Question

The human threshold of pain is about how many decibels?

	140 dB
	120 dB
	100 dB
	80 dB

When astronomers observe the spectra of distant galaxies, they notice that the hydrogen emission lines are shifted noticeably toward the red end of the visual spectrum, a phenomenon called red shift. Red is the lowest frequency of visible light. What does red shift indicate about the movement of the distant galaxies?

	They are moving away from us at high speed.
	They are motionless, while we are moving toward them at high speed.
	They are moving in the same direction as we are.
	They are moving towards us at high speed.

Answer 1

Some sources says that 120 dB as the pain threshold and define the audible sound frequency range which ending at about 20,000 Hz which is the threshold of hearing.

If an object is moving away from us then it's frequency decreases it means it is moving towards the red side of the visible spectrum. This is the reason it is called as redshifted. If it moves towards us then it's frequency increases this is called as blue shifted.

So the correct answer is a. Object is moving away from us at higher speed.