Question

What do you call a device that produces a very focused beam of visible light of just one wavelength and color?

Answer 1

laser

Did you ever see a cat chase after a laser light, like the one in Figure below ? A laser is a device that produces a very focused beam of visible light of just one wavelength and color. Waves of laser light are synchronized so the crests and troughs of the waves line up. The diagram in Figure below shows why a beam of laser light is so focused compared with ordinary light from a flashlight.