What can scientists measure to provide evidence that electrons can move from one energy level to another?

Several mountains together form a mountain . a few of these combined form a mountain system. several systems combined form a mountain , which can stretch thousands of miles in length.

As seen in a us weather map, the h (or high) and l (or low) symbols indicate regions where surface air pressures are high or low, respectively, compared to surrounding areas. an h is plotted where the air pressure is highest and an l is plotted where the air pressure is lowest. proceeding outward and away from the center of a high pressure system, the air pressure "

Which wave would cause destructive interference at every point of the wavelength and completely cancel out the wave?