When would the velocity of a wave traveling in a medium change?

What produces light and all other electromagnetic waves? a) the photoelectric phenomena b) heated metals c) charged particles d) radioactive samples (pls explain if you can : )

Bernie drove home from a vacation in 1.5 hours. he traveled a total distance of 60 miles. what was his average speed in miles per hour? a. 60 mph b. 90 mph c. 40 mph d. 55mph

Which subatomic particle has a negative charge