this during the day or night, sunlight doesn't need to be present. 

Remember that the amount of energy emitted by an object depends strongly on temperature (temperature to the 4th power in the Stefan-Boltzmann law).  Thus it is possible to see hot spots that emit a lot of energy and appear "bright" and cold spots.  Photographs like these are often used to perform an "energy audit" on a home, i.e. to find spots where energy is being lost.  Once you locate one of these hot spots you can add insulation and reduce the energy loss.  This photograph has been color coded.  Reds and orange mean more intense emission of IR radiation (warmer temperature) than the blues and greens.  The reds show you were energy is being lost, escaping from the warm interior of the house (often through poorly insulating windows).  Many of the roof tops are blue, they are cool.  There is probably a lot of insulation in the attic and little energy is being lost out the roof.

Later in the semester we will looking at satellite photographs of clouds.  Satellites take pictures of both the visible light reflected by clouds and also the IR radiation emitted by clouds.