PhysicsICSE Class 10 · Reflection of Sound and Echoes

SONAR and Uses of Echoes: Bats, Dolphins, Fishermen and Medicine (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

SONAR (Sound Navigation And Ranging) sends pulses of ultrasonic waves into water and times their echoes from the sea bed, a submarine or a shoal of fish; the distance is d = vt/2. Bats and dolphins locate objects the same way, fishermen use echo sounders, and doctors use ultrasound echoes to image organs.

The echo travels down and back, so the time t covers twice the depth; halve vt to get the distance d.

Illustration drawn for QuestionBuddy.

Understand SONAR and Uses of Echoes

Ultrasonic waves. Human ears hear sound from about 20 Hz to 20 kHz. Sound below 20 Hz is infrasonic; sound above 20 kHz is ultrasonic. Ultrasonic waves are ordinary mechanical (sound) waves: they need a medium and travel at the speed of sound in that medium. They are not electromagnetic, so do not confuse ultrasonic with ultraviolet.

Why echo-ranging uses ultrasonic waves.

Their wavelength is very short (millimetres to centimetres), so they can be sent as a narrow, directed beam that does not spread much.
Because the wavelength is short, they are reflected even by small objects (a fish, an insect, a kidney stone).
They carry more energy and are not easily absorbed, so they can travel long distances through water.
They are inaudible to people, so they do not disturb anyone.

Radio waves and light are strongly absorbed by sea water, which is why sound, not light, is used under the sea.

SONAR: Sound Navigation And Ranging. A ship (or submarine) carries a transmitter and a receiver (detector).
1. The transmitter sends a short pulse of ultrasonic waves into the water.
2. The pulse is reflected by the sea bed, a submarine, a wreck, an iceberg or a shoal of fish.
3. The receiver picks up the echo and the time interval $t$ is measured electronically.
4. With the speed of sound in sea water $v$ (about 1500 m/s), the distance is

$d = \frac{v t}{2} .$

Repeated pings show whether an object is approaching or moving away: if the echo time falls, the object is getting closer.

Uses of SONAR. Measuring the depth of the sea (echo sounding) and mapping the sea bed; locating submarines, sunken ships and icebergs; finding shoals of fish (fishermen's echo sounders); guiding ships safely through shallow channels.

Echoes in nature: echolocation.

Bats fly and hunt at night by sending out short bursts of ultrasonic squeaks. From the time and direction of the echoes they judge where obstacles and insects are, so they can fly in complete darkness without colliding.
Dolphins (and some whales) use ultrasonic clicks under water to navigate and to find fish.

Echoes in medicine: ultrasonography. A probe placed on the skin sends ultrasonic pulses into the body. Echoes from boundaries between different tissues return at different times, and a computer builds an image from them. Uses: watching the growth of a baby in the womb, detecting stones in the kidney or gall bladder, examining the heart (echocardiography) and other organs. It uses no harmful ionising radiation, unlike X-rays.

Other uses of echoes. Finding the speed of sound (by timing an echo from a known distance); estimating the distance of a cliff or the depth of a well; designing halls with curved ceilings and soundboards that reflect sound towards the audience.

SONAR and Uses of Echoes: formulas and key facts

SONAR range: $d = \frac{v t}{2}$; t is the time from sending the pulse to receiving its echo.
Wavelength of an ultrasonic wave: $λ = \frac{v}{f}$; Use the speed of sound in the medium (air about 340 m/s, sea water about 1500 m/s).
Ranges of sound frequency: Infrasonic below 20 Hz
Audible 20 Hz to 20 kHz
Ultrasonic above 20 kHz; Ultrasonic and infrasonic waves are sound, not electromagnetic radiation.
Users of echoes: Bats ultrasonic squeaks to find insects and obstacles
Dolphins ultrasonic clicks to navigate and hunt
Fishermen echo sounders to find shoals
Navy SONAR to locate submarines and wrecks
Doctors ultrasound scans of organs and babies; All work by timing reflected ultrasonic pulses.

Infrasonic	below 20 Hz
Audible	20 Hz to 20 kHz
Ultrasonic	above 20 kHz

Bats	ultrasonic squeaks to find insects and obstacles
Dolphins	ultrasonic clicks to navigate and hunt
Fishermen	echo sounders to find shoals
Navy	SONAR to locate submarines and wrecks
Doctors	ultrasound scans of organs and babies

How to solve SONAR and Uses of Echoes questions

SONAR questions mix a short explanation with an echo calculation.

1Name the waves: ultrasonic (frequency above 20 kHz).
2Give the reason for using them: short wavelength, narrow beam, reflected by small objects, not easily absorbed, inaudible.
3For a distance, use $d = \frac{v t}{2}$ with the speed of sound in water (or tissue, or air) given; convert minutes or milliseconds to seconds.
4For two echoes (a shoal and the sea bed), find each distance and subtract to get the separation.
5For repeated pings, compare the two distances and divide the change by the time between pings to get the speed of approach or recession.

