The speed of electromagnetic waves in a vacuum is the same as the speed of light. It can be measured by finding the frequency and wavelength of two different waves, and then by that correlation, the speed of the waveform.
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Answer: Electron flow / Electric current
Whenever light goes from transparent to another medium or from rarer to denser medium or vice versa, it's speed changes and hence light bends.
PCBs were found in greater concentrations in lakes where more salmon spawned.