The apparent change in the frequency due to the relative
motion of source and observer is called Doppler Effect. We can experience the change
in the frequency only when there is a relative motion. The original frequency
of the source is not actually changing. Due to the relative motion it is
appearing like changing and that’s why it is called as apparent change in frequency.

We can derive the equation for the apparent frequency in
different possible cases. When the observer is in the motion he will receive
more number of the waves than when he is in the state of rest. It is simply
because waves are not only crossing him and he is also crossing the waves.

When the observer is crossing the stationary source there
will be difference in the frequencies. If the observer is approaching the
apparent frequency increases and when the observer is receding the apparent
frequency decreases. The difference between these frequencies can be heard like
beats to the observer. We can calculate the number of the beats as shown below.

There will be apparent change in the frequency even when the
observer is the state of rest and the source is moving towards the Observer.
Here is the source is approaching the observer, its wavelength towards the
observer decreases and hence frequency increases. We can derive the equation
for the apparent frequency in this case as shown below.

When the observer is in the state of the rest and source is
approaching him, apparent frequency increases. When the source is moving away
from the stationary observer, apparent frequency decreases. The difference
between these two frequencies can be heard like the beats to the observer. We
can derive the equation for the number of the beats as shown below.

When a source is revolving around the stationary observer, he
is not going to have any Doppler effect experience. It is simply because there
is no relative motion between the source and observer. No component of the
velocity of the source is acting towards the observer and hence we cannot find any
change in the frequency.

If the source is moving in the circular path and observer is
far away from the Centre of the circular path, he can hear apparent frequency with
different possible frequencies. When the source is moving away from the
observer, apparent frequency decreases and vice versa. We can write the
equation is as shown below.

When a source is moving by making an angle to the direction of the observer, still there
will be apparent change in the frequency due to the component of the velocity
of the source towards Observer. We can write the equation for it as shown
below.

**Problem and solution**

**Problem and solution**

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The frequency of a radar is 780Hz. The frequency of the reflected wave from an aeroplane is increased by 2.6Khz then what is the velocity of plane?????

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