The engine of a train sounds a whistle at frequency $\nu$. The frequency heard by a passenger is

An observer moves on a circle as shown in the figure and a small sound source is at $S$. Let at ${\upsilon }_1$, ${\upsilon }_2$, ${\upsilon }_3$ be the frequencies heard when the observer is at $A$, $B$, and $C$, respectively.

A star is emitting yellow light. If it is accelerated towards earth, then to an observer on earth, it will appear

When the light source is moving towards an observer,