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

Diffraction Q & A

Q1. SPM2003/P1
The diagram shows an arrangement of apparatus for light diffraction experiment.

Part C: Essay

Q2.
Figure 6.0 shows the arrangement of the apparatus for an experiment to study a
wave phenomenon.

Figure 6.1 shows the bright and dark bands of the waves pattern formed on the white paper when plane waves passes through the narrow and wide gaps.

(a) Name the wave phenomenon shown in figure 6.2. [1 mark]

(b) (i) Explain how the dark and light bands are formed on the white paper. [4 marks]

(ii) Observe Figure 6.2(a) and (b). Compare the waves patterns and the wavelength of

the waves before and after they pass through the gaps.

Relate the size of the gaps, the waves patterns and the wavelengths to deduce a

relevant physics concept. [5 marks]

....to know the answer email me fizbro@gmail.com

Recall back:
Diffraction Phenomena
Refraction Phenomena

Diffraction Phenomena

The next phenomena I’m gonna discuss here is DIFFRACTION.

So what is diffraction??

Diffraction is the spreading of a wave when it travels through a gap or an obstacle.

Water Wave Diffraction:

Now lets visualize this, what will happen when water waves are passing through a gap…Watch this!

The effect of diffraction depends on the size of gap, obstacle & the wavelength.

The smaller the gap, the bigger the diffraction . Now try to watch this one pulak..

Meanwhile..The bigger the wavelength, the bigger the diffraction effect can be seen.

U should try this out..click here now!

Diffraction also occurs when wave passed through a barrier/ obstacles.

This is IMPORTANT! Please remember this!.. the effect of diffraction is bigger when:

• barrier/obstacle is bigger
• wavelength is bigger
• frequency is lower
• slit/gap is smaller