5.6 Waves Interferens

MODUL PDP SPM PASCA PKP
NEGERI PERAK 2020

PHYSICS Form 4

5.6: Interference of Waves

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5.6 Interference of Waves

Learning Standard:

Pupils are able to:

• explain the principle of superposition of waves.
• describe the pattern of interference for:

(i) water waves
(ii) sound waves
(iii) light waves
• relate λ, a, x and D for the wave interference pattern.
• solve problems involving interference of waves.
• communicate on the applications of interference of waves in daily life.

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5.6 notE

1. Principle of Superposition of Waves
The principle of superposition states that when two waves overlap, the resultant
displacement is the sum of the individual displacements of the two waves.

The photo above shows two waves on the water surface in a lake. Two circular waves
formed overlap or in superposition.
The effect of superposition produces interference of waves.

2. Interference with Coherent Source of Waves
Interference of waves is the superposition of two or more waves from a coherent source
of waves.
Two sources of waves are coherent when the
frequency of both waves is the same
phase difference is constant.
Superposition of waves produces :
 constructive interference
 destructive interference.

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3.
ConstruIncttievrefeinrteenrsfemreenmcebina

occurs when two crests are in superposition to also occurs when two troughs are in

produce a higher crest. superposition to produce a deeper

trough.

Constructive interference between two crests

Constructive interference between two troughs
4.

Destructive interference
Destructive interference occurs when a crest and a trough are in superposition to produce
zero combined displacement.

Destructive interference between a crest and a trough

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Interference of water waves
5.

S1 dan S2 : coherent sources
Interference pattern for water waves

Interference of light waves

6. Figure shows the interference pattern formed on the screen with light from a laser pen.

Laser pen is used because
it emits monochromatic
light which is light waves
with one colour and one

wavelength only.

Interference pattern for light waves

 Diffracted light waves that appear from the double-slit are coherent.

 Superposition of waves from the double-slit produces a pattern made up of with
bright fringes and dark fringes.

 Constructive interference produces bright fringes while destructive interference
produces dark fringes.

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Interference of sound waves
7. • Loud sound is heard in constructive interference region and

• soft sound is heard in destructive interference region.

Pattern of interference for sound waves
Drawing patterns of wave interference

8. Points P and Q are antinodes, the points where constructive interference occurs. Point R is a
node the point where destructive interference occurs.

S1 dan S2 : coherent sources

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Relationship between Variables of Interference of Waves

9. In the pattern of interference of waves, there are four variables namely λ, a, x dan D.

λ = wavelength
a = distance of separation between two coherent sources
x = distance of separation between two adjacent antinodal lines or

two adjacent nodal lines.

D = the perpendicular distance from the sources and the position

where the value of x is measured

The relationship between a and x
a is inversely proportional to x

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The relationship between λ and x
λ is directly proportional to x

Solving Problems Involving Interference of Waves

Figure shows Young’s double-slit experiment which produces interference pattern on the
screen. The distance between two slits is 0.25 mm and the distance between the screen and
the double slit is 3.0 m. What is the wavelength of the light used?
Jarak antara dwicelah ialah 0.25 mm dan jarak di antara skrin dengan plat dwicelah ialah 3.0
m. Berapakah panjang gelombang cahaya yang digunakan?

a = 0.25 mm

x = 14 mm

Figure: Young’s double-slit experiment

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Solution:

Given 7x = 14 mm
x = 2 mm = 2 x 10-3 mm

D = 3.0 m
a = 0.25 mm = 2.5 x 10 -4 m

Use λ = ax = 2.5 x 10−4 x 2 x 10−3
D 3

= 1.67 x 10-7 m

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Active noise control (ANC) system in the cabin of the airplane. When noise is detected, the
system will generate a cancelling waveform equal in amplitude and frequency, but exactly
opposite in phase to eliminate the noise.

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5.6 MIND MAP

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5.6 FAQ

No Questions and Answers
1Q What is interference of a wave?

A Wave interference is the phenomenon that occurs when two waves meet
while traveling along the same medium. The interference of waves causes

the medium to take on a shape that results from the net effect of the two

individual waves upon the particles of the medium.

2 Q What are the two types of interference?

A There are two types of interference: constructive and destructive.
1. Constructive interference occurs when two crests / trough superposition
to produce a greater amplitude.
2. Destructive interference occurs when crest meet trough or trough meet
crest, produce zero resultant displacement.

3 Q What is the principle of superposition and to what does it apply?

A The principle of superposition states that, when two or more waves of the
same type cross at some point, the resultant displacement at that point is

equal to the sum of the displacements due to each individual wave.

4 Q What is the application of interference of waves?

A A common example of this involves the interference of radio wave signals
that occur at the antenna of a home when radio waves from a very distant
transmitting station take two different paths from the station to the home.

5 Q Does destructive interference destroy energy?

A Destructive interference destroys the potential energy, but doubles the
kinetic energy.

6 Q What is an example of a real life application of wave interference that deals
with sound waves?

A
When music is played in a concert hall, it reverberates off the walls of the
auditorium. Assuming the place is well designed acoustically, these bouncing
sound waves will interfere constructively, and the auditorium comes alive
with the sound of the music.

