Small forces at natural frequency cause large oscillations. Thanks to the several folks who talkd me about problems. Phase of a wave - no definition of absolute phase, only relative In phase and out of phase. Lagging in phase. Draw linear scale along x-axis.
Electronic circuits can modify the natural frequency.
Radiation for humans - visible portion Show full spectrum. Generation of radiation by making something hot Standard way of creating visible radiation. Must make charges wiggle. So heat up material, charges inside wiggle. Scientific ideal - blackbody radiation.