What is different about light compared to other waves such as sound waves or waves on water?
a. If light passes through two closely-spaced openings, the waves leaving the openings spread out and interfere with each other.
b. If we measure the speed of light in a vacuum, we get a different value depending on how we or the light source are moving.
c. Light does not need any medium to travel, whereas sound or water waves need some medium to travel.
d. If a source of light is moving relative to us, the light waves emitted in the direction of motion of the source move faster than if the source were at rest, increasing the observed wavelength of the waves.