IB Physics-Theme C--Paper-2 Qc#8-9-Standing waves-interference of waves and Doppler effect

This question is about sound waves.
A whistle on a steam train consists of a pipe that is open at one end and closed at the other. The sounding length of the whistle is 0.27 m and the steam pressure in the whistle is so great that the third harmonic of the pipe is sounding. The speed of sound in air is .
Show that there must be a node at a distance of 0.18 m from the closed
end of the pipe.
b. Calculate the frequency of the whistle sound.
c. . The train is moving directly away from a stationary observer at a speed
of while the whistle is sounding. Calculate the frequency heard
by the observer.
Qc#2
This question is about interference of light.
Coherent monochromatic light is incident on two narrow slits and a distance
apart. A screen is placed a distance from the slits. An interference pattern of
bright fringes and dark fringes appears on the screen. The central maximum is at Q.
State one way to ensure that the light incident on the slits is coherent.
b. Light emerging from and reaches the screen. Explain why the
screen appears dark at point P.
When red light of wavelength 660 nm is used the first fringe at P subtends an angle 0.0045 rad from midpoint of and .
c. Determine the change in angle when blue light of wavelength 440 nm is
used.
Topic 4 and topic 9 oscillation and waves: