Doppler's effect can help in deciding whether a star is approaching or receding from the earth. Explain how.

How far is the sun from the earth?

How is the electro-chemical series helpful in determining whether a metal can displace hydrogen from acid or not ? Explain.

Doppler's effect in sound is different depending on whether the source or observer is in motion. However in case of light, the Doppler's shift is the same irrespective of whether the source or observer is in motion, explain how?