Class 12 physics presentation on doppler effect.

1. waves
waves

2. waves
In this module, we learn about

3. waves
The phenomenon of apparent change in frequency of the source of
sound due to the relative motion between the source and the observer is
defined as Doppler effect.
Doppler Effect:
Doppler Shift:
The change in the frequency (n) and apparent
frequency of sound heard (n) is called Doppler
shift.
n = n – n

4. waves

5. waves
(L + VsT0)
S1
L
O
Vs
S2
VsT0
Vs
t=0 t= T0
(L – V0T0)
O2
L
S
V0
O1
V0T0
V0
t=0
t= T0
When source is moving away from
stationary observer
When observer
is approaching
the stationary
source

6. waves
 Let us choose the convention to take the
direction from the observer to the source as
the positive direction of velocity.
Case1:
Apparent frequency when the source is in motion and observer is at rest:
 Consider a source S moving with velocity
VS and an observer who is stationary in a
frame in which the medium is also at rest.

7. waves
Doppler effect (change in frequency of wave)
detected when the source is moving and the
observer is at rest in the medium.
L
S1
L + nsT0
O
n0
S2
nsT0

8. waves
 Let the speed of a wave of angular frequency  and period To, both
measured by an observer at rest with respect to the medium, be V.
 We assume that the observer has a detector that counts every time a
wave crest reaches it.
 As shown in figure, at time t = 0 the source
is at point S1, located at a distance L from
the observer, and emits a crest.

9. waves
 At time t = T0 the source has moved a distance VsT0 and is at point S2,
located at a distance (L + VsT0) from the observer.
 At point S2, the source emits a second crest.
 This reaches the observer at time
t2 = T0 +
(L + VsT0)
V
 This reaches the observer at time t1 =
L
V
.

10. waves
At time nT0, the source emits its (n+1th ) crest and this reaches the
observer at time.
tn+1 = nT0 +
(L + nVsT0)
V
The interval between 1st crest and (n+1)th crest
to reach the observer.
t = tn+1 – t1 = nT0 +
(L + nVsT0)
V
−
L
V
= nT0
V + Vs
V

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The observer’s detects crests in time interval t then the observer
records the period of the wave as T =
t
n
.
T = T0 𝟏 +
Vs
V
(1)
Equation (1) may be rewritten in terms of the
frequency n0 that would be measured if the
source and observer were stationary, and the
frequency n observed when the source is
moving, as
n = n0 𝟏 +
Vs
V
−𝟏
(2) T=
1
n

12. waves
If Vs is small compared with the wave speed V, taking binomial
expansion to terms in first order in Vs/V and neglecting higher power,
Eq.(2) may be written as
n = n𝟎 1 −
Vs
V
(3)
For source approaching the observer, we
replace Vs by –Vs and we get,
n = n𝟎 1 +
Vs
V
(4)
n > n𝟎
n < n𝟎

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By equation (3), the observer thus measures a lower frequency when the
source recedes from him as compared to when he is at rest.
By equation(4), the observer measures higher frequency when the source
approaches him.

14. waves
Thank you…