5.3 Reflection of waves

MODUL PDP SPM PASCA PKP
NEGERI PERAK 2020

PHYSICS Form 4

5.3: Reflection of Waves

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5.3 Reflection of Waves

Learning Standard:

Pupils are able to:

• describe reflection of waves from the following aspects:
(i) angle of incidence, i
(ii) angle of reflection, r
(iii) wavelength, λ
(iv) frequency, f
(v) speed, g
(vi) direction of propagation of waves.

• draw a diagram to show the reflection of plane water waves through a plane reflector.
• justify the applicaton of reflection of reflection of waves in daily life.

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5.3 Reflection of Waves

Wavefront

wavefront

Wavefront is the line that joins all the
points which are in phase and are of the
same distance from the source of
vibration and have the same
displacement.

WAVEFRONT

Plane wavefronts Circular wavefronts

vibrating Dip finger in circular
wooden beam the water wavefronts

plane wavefronts

direction of direction of
propagation propagation

direction of
propagation

wavefronts Direction of propagation of the wave is
perpendicular to the wavefront.
direction of Wavelength, λ is the same as the distance
propagation between two consecutive wavefronts.

λ

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Reflection of waves occurs
when the waves hit a
reflector.

Normal Characteristics of reflection of waves :
Frequency (f) of incident wave = frequency (f) of reflected wave
Incident reflected Incident wavelength (λ) = Reflected wavelength (λ)
ray ray Speed (v) of incident wave = Speed (v) of reflected wave
Direction of propagation = Changes
Incident angle reflected angle mirror Velocity of wave (v) = Changes because of direction changes

REFLECTION
OF WAVES

Law of Reflection : Reflection of waves occurs for all types
1. incident angle, i = reflected angle, r of waves such as water waves, light
2. Incident ray, reflected ray and normal waves and sound waves.

are on the same plane. Reflection of water waves can be investigated and
observed using ripple tank and plane reflector.
Reflection of water waves

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Steps to draw reflection of water waves

Plane reflector B Pemantul satah

INCIDENT INCIDENT ir
WAVES WAVES
REFLECTED
WAVES

AN C

Step 1 : Draw an arrowed line AB perpendicular to incident wavefront to represent the direction of
propagation of incident wave.

Step 2 : Draw the normal BN which is perpendicular (with angle 90o) to the plane reflector.
Step 3 : Draw an arrow BC with the condition angle CBN (r) is the same as angle ABN (i) to represent

the direction of propagation of reflected wave.
Step 4 : Draw lines perpendicular to BC to represent reflected wavefronts. The reflected

wavelength, λ2 must be the same as the incident wavelength, λ1.

Circular Reflected The pattern of circular incident waves reflected by a
incident circular plane reflector.
waves waves The waves seems to be diverged from point of
image P which is P’.

APPLICATIONS OF REFLECTION OF WAVES IN DAILY LIFE

Ultrasonic waves are reflected by the Radio waves from communication satellites are
internal organs or foetus in medical check- reflected by the parabolic dish and focused onto the
up. antenna on the feed horn.

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Technology of ultrasonic reflection which is Patterns of reflected sound waves caused by
known as SONAR helps to detect areas which different rocks enable the location, depth and
have a lot of fish. Transducer transmits structure of the seabed which contain sources of
waves into the water and these waves are natural gas to be identified.
reflected by the fish to the transducer.

Depth, =
2

Solving Problems Involving Reflected Waves

Example :
Ultrasonic waves of frequency 25 kHz are transmitted from a ship to the seabed to determine
the depth of the sea. These waves travel at a speed of 1500 m s-1 in the water. Time between
sending the ultrasonic wave and receiving the reflected wave is 100 ms. Determine
(i) depth of sea, and
(ii)ultrasonic wavelength

Solution :
(i) The ultrasonic wave takes 100 ms to travel from the ship to the seabed and back to the

ship. Distance travelled by the wave is two times the depth of the sea.

Step 1 : List the given information in symbols.
Speed of wave, v = 1500 m s-1
Time interval, t = 100 ms

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Step 2 : Identify and write down the formula used.

Distance travelled = Speed × time
2d = vt

Step 3: Substitute numerical values into the formula and perform the calculations

Depth of sea, =
2

= 1500 (100 10−2)
2

= 750 m

(ii) =

1500 = (25 103)

= 1500
(25 103)

= 0.06 m

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5.3 MIND MAP

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5.3 FAQ

No Questions and Answers
1 Q What is the reflection of wave?

A The reflection of a wave is simply a process by which a wave, whether light,
sound, infrared, or radio waves, hits an object and bounces off it.

