There are several mechanisms to create/cause electromagnetic waves:

• Macroscopically: accelerating charge (just moving at constant velocity is not enough; this is why we drive antennae with an alternating current pushing electrons back and forth.)
• Microscopically: spontaneous (i.e. without cause); think of exicted atoms emitting a photon. This is what happens in phosphorous materials that you shone light on.
• Microscopically: stimulated by an already present wave; this is what happens in a laser when a photon causes an excited atom to emit another photon

There are likely other mechanisms that my physics colleagues can enumerate.

There are several mechanisms to create/cause electromagnetic waves:

• Macroscopically: accelerating charge (just moving at constant velocity is not enough; this is why we drive antennae with an alternating current pushing electrons back and forth.)
• Microscopically: spontaneous (i.e. without cause); think of exicted atoms emitting a photon. This is what happens in phosphorous materials that you shone light on.
• Microscopically: stimulated by an already present wave; this is what happens in a laser when a photon causes an excited atom to emit another photon

There are likely other mechanisms that my physicist colleagues will enumerate.