SONAR and Uses of Echoes in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

A bat catching a moth

Suppose a bat's 80 kHz squeak bounces off a passing moth and returns to the bat 0.02 s after it was made. Sound travels at 344 m/s in the warm night air.

How far away is the moth, and what is the wavelength of the squeak?

Try it first, then show the solutionSolution

$d = \frac{344 \times 0.02}{2} = 3.44$ m.
$λ = \frac{344}{80 000} = 0.0043$ m $= 4.3$ mm, far smaller than the moth, so the moth reflects it well.

Answer: 3.44 m away; wavelength 4.3 mm.

Memory hook: Short squeak, short wavelength, small prey found.

A fisherman's echo sounder

Imagine a trawler's echo sounder shows two echoes from a single ping: a weak one after 0.3 s and a strong one after 2.0 s. (Speed of sound in sea water 1500 m/s)

What are the two echoes, and how far apart are they?

Try it first, then show the solutionSolution

Weak echo: a shoal of fish at $\frac{1500 \times 0.3}{2} = 225$ m.
Strong echo: the sea bed at $\frac{1500 \times 2.0}{2} = 1500$ m; the shoal is $1500 - 225 = 1275$ m above the sea bed.

Answer: A shoal at 225 m and the sea bed at 1500 m; the shoal is 1275 m above the bottom.

Memory hook: First echo, nearest object.

Tricks to remember SONAR and Uses of Echoes

Short memory aids. Say each one aloud once, then use it on a question the same day.

Spell out SONAR
SOund NAvigation and Ranging: the 'Ranging' part is the distance, d = vt/2.
Use it when: Writing the full form.
Ultra- vs infra-, sonic vs violet
'-sonic' means sound (needs a medium); '-violet' and '-red' mean light-family radiation. SONAR is sonic.
Use it when: Avoiding the ultraviolet trap.
Four reasons: Narrow, Small, Strong, Silent
Ultrasound travels in a Narrow beam, reflects from Small objects, is Strong (not easily absorbed) and Silent to us.
Use it when: Why ultrasonic waves are used.
BaDFiM
Bats, Dolphins, Fishermen, Medicine: the four users of echoes named in the syllabus.
Use it when: Listing uses of echoes.

SONAR and Uses of Echoes: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Waves used in SONAR · 1 mark · MCQBoard level

SONAR uses:
(a) infrasonic waves
(b) ultraviolet rays
(c) ultrasonic waves
(d) microwaves

Try it first, then show the solutionSolution

SONAR sends sound pulses of frequency above 20 kHz into water: ultrasonic waves.

Answer: (c) ultrasonic waves

Example 2: Why ultrasonic waves? · 3 marks · Short answerBoard level

(a) What is meant by ultrasonic waves? (b) Give two reasons why ultrasonic waves, and not audible sound, are used in SONAR. (c) Why is light not used to find the depth of the sea?

Try it first, then show the solutionSolution

(a) Sound waves of frequency above 20 kHz, which humans cannot hear. (1)
(b) Any two: their short wavelength lets them travel as a narrow beam without spreading; they are reflected even by small objects; they are not easily absorbed by water; they are inaudible. (1)
(c) Light is strongly absorbed by sea water and cannot reach great depths. (1)

Answer: (a) Sound above 20 kHz (b) Narrow beam / reflected by small objects / not easily absorbed (c) Light is absorbed by sea water

Example 3: Depth from a SONAR ping · 3 marks · Short answerHarder board level

A ship's SONAR works at 30 kHz, and sound travels through the local sea water at 1520 m/s. (a) Find the wavelength of the waves in water, in cm. (b) The echo from the sea bed arrives 1.8 s after the ping. Find the depth of the sea.

Try it first, then show the solutionSolution

(a) $λ = \frac{1520}{30 000} \approx 0.0507$ m $\approx 5.07$ cm. (1)
(b) $d = \frac{v t}{2} = \frac{1520 \times 1.8}{2}$ (1)
$= 1368$ m. (1)

Answer: (a) About 5.07 cm (b) 1368 m

Example 4: Echolocation by a bat · 2 marks · Short answerHarder board level

Explain how a bat is able to fly in complete darkness without bumping into walls and still catch insects. Name the kind of sound it uses.

Try it first, then show the solutionSolution

The bat emits short bursts of ultrasonic squeaks, which are reflected by obstacles and insects. (1)
From the time delay and direction of each echo it judges the distance and position of the object (echolocation), so it avoids obstacles and homes in on prey. (1)

Answer: It uses echoes of its own ultrasonic squeaks (echolocation)

Example 5: Ultrasound in hospitals · 3 marks · Short answerHarder board level

(a) Name the medical technique that forms images of internal organs using echoes. (b) State two of its uses. (c) Why is it preferred to X-rays for checking a baby in the womb?

