To reduce friction between two surfaces, you have to
make the surfaces smoother.
The best way to do that is to introduce a fluid between them,
like grease, oil, or air.
A. is the answer for this question
“a point at which rays of light, heat, or other radiation meet after being refracted or reflected.” Meaning multiple light rays or heat (and other forms of radiation) are all being refracted or reflecting to a certain point
Radio waves, gamma-rays, visible light, and all the other parts of the electromagnetic spectrum are electromagnetic ... The different types of radiation are defined by the the amount of energy found in the photons.
