SPM: Interference Q & A

Q1 : SPM 2003/P1

The diagram shows the fringe pattern produced from Young’s double slit experiment using the monochromatic red light source.
What happens to the fringes if monochromatic yellow light source is used?

A. The bright fringes are wider than the dark fringes
B. The dark fringes are wider than the bright fringes
C. The distance between consecutive fringes decrease
D. The distance between consecutive fringes increase

Ok to answer this one..u must remember the sequence of visible light (pelangi) spectrum.
it starts with red,orange,yellow,green,blue and so on.

from red to blue...the wavelength is decreasing,
So yellow wavellength is smaller,... by referring equation λ=(ax) /D ...

λ is directly proportional with x, thus x (distance between fringes)will also decrease.

Answer = C

Q2 : SPM 2004/P1

The diagram shows an interference pattern of two coherent water waves of sources, P and Q.
Which of the positions A, B, C or D, does destructive interference occur?

Answer: D

Q2 : SPM 2006/P1

Diagram 20 shows the interference patterns for water waves from two coherent source, S1 and S2.

Which of the following superposition of the waves at
shows the point Y?




answer= A (crest +trough = destructive interference)

SPM: Sound Interference

I just bought a surround home theatre speaker lately..now watching movie at home is awesome & more comfortable too.. thanx to those engineer who understands well the interference concept..

Now lets analyze the sound interference..all u need is two speakers which produce sound with same frequency (i.e coherent) and separated at distance, a

Ok..most important observation was that..u’ll hear loud & soft sound along the path

Loud = constructive interference

Soft = destructive interference

x = distance between 2 loud sound (or 2 soft sound)

Example:

Example:

Two loud speakers placed 2 m apart are connected to an audio signal generator that is

adjusted to produce sound waves of frequency 550 Hz. The detection of loud and soft

sounds as a person moves along a line is at 4.0 m from the loudspeakers. Distance between two loud places is 1.2 m

a) the wavelength

b) the speed

Tips: 1^st u identify & extract the information given..such as

a = 2 m

f = 550hz

x = 1.2 m

Then apply in equation λ = ax …. u’ll get the wavelength

D

Next use v = f x λ to find the speed..try it out

(Answer: wavelength = 0.6m, speed = 330m/s)

Revise other topics:
Superposition
Light Interference
Water Interference

SPM : Light Interference

For light intereference, u must be able to describe what u’ve observed while doing the experiment. But majority of my student didn’t do the experiment..may b cikgu u all quite bz

..so here I am to explain the important observation need to be understood…

First..lets look at this experiment diagram..

From this experiment, we can measure the wavelength..but please watch out!..

the distance D =3m, not 3+1 m.

Now look at the next diagram..

Important things to observe from the diagram….

i. on the screen a uniform bright + dark fringes are formed

ii. bright fringes(white) = constructive interference

iii. dark fringes(black) = destructive interference

iv. x = is the distance between 2 bright fringes

v. a= distance between double slits ( i.e coherent sources)

To find wavelengh ,..again we apply

λ = ax

D

Ok..one more thing, u must be able to differentiate the fringes pattern between diffraction & interference…

(* famous in SPM)

(interference pattern)

.............look & Compare........

(diffraction pattern)

I leave u guys with this video..some might not covered in our SPM syllabus, but have a think about it..

SPM: Water Interference

Now lets examine what happen during water interference in a ripple tank..Look at this video



Observe the water ripples in the video above ..uniform water patterns are formed by 2 sources of ripple. These sources are coherent to each other. What does it mean by coherent sources???

coherent = waves produced at same frequency, amplitude and in phase

This interference pattern can be illustrated in diagram below..
Ok now..its time to describe what can be seen in the diagram..

i. - points where the crest meet crest or trough meet trough is known as antinode.
line joining these points is known as antinode line

ii. - points where crest meet trough is known as node.
line joining these points is known as node line

From this interference phenomena we can also calculate the wavelength (λ). This is given by formula

λ = ax

D

a = distance between 2 coherent sources

x = distance between 2 consequtive node lines (or antinodal lines )

D = distance from coherent sources to the point of measurement of x

Ok..usually in SPM they will test you whether u know how to relate changes in a with respect to x & D.

Its about your maths skill..if u r gud in algebra..than it would be easy.

To let u understand this better, try this animation! (u need to have java applet installed)

Interference Phenomena

In order to understand interference, u need to understand the concept of superposition.

The principle of superpositionstates that;

When two waves meet or overlap, the resultant displacement is equal to sum of the displacement of each wave.

Look at the figure below;

Check this video too..a creative illustration of wave superposition

the green wave meet the brown wave..yellow wave is produced

So... we can conclude that..interference will occur if two wavefront meets.

This will produce either constructive or destructive interference as shown in diagram above.

U should also check the following:

Water Interference

Light Intererence

Sound Interference