# Transverse Relativsitic Doppler Effect

I have a question regarding relativistic Doppler effect. The following page is taken from "Introduction to Special Relativity" by Robert Resnick.

My question is why Eq. (2.25b) needs to be rearranged to analyze the effect? The equation that is finally used has $$\nu'$$ that belongs to the moving frame whereas $$\theta$$ belongs to the stationary frame. What is the rationale behind this?

Relativity is not my field of expertise. Excuse me if this question sounds stupid. Thanking in advance.

• What has your question got to do with the transverse relativistic Doppler effect? Jun 6 at 14:37

• In that case why not Eq. (2.25a)? Thanks for your answer and edit. Because, depending on which form you choose, factor $\gamma$ changes the position. And, the position of this factor determine blueshift/redshift. Jun 6 at 13:28