The distance between a lens or mirror and its focal point. A point, or focal point, where a lens or mirror directs light . The number of oscillations, or cycles, within one second.
Frequency can refer to the number of times a piano or guitar string vibratesto produce a sound.
When the electron vibrates about 600 thousand billion times in one second, it emits green light. Ocean waves also have a frequency that you can measure by counting the number of times a floating buoy bobs up and down.