Question

A nearby galaxy emits distinctive electromagnetic radiation which is detected by optical telescopes at the Keck Observatory in Hawaii. Given that the measured light spectrum is shifted towards higher frequencies, which one of the following statements is true?

A. The galaxy's relative motion is toward the Earth

B. The change in frequency comes from the conservation of total angular momentum of the Earth and the galaxy.

C. The galaxy's relative motion is away from the Earth.

D. The distance between the galaxy and the Earth is constant.

E. The change in apparent light frequency is due to the index of refraction of the earth's atmosphere.

The answer is A but I don't understand why.

Answer 1

Here, higher frequencies is the keyword.

There are two type of shifts.

Red shift and Blue shift

The highest frequencies of light are at the blue end of the visible spectrum whereas lower frequencies are at red end. In this way,  An object travelling towards us will appear blue-shifted and object travelling away from us will appear red-shifted.

As the question mentions higher frequencies, it means the object is moving towards us, means galaxy's relative motion is toward the Earth.

Therefore,

Option A is correct.