2 Q What happens when waves are reflected?

A When waves strike an obstacle at any other angle, they bounce back at the
same angle but in a different direction.

3 Q What are the two laws of reflection?

A Following are the two laws of reflection :
1. The incident ray, the reflected ray and the normal are all in the same
plane.
2. The angle which the incident ray makes with the normal (i) is equal to the
angle which the reflected ray makes to the same normal (r).

4 Q Why is reflection of light important?

A Reflection of light enables us to see everything around us. Take a look out
your window: you see everything in the natural world (that doesn't produce
its own light) because it reflects the light of the Sun .

5 Q What are the uses of reflection?

A Convex mirrors are often used in shops as security mirrors because of the
type of image produced by that mirror. The image is smaller but shows a
wide angle view of the shop. Concave mirrors are often used in torch lights
or in the headlights of cars.

6 Q What is an example of reflection of sound?

A It is the sound heard when reflections occur from a firm surface, for example,
a wall or cliff. Echo is the repetition of sound even after the source has
stopped vibrating. This is used by bats as well as dolphins for the detection
of obstacles or navigation.

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5.3 EXERCISE

SECTION A
1. Diagram shows water waves moving towards a barrier.

Which physical quantities will be changed after the water waves hit the barrier?
[MEMAHAMI]

A Speed
B Frequency
C Wavelength
D Direction of propagation

2. Diagram below shows plane water waves travelling towards an L-shaped barrier in a
ripple tank.

L-shaped barrier
Halangan berbentuk L

Which diagram shows the reflected wave pattern? [MEMAHAMI]
AC

BD

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3. Diagram shows a fisherman used sonar to determine the position of a shoal of fish in
the sea water.

The wave phenomenon involved is [MENGETAHUI]

A reflection
B interference
C diffraction
D refraction

4. Diagram shows a dolphin creates ‘clicks’ sound to detect a school of fish by the
reflection of sound phenomenon.

Which statement about the reflected sound is correct? [MEMAHAMI]

A The speed of the sound waves is unchanged
B The amplitude of the sound waves increases
C The frequency of the sound wave decreases
D The wavelength of the sound waves increases

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5. Diagram below shows a ship is sending out a pulse of ultrasound to the seabed.

Kapal
Boat

If the reflected ultrasound is received 0.36 s later, what is the depth of the sea?
[ assume : the speed of sound in sea water = 1500 m s-1]

[MENGAPLIKASI KUANTITATIF]
A 9.3 m
B 270 m
C 540 m
D 4166 m

SECTION B
1. Complete the light rays and determined the bright fringe and dark fringe that form

on the screen.
[MEMAHAMI]

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2. Complete the diagram below to show the reflection of water waves and determine
the direction of the reflection water waves.
[MEMAHAMI]

3. States the effects of reflection on characteristics of waves

[MENGETAHUI]

Characteristic of wave Effect after reflection of wave
Wavelength
Frequency
waves speed
Direction of propagation
Angle of incidence and angle of
reflection

4. Diagram shows a submarine transmitting ultrasonic waves directed at a big rock
on the sea bed. After 8.3 seconds, the submarine detects the reflected wave.

Calculate the distance between the submarine and the big rock.
[ Velocity of ultrasonic wave = 1560 ms-1 ]

[MENGAPLIKASI KUANTITATIF]

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5.3 QR CODE

Title 5.3 Reflection of waves QR CODE
link
Reflection of https://youtu.be/YdOzyAcxjz0
water waves

Waves https://youtu.be/1PsGZq5sLrw
Reflections

Ripple Tank https://youtu.be/R5EdLv3NS7Y
Reflection

REINFORCEMENT https://drive.google.com/file/d/1t3h6YElNfTL26tsfeHJPlu
pXkxaF5a7y/view?usp=sharing
TEST

ANSWER https://drive.google.com/file/d/1bL670F7wMZVw62cC8R
REINFORCEMENT Ef8Fy3TgFmt-3x/view?usp=sharing
TEST

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5.3 ANSWER

SECTION A
1D
2D
3A
4C
5B

SECTION B
1

2.

3. Characteristic of wave Effect after reflection of wave

Wavelength, λ No change

Frequency, f No change

waves speed, v No change

Direction of propagation Changes with the condition that the angle of

incidence is the same as angle of reflection

Angle of incidence and angle of Angle of incidence = angle of reflection

reflection

4. v = 1560 ms-1

t = 8.3 s

Distanced travelled = speed x time

2d = vt

d = vt/2 = 6474 m /6.474 km

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