You look at a photo on your phone and suddenly the bottom of the yellow image becomes green. Explain what is missing and why it went from yellow to green. 

Expert Answers

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The most likely explanation is that your phone has a low enough resolution that individual subpixels are important, and the bottom of the image ended at a green or blue subpixel and therefore didn't have enough red. The red subpixels could also be damaged, but that sort of specific, precise damage is less likely.

Every pixel on an LCD screen is actually three subpixels, one red, one blue, and one green. Each of these is calibrated to stimulate a specific set of vision cells in the human retina called cone cells, which come in three corresponding types, one that primarily senses red, another that primarily senses green, and a third that primarily senses blue. (Colorblind people are usually missing one type of cone cell, most frequently the red-sensing one.) In this way, the image appears to have the color it would have in the real world, at least to our eyes. To the eyes of a species such as bees that have ultraviolet-sensing cells, the colors on a screen would look wrong.

Rendering methods for low-resolution screens actually take advantage of this, by turning on some subpixels and not others to smooth lines and edges. This adds a bit of color even to black-and-white images, but usually that is less of a problem than the hard, jagged edges that would otherwise result. Often, the color is imperceptible anyway.

Yellow is shown on an LCD as a roughly equal combination of red and green, but if a yellow object covers the whole screen, it may hit the edge in such a way that the last green subpixel is triggered but not the corresponding red subpixel. Thus, the very edge of the object may appear green.

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