Why does a movie projector have a convex lens?

How do we measure or think of speed in the united states?

State the below theories and give the name of the writer of the theory 1. wave theory 2. quantum theory 3. corpuscular theory 4. electromagnetic theory

Select the best definition for wavelength. the height of an oscillating electromagnetic wave the rate at which electromagnetic waves oscillate. the distance between two crests of an electromagnetic wave. the oscillations of electric and magnetic fields select the best definition for frequency. the oscillations of electric and magnetic fields. the height of an oscillating electromagnetic wave. the distance between two crests of an electromagnetic wave. the rate at which electromagnetic waves oscillate