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5.6 EXERCISE

SECTION A
1. In which diagram will destructive interference occur when the waves meet?

[MENGETAHUI]
AC

BD

2 Diagram 2 shows an interference pattern of water waves from two sources, S1 and
S2.

Diagram 2 [MEMAHAMI]
Which form of wave happens at point X from sources S1 and S2.?

AC

BD

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3 The diagram below shows an interference pattern of two coherent water waves of
sources, P and Q.

Which of the positions A, B, C and D does the amplitude of waves are zero?
[MENGETAHUI]

4 Diagram below shows the arrangement of apparatus used to study the effect of
waves. Given that the distance between two consecutive weak sounds is 1.0 m

What is the wavelength of the sound wave generated?
[MENGAPLIKASI KUANTITATIF]

A 0.05 m
B 0.10 m
C 0.50 m
D 0.75 m

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5 Diagram below shows the red fringes formed in a Young’s double-slit experiment.

The separation between the fringes can be decreased by
[MEMAHAMI]

A decreasing the separation between the slits
B increase the separation between the slits
C decreasing the intensity of the red light
D increasing the distance between the double slit and the screen

SECTION B

Complete the following statements about interferens of waves by filling in the blanks.
1. The principle of superposition states that when two waves overlap, the resultant

_____________________ is the _________________ of the individual displacements
of the two waves.

[MEMAHAMI]
2. Interference of waves is the superposition of two or more waves from a

_____________ source of waves. Two sources of waves are coherent when the
____________ of both waves is the same and the ____________ ce is constant.

[MEMAHAMI]
3. ___________________ occurs when the crests or troughs of both waves coincide

and produce a wave with a crest or trough of maximum ________________.
[MENGETAHUI]

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4. Destructive interference occurs when the ____________ of a waves coinsides with
the _____________ of another waves and produces a waves with _____________
amplitude.

[MENGETAHUI]

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5. Label the diagram.

[MENGETAHUI]

The diagram above shows the interference pattern observed when two coherent
waves sources coincide. Label the diagram.

6 Figure shows Young’s double slit experiment which produces interference pattern on
the screen.
The distance between adjacent bright fringes is 3.2 mm. What is the wavelength of
the light used?

[MENGAPLIKASI KUANTITATIF]

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7. Diagram 7 shows an experiment to determine the wavelength of monochromatic light
waves using a red light source.

(a) What is meant by monochromatic light? [MENGETAHUI]
…..……………………………………………………………………………………………………….. [1 mark]
(b) Why are red fringes formed on the screen ?
[MEMAHAMI]

....................................................................................................................................................
[1 mark]

(c) The distance between the Young double slit and the screen is 3.0 m. The double slit used
has the slit separation of 5 x 10-4 m. Calculate the wavelength of the red light.

[MENGAPLIKASI KUANTITATIF]

[2 marks]
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(d) The red light is replaced by the green light.

(i) What happen to distance between two consecutive dark fringes.

[MEMAHAMI]

………………………………………………………………………………………………………….. [1 mark]
(ii) Give the reason for your answer in (d)(i). [MEMAHAMI]

...........................................................................................................................................

...........................................................................................................................................
[1 mark]

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5.6 QR CODE

5.6 Interference of waves QR CODE
Title link
Interference of waves https://youtu.be/IgUyHW5f7rk

Interference | Waves https://youtu.be/GuIS8lBhbF8

Interference of sound https://youtu.be/K2H5CaVKbSM
waves

Interference of Light https://youtu.be/cuidzly4NVQ
Waves

REINFORCEMENT https://drive.google.com/file/d/1t3h6YElNfTL26t
TEST sfeHJPlupXkxaF5a7y/view?usp=sharing

ANSWER https://drive.google.com/file/d/1bL670F7wMZV
REINFORCEMENT w62cC8REf8Fy3TgFmt-3x/view?usp=sharing
TEST

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5.6 ANSWER

SECTION A Answer
A
No C
1 C
2 B
3 B
4
5

SECTION B
1 displacement, sum
2 coherent /frequency/phase differen
3

Constructive interference/ amplitude

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4

crest/trough/zero

5 Garis antinod

. Garis nod

6 Separation of slit, a = 0.5 mm

= 0.5 x 10-3 mm

Distance between adjacent bright fringes, x = 3.2 mm

x =3.2 x 10 -3 mm

Distance between screen and double slit, D = 3.0 m

Wavelength, λ =

= 5 x 10−4 x 3.2 x 10−3
3

= 5.33 x 10-7 m.

7(a) Wave with only one colour or one wavelength 1
(b) Constructive interference//Crest meet crest//trough meet 1

trough//diagram 1
2
(c) x=(1.5x10-2 m)/4
=3.75 x 10 -3 m 1
1
(d)(i) λ=ax/D 1
(ii) λ=(5x10-4 x 3.75x10-3)/3

=6.25x 10-7 m
Decreases
Wavelength of red > green //λred >λgreen //vice versa

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