When the source is moving towards the stationary listener, the expression for apparent frequency is
\(n'=\left( \frac { v }{ v-{ v }_{ s } } \right) n\)
\(1000=\left( \frac { 330 }{ 330-50 } \right) \times n\)
\(n=\left( \frac { 1000 \times 280 }{ 330 } \right)\)
n = 848.48 Hz
The actual frequency of the sound is 848.48 Hz. When the source is moving away from the stationary listener, the expression for apparent frequency is
\(n'=\left( \frac { v }{ v+{ v }_{ s } } \right) n\)
=\(\left( \frac { 330 }{ 330+50 } \right) \times 848.48\)
= 736.84 Hz

When the source is moving towards the stationary listener, the expression for apparent frequency is
\(n'=\left( \frac { v }{ v-{ v }_{ s } } \right) n\)
= \(\left( \frac { v }{ v-\left( \frac { 1 }{ 10 } \right) v } \right) n=\left( \frac { 10 }{ 9 } \right) n\)
= \(\left( \frac { 10 }{ 9 } \right) \times 90=100\) Hz
n'=100 Hz

a) Ultrasonic Vibrations:
(i) The Vibrations produced by sound waves with a frequency greater than 20 KHz are called ultrasonic vibrations.
(ii) Human ear cannot detect these waves.
b) Uses of Ultrasonic vibration:
(i) Ultrasonic devices are used to detect objects and measure distances.
(ii) Ultrasound imaging or sonography is often used in medicine.
(iii) In the non destructive testing of products and structures. Ultra sound is used to detect invisible flaws.
c) Certain creatures like mosquitoes, dogs, bats and dolphins can detect these waves.

Factors affecting velocity of sound:
(i) The following factors affect the velocity of sound waves.
Effect of density:
(i) The velocity of sound in a gas is inversely proportional to the square root of the density of the gas.
(ii) Hence, the velocity decreases as the density of the gas increases.
\(V \propto \frac{1}{\sqrt{d}}\)
Effect of temperature:
(i) The velocity of sound in a gas is directly proportional to the square root of its temperature.
(ii) The velocity of sound in a gas increases with the increase in temperature. \(v \propto \sqrt{\mathrm{T}}.\)
(iii) Velocity at temperature T is given by the following equation:
\(\mathrm{v}_{\mathrm{T}}=\left(\mathrm{v}_{0}+0.61 \mathrm{~T}\right) \mathrm{m} \mathrm{s}^{-1}\)
(i) Here, v_o is the velocity of sound in the gas at \(0^{\circ} \mathrm{C}\).
(ii) For air, \(\mathrm{v}_{\mathrm{o}}=331 \mathrm{~m} \mathrm{~s}^{-1}\).
(iii) Hence, the velocity of sound changes by 0.61 ms^-1 when the temperature changes by one degree celsius.
Effect of relative humidity:
(i) When humidity increases, the speed of sound increases.
(ii) That is why you can hear sound from long distances clearly during rainy seasons.

b) Light
The light wave has a longer wave length. Because, it has much greater speed.