White light is made up of blue, red, and green light. When white light strikes red glass, the green and blue light's frequency matches the natural frequency of the electrons in the red glass. Resonince occurs and the electrons in the red glass vibrate more intensely than they normally would, causing the green and blue light to be absorbed by the glass. Due to the absorption, thermal energy is created and the glass heats up. The red light does not get absorbed however, instead the red light is transmitted. The red light causes electrons to vibrate and make the neighboring electrons vibrate until red light is transmitted out the other side.
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