Try it first, then show the solutionSolution

(a) Ultrasonography (ultrasound scanning). (1)
(b) Any two: watching the growth of a baby in the womb; detecting kidney or gall-bladder stones; examining the heart and other organs. (1)
(c) It uses sound waves, not ionising radiation, so it does not harm the growing baby's cells. (1)

Answer: (a) Ultrasonography (b) Foetal scans, detecting stones (c) No harmful radiation

Example 6: Is the submarine approaching? · 3 marks · Short answerHOTS / competency

A warship sends two SONAR pings exactly 60 s apart towards a submarine. The first echo returns after 0.8 s and the second after 0.6 s. (Take sound to travel at 1500 m/s in sea water.) (a) Find the distance of the submarine at each ping. (b) Is it approaching or moving away? (c) Find its average speed along the line of sight.

Try it first, then show the solutionSolution

(a) $d_{1} = \frac{1500 \times 0.8}{2} = 600$ m; $d_{2} = \frac{1500 \times 0.6}{2} = 450$ m. (1)
(b) The distance has decreased, so it is approaching. (1)
(c) Speed $= \frac{600 - 450}{60} = 2.5$ m/s. (1)

Answer: (a) 600 m and 450 m (b) Approaching (c) 2.5 m/s

Example 7: Case study: a survey ship · 4 marks · Case-basedHOTS / competency

A survey ship maps the sea bed with an echo sounder. Over a flat region the echo time is steady at 2.0 s; then, over a few minutes, it falls to 0.4 s and later rises back to 2.0 s. Sound travels at 1500 m/s in this sea water.
(a) Find the depth over the flat region.
(b) What does the fall to 0.4 s show? Find the depth there.
(c) Find the height of the feature above the surrounding sea bed.
(d) A student suggests using a powerful torch instead. Give two reasons why this would not work as well.

Try it first, then show the solutionSolution

(a) $d = \frac{1500 \times 2.0}{2} = 1500$ m. (1)
(b) The sea bed rises: an underwater hill. Depth $= \frac{1500 \times 0.4}{2} = 300$ m. (1)
(c) Height $= 1500 - 300 = 1200$ m. (1)
(d) Light is absorbed by sea water within a short distance, and its speed is so high that the time could not be measured simply; ultrasonic waves travel far in water as a narrow beam and are easily timed. (1)

Answer: (a) 1500 m (b) An underwater hill; 300 m (c) 1200 m (d) Light is absorbed by water; ultrasound travels far as a narrow beam

Where students lose marks in SONAR and Uses of Echoes

The slips examiners see most often, and what to write instead.

Mistake: Naming the wrong waves for SONAR
SONAR is based on sound; ultraviolet, infrared and radio waves are electromagnetic.
Wrong working
'SONAR uses ultraviolet (or infrared, or radio) waves.'
Right working
'SONAR uses ultrasonic waves.'
Do this: Remember 'SO-nar is SOund'.
Usually costs: Reported by CISCE examiners for 2024 Q7(i)(a).
Mistake: Forgetting that the pulse goes down and back up
The echo time includes the journey to the object and back.
Wrong working
$d = 1500 \times 1.8 = 2700$ m.
Right working
$d = \frac{1500 \times 1.8}{2} = 1350$ m.
Do this: Write $v = \frac{2 d}{t}$ first.
Usually costs: Reported by CISCE examiners for the 2025 Q7(ii) submarine problem.
Mistake: Giving a use instead of a reason
'Why are ultrasonic waves used?' asks for a property (narrow beam, small wavelength), not where they are used.
Wrong working
'Because they are used to find fish.'
Right working
'Because their short wavelength lets them travel as a narrow beam and be reflected by small objects.'
Do this: Start your answer with 'because their wavelength is short…'.
Usually costs: The reason mark.
Mistake: Wrong unit conversion in ultrasound problems
Times in scans are in milliseconds or microseconds, and frequencies in kHz or MHz.
Wrong working
Using 2 for 2 MHz, giving $λ = 770$ m.
Right working
$f = 2 \times 1 0^{6}$ Hz, so $λ = 0.77$ mm.
Do this: Write every quantity in SI units before substituting.
Usually costs: The calculation mark.

Exam tips

Write 'ultrasonic waves' in full; 'ultra waves' or 'UV' will not be accepted.
When asked for uses of echoes, name the user and the purpose: 'bats, to locate prey in the dark'.
For a SONAR calculation, state the formula $d = \frac{v t}{2}$ , substitute, and give the answer in m (and km if large).
If a question asks why sound and not light is used under water, the key point is that sea water absorbs light strongly.

