# phyiscs help needed!!

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#1
Could someone pls explain to me how to solve phase difference and path difference questions?
I have no idea
1
1 year ago
#2
Perhaps you can post the question?
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1 year ago
#3
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#4
I don't understand how to solve this or questions like this. COuld you help me!
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1 year ago
#5
(Original post by ThirstyLearner)
I don't understand how to solve this or questions like this. COuld you help me!
Trying...
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#6
(Original post by Spannerin'moi)
Trying...
the answer is C, but idk how.
With the formula of 2pi times path difference divided by wavelength gives 3pi
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1 year ago
#7
(Original post by ThirstyLearner)
the answer is C, but idk how.
With the formula of 2pi times path difference divided by wavelength gives 3pi
I see..was the path difference 0?
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#8
(Original post by Spannerin'moi)
I see..was the path difference 0?
Mark scheme says- π radians is the phase difference between the two waves at A – the path difference between the two waves is (8cm – 5cm) = 3cm = 1.5λ. This results in the waves arriving in antiphase (π radians out of phase) which also occurs when the path difference is 1cm.
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1 year ago
#9
(Original post by ThirstyLearner)
Mark scheme says- π radians is the phase difference between the two waves at A – the path difference between the two waves is (8cm – 5cm) = 3cm = 1.5λ. This results in the waves arriving in antiphase (π radians out of phase) which also occurs when the path difference is 1cm.
Wait...(8cm-5cm?) how did that happen tho thought it was 6.5-6.5...
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#10
(Original post by Spannerin'moi)
Wait...(8cm-5cm?) how did that happen tho thought it was 6.5-6.5...
4+4=8
and 2.5+2.5=5
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1 year ago
#11
(Original post by ThirstyLearner)
4 4=8
and 2.5 2.5=5
Ooh in that case I'm getting 3λ/2 too
Thanks for the sharing the question...helped me revise
Last edited by username2889812; 1 year ago
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1 year ago
#12
(Original post by ThirstyLearner)
Mark scheme says- π radians is the phase difference between the two waves at A – the path difference between the two waves is (8cm – 5cm) = 3cm = 1.5λ. This results in the waves arriving in antiphase (π radians out of phase) which also occurs when the path difference is 1cm.
nice ...I knew only until anti phase occurred when path difference was (n+ 1/2)λ ... but that 1cm rule though...hope someone clarifies...
Last edited by username2889812; 1 year ago
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1 year ago
#13
BobbJo
Do you think the phase difference is π?
for yer time!
Last edited by username2889812; 1 year ago
0
1 year ago
#14
(Original post by Spannerin'moi)
BobbJo
Do you think the phase difference is π?
for yer time!
Yes. We ignore any phase change during reflection [if interested on this : https://physics.stackexchange.com/qu...-on-refraction, https://physics.stackexchange.com/qu...denser-medium]

The path difference is (4+4)-(2.5+2.5) = 8 - 5 = 3 cm

3 cm is equal to 1.5 wavelengths
The path difference is 1.5 wavelengths. We have destructive interference and the phase difference is pi.
[1.5 wavelengths corresponds to a phase difference of 3 pi which is equivalent to a phase difference of pi]
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1 year ago
#15
(Original post by BobbJo)
Yes. We ignore any phase change during reflection [if interested on this : https://physics.stackexchange.com/qu...-on-refraction, ]https://physics.stackexchange.com/questions/32122/phase-shift-of-180-degrees-of-transversal-wave-on-reflection-from-denser-medium]

The path difference is (4+4)-(2.5+2.5) = 8 - 5 = 3 cm

3 cm is equal to 1.5 wavelengths
The path difference is 1.5 wavelengths. We have destructive interference and the phase difference is pi.
[1.5 wavelengths corresponds to a phase difference of 3 pi which is equivalent to a phase difference of pi]
for extra links and yer patience
I got it till the destructive interference part....
Is it a rule of thumb that all destructive interferences should have a phase difference of π?
And also how do I plug in values into this equation?
Φ = (2π/λ) * x ?
I thought this was to be done...
(2π/λ) * 1.5λ but it results in 3 π
0
1 year ago
#16
(Original post by Spannerin'moi)
for extra links and yer patience
And also how do I plug in values into this equation?
Φ = (2π/λ) * x ?
I thought this was to be done...
(2π/λ) * 1.5λ but it results in 3 π
[Note: the reflection can be ignored because both will undergo reflection and hence both will have the pi phase change, hence it cancels out]

For the equation, plug in values of λ and x
Φ = (2π/λ) * x
in this question, λ = 2 cm
hence Φ = (2π/2) x 3 = 3π
or convert x = 3 cm to x = 3/2 λ and replace, thus
Φ = (2π/λ) * 3/2 λ = 3π

I got it till the destructive interference part....

Is it a rule of thumb that all destructive interferences should have a phase difference of π?
Yes, all destructive interferences occur when the phase difference is π.
In fact, the path difference must be half a wavelength, or (n+1/2)λ where n is an integer.
This corresponds to a phase difference of Φ = (2π/λ) * x = (2n+1)π for integer n.
Destructive interference therefore occurs for a phase difference: π, 3π, 5π, 7π
But all of these are equivalent to a phase difference of π

The displacement of a point on a wave varies sinusoidally:
x = x0sin(ωt+Φ), Φ denotes the phase angle
Look at the following:
https://www.desmos.com/calculator/epjylr2j7v
Notice that the curves are identical for Φ = π, 3π, 5π, ...
This is due to the periodicity of the sine curve. Hence phase difference of 3 pi and 5 pi are equivalent to a phase difference of pi
0
1 year ago
#17
(Original post by BobbJo)
[Note: the reflection can be ignored because both will undergo reflection and hence both will have the pi phase change, hence it cancels out]

For the equation, plug in values of λ and x
Φ = (2π/λ) * x
in this question, λ = 2 cm
hence Φ = (2π/2) x 3 = 3π
or convert x = 3 cm to x = 3/2 λ and replace, thus
Φ = (2π/λ) * 3/2 λ = 3π

Yes, all destructive interferences occur when the phase difference is π.
In fact, the path difference must be half a wavelength, or (n+1/2)λ where n is an integer.
This corresponds to a phase difference of Φ = (2π/λ) * x = (2n+1)π for integer n.
Destructive interference therefore occurs for a phase difference: π, 3π, 5π, 7π
But all of these are equivalent to a phase difference of π

The displacement of a point on a wave varies sinusoidally:
x = x0sin(ωt+Φ), Φ denotes the phase angle
Look at the following:
https://www.desmos.com/calculator/epjylr2j7v
Notice that the curves are identical for Φ = π, 3π, 5π, ...
This is due to the periodicity of the sine curve. Hence phase difference of 3 pi and 5 pi are equivalent to a phase difference of pi
PRSOM
a ton for the detailed explanation! I understand better now and hopefully OP
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