As the capabilities of computers grew, it became more and more important to have an easy means of interacting with them: providing inputs and seeing outputs. Paper was used to this end for many years, but another technology already existed in television and was co-opted for computer displays: the cathode ray tube.
Cathode ray tubes (usually abbreviated CRTs) are pretty similar in construction to the vacuum tube, the early logic switch component. Inside a glass tube which has had the air removed, we have a filament that can emit electrons when heated. But instead of the electron collector in the vacuum tube, there is a screen which immediately emits light when struck by electrons, a behavior called fluorescence. Now, in order to create a two-dimensional image, you need a very focused electron beam, as well as some way to move the electron beam across the screen. You can use an electric field for both these tasks, to create a focused beam that can be scanned around the screen in order to light up different sections. But the larger the screen, the deeper the display has to be to give the beam enough room to scan the screen with reasonable resolution. This accounts for the large size of CRT monitors, which you either remember or occasionally spot in movies with computers between about 1980 and 2000.
The main display technology that displaced the CRT is based on a pretty amazing material called liquid crystals (and you may recognize the acronym LCD, which stands for liquid crystal display). The name comes from the fact that if you create a collection of small crystals that have one axis significantly longer than the others, this assembly of crystals acts like a hybrid of two phases of matter, liquids and elongated solid crystals. The crystals themselves can move around and are not fixed in one place, but the orientation of one crystal can affect those around it, and it’s possible to have liquid crystal assemblies with various degrees of ordering. The key to using liquid crystals in a display is controlling their orientation in order to control what orientations of light can pass through them. Using an external electric field, we can either align the crystals with the direction of motion for specific kinds of light passing through, which makes the liquid crystal cell transparent to that light, or we can orient them perpendicular to the direction of motion, which causes them to absorb the light so that none is transmitted. If you have an array of light emitting diodes, with each diode covered by a liquid crystal cell, then you’ve created a basic liquid crystal display.
Now, one thing to note about both the above technologies is that they are emissive: they generate light, so they can be read in a darkened room, used as a flashlight or as a source of eyestrain. But this also means that whenever an emissive display is switched on, it’s consuming power, and when it’s switched off there is nothing displayed. Contrast this with that ancient display technology, paper, which is reflective: it can only be read with enough ambient light bouncing off its surface, and once created it costs nothing to display its content continuously. Of course, you can’t rewrite what’s on paper very easily, so you end up using a lot of it. Additionally, the same natural resources we use for paper are in high demand for many other things, and transporting a library as a huge collection of papers is quite onerous, and for these and many other reasons the move to electronic displays is thus pretty strongly motivated. But, is there a way to make a reflective electronic display?
If you have ever used a Kindle, or any other device with an e-ink display, then you know the answer is yes. The surface of an e-ink display has millions of tiny cells, which contain miniscule beads that are either white and positively charged, or black and negatively charged. An electric field can be applied to each cell, bringing either white or black beads to its surface. Light coming into the surface from outside sources is then either reflected off the white beads or absorbed by the black beads, creating a rewritable reflective display.
Of course, paper is still the cheapest display technology by far. But in some cases, for example when you want to make small corrections in content, display something outside, ship information somewhere, or carry an entire library on something the size of a pad of paper, newer inventions are getting the edge on paper.