Wave–particle duality
Physicists who studied light in the 1700s and 1800s had an argument about whether light was made of particles or waves. Light seems to act like both. At times, light seems to go only in a straight line, as if it were made of particles.  But other experiments show that light has a frequency and wavelength, just like a sound wave or water wave.  Until the 20th century, most physicists thought that light was either one or the other, and that the scientists on the other side of the argument were simply wrong.