Hydrogen Emits Light at Certain Discrete Frequencies

Multiple Choice

Hydrogen emits light at certain discrete frequencies. One particular emission has a frequency of 4.57 × 10¹⁵ s^-1, as measured in the frame of reference of the hydrogen atom. Light from this emission is observed coming from a star that is moving away from Earth at 0.900c. What is the observed frequency? Note that, in addition to time dilation, one must take into account that successive wavefronts have to travel longer distances because the star is moving away (Doppler Effect) .

A) 1.99 × 10¹⁵ s^-1
B) 7.62 × 10¹⁵ s^-1
C) 1.99 × 10¹⁶ s^-1
D) 1.05 × 10¹⁵ s^-1
E) 0.90 × 10¹⁵ s^-1