MOTION MOUNTAIN

displaying images 189
displaying images
F I G U R E 139 A cathode ray tube in older
televisions: the first way – now obsolete –
to produce changing colour images using
electric signals. Television tubes emit an
electron beam, deflect it, and generate
light by electroluminescence on a
coloured screen covered with patterned
phosphors.
Systemsthatdisplayimagesareofimportanceintechnical devices and, to a smallerdegree,innature.Innature,thesedisplaysareoftwotypes:Thefirsttypeisusedbysquids
living in shallow water: they are able to produce moving colour patterns on their skin,
and they use these patterns to confuse prey. The second type is found in the deep sea,
where there is no ambient light: there, many living beings produce moving light displays
to attract prey or to confuse predators.
In short, images can be generated by changing surface colours – passive displays – or
by emitting light. Also human-madesystemscandivided into these two classes.
At present, the most common passive displays are liquid crystal displays – or LCDs –
and electronic ink displays.The former are used in watches and mobile phones, the latter
in electronic book readers.
The most common light emitting displays are the dated cathode ray tube, plasmadisplays,
the light emitting diode displays and projection displays. Thesedisplays are used
mostlyinentertainmentdevices.
Hopping electrons and the biggest disappointment of the
television industry
It is well known that when an electric field in a vacuum points along a glass surface,electronscan
hop along theglasssurface.The general effectis shownin Figure 140; usually,
the effect is unwelcome. Among others, the hopping effect is responsible for sparks in
vacuum systems that contain high voltage. To avoid the effect, the glass insulators on
high voltage lines have complex shapes.
When this effect was studied in more detail, it turned out that reasonably low electricfieldsaresufficienttocreatesizeable
electrichoppingcurrentsinhollowglasstubes
with an internal diameter around a millimetre. The low elctric field can also lead elec-
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net