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   Infrared Sunset
   
  
 
 











On a flight from Geneva to Warsaw, Philip Laven (see his excellent Optics of a water drop ) saw the low sun disappear behind clouds. But his camera viewfinder showed a different scene - a purple sunset. Image ©Philip Laven

"The amazing thing was that, with my unaided eyes, the sun was completely invisible.  However, the purple sun remained clearly visible on my camera for about 2 minutes "

Digital cameras are sensitive to near infrared radiation. Here, the water droplets in the cloud bank have scattered and obscured the direct visible light of shorter wavelength but still transmit a small fraction of the sun's infrared.

The solar image is quite flattened because the picture was taken at fairly high altitude and rays from the sun then undergo differential refraction twice - on entering and leaving the dense lower atmosphere.

    
The peculiar violet colour is an artifact of the camera sensor. CCD sensors respond to infrared in the range 750-1150 nm (visible light is about 400 - 700nm). Cameras therefore have to contain infrared blocking filters but these usually allow some residual rays through. The infrared sensitivity depends on the camera, test your own by pointing it at your TV remote control from a distance of 6 inches or so.