Why were the first lightbulbs (I assume from vague general knowledge) yellow and what property changes inside lightbulbs so that we can have blue, green or red colours.
In: Physics
For a incandescent light bulb that is red you would put filter around the bulb to create the light. So for example put a layer of red plastic around the bulb and it will let red light trough and absorb all other colors of light
For LED light that is common today we have build LEDs that emit light in different colors depending on what type of material is used in the semiconductor. There is also white LED. A LED lamp that you can control the color of contain red, blue and green leds that you can change the brightness of each color individual.
Because of how our eye work you can create almost any color with red, blue and green light the same way as it is done on the screen you read this. So in a way a lamp you can change the color off is a one pixel display.
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