Practice questions on SONAR and Uses of Echoes

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The frequency of ultrasonic waves is:
(a) less than 20 Hz
(b) between 20 Hz and 20 kHz
(c) more than 20 kHz
(d) more than 20 MHz only
Hint
Above the audible range.
Answer
(c) more than 20 kHz
Q2 · 1 mark · Very short answerBoard level
Write the full form of SONAR.
Hint
Sound …
Answer
Sound Navigation And Ranging.
Q3 · 2 marks · Short answerHarder board level
A dolphin emits clicks of frequency 120 kHz under water, where sound travels at 1500 m/s. Find the wavelength of the clicks in mm. Why can the dolphin detect small fish with them?
Hint
$λ = \frac{v}{f}$ .
Answer
$λ = \frac{1500}{120 000} = 0.0125$ m $= 12.5$ mm. The wavelength is short compared with a fish, so even small fish reflect the clicks well.
Q4 · 2 marks · Short answerBoard level
Draw a labelled diagram to show how SONAR is used to measure the depth of the sea.
Hint
Two sets of arrows.
Answer
A ship on the sea surface with a transmitter and a receiver on its hull; ultrasonic pulses (arrows) going down to the sea bed; reflected pulses (arrows) coming back up to the receiver; depth $d$ marked between the ship and the sea bed, with $d = \frac{v t}{2}$ .
Q5 · 2 marks · Short answerHarder board level
At a place where the sea is known to be 2280 m deep, a SONAR echo from the bottom returns in 3 s. Find the speed of sound in that sea water.
Hint
$v = \frac{2 d}{t}$ .
Answer
$v = \frac{2 \times 2280}{3} = 1520$ m/s.
Q6 · 2 marks · Short answerHarder board level
An ultrasound probe works at 2 MHz. Taking the speed of sound in body tissue as 1540 m/s, find the wavelength in mm. Why must the frequency be so high for a clear image?
Hint
Short wavelength, fine detail.
Answer
$λ = \frac{1540}{2 \times 1 0 ^{6}} = 7.7 \times 1 0^{- 4}$ m $= 0.77$ mm. A very short wavelength is reflected by very small structures, so fine details of organs show up.
Q7 · 2 marks · Short answerHarder board level
A student writes: 'SONAR uses ultraviolet radiation because it can travel through water.' Point out two errors in this statement.
Hint
Ultrasonic vs ultraviolet.
Answer
(1) SONAR uses ultrasonic waves (sound above 20 kHz), not ultraviolet radiation, which is electromagnetic. (2) Electromagnetic radiation such as ultraviolet or light is strongly absorbed by sea water; sound travels far through water.
Q8 · 3 marks · Short answerHOTS / competency
A ship's SONAR detects an iceberg straight ahead; the echo returns 6 s after the ping. (Speed of sound in sea water 1500 m/s) (a) How far away is the iceberg? (b) If the ship keeps moving towards it at 5 m/s, how many minutes does the captain have before reaching it? (c) What will happen to the echo time as the ship gets closer?
Hint
Halve the path; time = distance ÷ speed.
Answer
(a) $d = \frac{1500 \times 6}{2} = 4500$ m. (b) $t = \frac{4500}{5} = 900$ s $= 15$ min. (c) It becomes steadily shorter, since $t = \frac{2 d}{v}$ and $d$ decreases.
Q9 · 3 marks · Short answerHarder board level
Name the waves used for each and state whether a human can hear them: (a) a bat locating a moth, (b) an elephant's long-distance rumble, (c) a TV remote control.
Hint
Two are sound, one is electromagnetic.
Answer
(a) Ultrasonic sound waves; not audible. (b) Infrasonic sound waves (below 20 Hz); not audible. (c) Infrared radiation (electromagnetic, not sound); it cannot be heard or seen.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Frequency of ultrasonic waves?Show answer
Above 20 kHz.
2.Full form of SONAR?Show answer
Sound Navigation And Ranging.
3.Animal that uses ultrasonic squeaks to hunt at night?Show answer
Bat.
4.Medical use of echoes?Show answer
Ultrasonography (ultrasound scanning).
5.SONAR distance formula?Show answer
$d = \frac{v t}{2}$ .

SONAR and Uses of Echoes: frequently asked questions

How does SONAR work?

A transmitter on a ship sends ultrasonic pulses into the water. They are reflected by the sea bed or an object, and a receiver detects the echo. The distance is found from d = vt/2, using the speed of sound in water.

Why are ultrasonic waves used in SONAR?

Their short wavelength lets them travel as a narrow beam and be reflected by small objects; they are not easily absorbed in water and cannot be heard by people.

How do bats use echoes?

Bats send out ultrasonic squeaks and listen for the echoes. The time delay and direction of each echo tell them where obstacles and insects are, so they can fly and hunt in the dark.