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
"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
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