Question

Explain how bats use ultrasound to catch a prey.

Answer 1

Hint: We need to know about the reflection of sound waves when we buy some objects. Then, we will know a new effect known as the Doppler effect. After understanding this effect, the preying techniques of a bat can be better understood.

Formula used:
$\nu_{eff}=\nu \dfrac{v+v_o}{v-v_s}$

Complete step-by-step solution:
We all know that bats cannot see very well. So, they totally depend on their ultrasound techniques to catch a prey. They use the reflection of sound waves and the Doppler effect in order to do this. We all know that light is reflected when it comes across the surface of another medium. The case is almost the same for a sound wave. It can also get reflected when it comes to a surface of some kind of object. The frequency of the sound wave does not change in case of reflection. But if the reflector or the observer is moving with a particular velocity, the effective frequency may appear to be changed. If $v_o$ and $v_s$ be the velocity of the observer and the source respectively, (in absence of any reflector), and if $\nu$ be the actual frequency of the sound wave, by Doppler effect, the effective sound frequency will be,
$\nu_eff=\nu \dfrac{v+v_o}{v-v_s}$
Now in case of a bat, the source itself is the observer. Let the velocity of the bat be $v_b$. Now, Let’s imagine its prey to be at rest. So, if it emits sound waves of frequency $\nu$, the effective frequency that it will get back is,
$\nu_eff=\nu \dfrac{v+v_b}{v-v_b}$
So, the bat will know if there is anything on its way or not. But it doesn’t know its exact position. If the prey moves with a particular velocity, then the bat will be able to know it more precisely. But in that case the formula will be much complicated which is not given here.

Additional information:
Doppler effect is equally valids for light waves as well. Using this effect, the scientist calculates the velocity of different stars.

Note: Remember that the frequency of sound is not changed by reflection only. To change the effective frequency either the reflector or the source or both has to move. Don’t use relative velocity between the source and the object, because sound travels in medium and both velocities must be considered separately.