Question

A driver in an automobile hears a siren behind her and pulls over to let a fire engine pass. The sound of the siren changes as the fire truck approaches, is beside her, and passes her. Which of the following is/are true under these conditions?
I. The pitch of the sound increases as the fire truck approaches.
II. The wavelength of the sound increases as the fire truck approaches.
III. The wavelength of the sound increases after the fire truck passes.

• A. I only
• B. II only
• C. I and III only
• D. II and III only
• E. I, II, and III only

Answer 1

The correct option is A I only

in doppler effect the wavelength of the sound waves doesn't change. $f=\frac{\upsilon \pm {\upsilon }_o}{\upsilon \mp {\upsilon }_r}{f}_0$. Frequency of the sound increases as the truck approaches,so as the pitch.