The document discusses the Doppler effect, which is the change in frequency of a wave due to the movement of the source relative to the observer. It describes how the Doppler effect causes changes in the observed frequency of sound waves from an approaching or receding ambulance siren. It then gives the example of Doppler radar guns used by police to detect speeding vehicles by measuring changes in radar frequency caused by the Doppler effect. The formula provided calculates the perceived frequency based on the actual frequency, speed of sound, and speed of the observer/source.

1. Lorem Ipsum Dolor
Doppler Effect LO5

2. History
❖ Doppler effect is named
after the Austrian physicist
Christian Doppler
❖ Christian Andreas Doppler
(29 November 1803 – 17
March 1853)

3. What is the Doppler Effect ?
❖ The Doppler Effect is an increase or decrease in the
frequency of sound waves as the sound-producing
object moves toward or away from an observer.

4. Doppler Effect Example:
Ambulance Siren
❖ As an ambulance approaches, the siren's sound waves
become more frequent (compressed together).
❖ As the ambulance drives away, the siren's sound waves
become less frequent (stretched apart).

5. Uses of Doppler Effect
Doppler Radar Gun - Police Speed Detector
❖ A radar speed gun (also radar gun and speed
gun) is a device used to measure the speed of
moving objects. It is used in law-enforcement to
measure the speed of moving vehicles and is
often used in professional spectator sport, for
things such as the measurement of bowling
speeds in cricket, speed of pitched baseballs,
athletes and tennis serves.
❖ It measures the speed of the objects at which it
is pointed by detecting a change in frequency
of the returned radar signal caused by the
Doppler effect, whereby the frequency of the
returned signal is increased in proportion to the
object's speed of approach if the object is
approaching, and lowered if the object is
receding.

6. ❖ The Doppler effect can be observed for any type of wave - water
wave, sound wave, light wave, etc. We are most familiar with the
Doppler effect because of our experiences with sound waves.
❖ Perhaps you recall an instance in which a police car or
emergency vehicle was traveling towards you on the highway. As
the car approached with its siren blasting, the pitch of the siren
sound (a measure of the siren's frequency) was high; and then
suddenly after the car passed by, the pitch of the siren sound was
low. That was the Doppler effect - an apparent shift in frequency
for a sound wave produced by a moving source.

7. Formula
❖ The equation that describes this effect can be used whether the source is approaching or receding from the
observer
❖ perceived frequency = actual frequency(speed of sound +speed of observer/speed of sound-speed of
source)
❖ or f = fo(V+Vo)/(V-Vs)
❖ Here, fo refers to the actual frequency being emitted by an object, while f is the frequency heard by the
observer as the source approaches or recedes. If a source approaches, the perceived frequency will be
higher than the actual frequency. If a source recedes, the perceived frequency is lower than the actual
frequency.
❖ In order for this equation to work properly, there is a standard convention to which you must adhere
whenever solving Doppler exercises.
❖ Vo is (+) if the observer moves toward the source.
❖ Vo is (-) if the observer moves away from the source.
❖ Vs is (+) if the source moves toward the observer.
❖ Vs is (-) if the source moves away from the observer.

8. Example Question
❖ Sitting on the beach, Sunny finds herself beneath the flight path of the
airplanes. What frequency will Sunny hear as a jet, whose engines emit sound
at a frequency of 1000. Hz, flies toward her at a speed of 100.0 m/s?
❖ Solution: First draw a diagram of the situation. Notice in the calculation below
that Sunny is sitting at rest and the plane is approaching. Therefore, the
source is moving toward the observer. The observer remains stationary.

9. ❖ In the previous example, what frequency will Sunny observe as the jet
travels away from her at the same speed?
❖ Solution: Again, draw a diagram of the situation. This time, the source
is moving away from the observer, so the value for vs must be
negative.

10. Example Question
❖ The following figure shows pressure vs. time for a sound wave when the source is stationary with
respect to the observer.
❖ Which of the following best represents the sound wave if the source is moving towards the observer?
Answer: d