if 2 waves result in a wave with smaller amplitude is it destructive superposition?

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thebrahmabull
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In many places I see written that for destructive superposition to occur, waves must be 180 degrees out of phase. Is that the only way destructive superposition occurs? Shouldn't the waves cancel out in such a situation? Under what condition would a wave he created which has less amplitude than each component wave?
And would the process of such creation be called destructive superposition?
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hannah-ri0
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Don't quote me on this coz I am not the greatest at physics, but hopefully this might help you:

If two waves are that have the same frequency and similar amplitude, that are 180 degrees out of phase total destructive interference will occur and the waves will completely cancel each other out. This is the only way destructive interference will occur. it is only called destructive interference if they are 180 degrees apart. However if they have different amplitudes, it is still destructive interference even though the will not cancel out completely.
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hannah-ri0
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can anyone answer these two questions?

Explain why you do not see any interference between the light from two light bulbs connected to the same power supply.

Explain why you can hear interference between sound from two loudspeakers that are connected to the same source

HELP!!
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sianasiggers
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May I ask where you found these questions?
I'm not 100% sure, but I think that the answer to the first question is because light is emitted in bursts of energy. Therefore the light from the two different light sources aren't coherent, so no visible interference is seen. Interference happens, but it's just all jumbled up.

I'm not too confident with the second question, sorry. I hope this helps a little
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hannah-ri0
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The answer to that first question sounds right. Thank you!!! my teacher gave me some photocopied questions out of one of the old texts books for practise. Im not sure which text book exactly it was though... Sorry
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Arbolus
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(Original post by thebrahmabull)
In many places I see written that for destructive superposition to occur, waves must be 180 degrees out of phase. Is that the only way destructive superposition occurs? Shouldn't the waves cancel out in such a situation? Under what condition would a wave he created which has less amplitude than each component wave?
And would the process of such creation be called destructive superposition?
There's a difference between complete and incomplete destructive superposition. If complete, the component waves must have the same amplitude and frequency and be 180 degrees out of phase, in which case they will cancel exactly and produce no resultant wave. This is probably what the examples you've seen have talked about.

If the destruction is incomplete, either the component waves have a phase difference that is not 180 degrees but is closer to 180 than it is to 0 or 360, or they have different amplitudes, or both. This will produce a resultant wave with a non-zero amplitude that has a smaller amplitude than either of the component waves.
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Arbolus
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(Original post by hannah-ri0)
can anyone answer these two questions?

Explain why you do not see any interference between the light from two light bulbs connected to the same power supply.

Explain why you can hear interference between sound from two loudspeakers that are connected to the same source

HELP!!
(Original post by sianasiggers)
May I ask where you found these questions?
I'm not 100% sure, but I think that the answer to the first question is because light is emitted in bursts of energy. Therefore the light from the two different light sources aren't coherent, so no visible interference is seen. Interference happens, but it's just all jumbled up.

I'm not too confident with the second question, sorry. I hope this helps a little
Coherence is certainly one factor. However, I think probably what they're looking for in this case is that interference is most visible if the distance between the two sources is close to the wavelength of the emitted signal. The wavelength of light is on the order of hundreds of nanometres whereas the wavelength of sound is on the order of several metres, so in a normal-sized room the effect is going to be more noticeable for sound than for light.
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hannah-ri0
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(Original post by Arbolus)
Coherence is certainly one factor. However, I think probably what they're looking for in this case is that interference is most visible if the distance between the two sources is close to the wavelength of the emitted signal. The wavelength of light is on the order of hundreds of nanometres whereas the wavelength of sound is on the order of several metres, so in a normal-sized room the effect is going to be more noticeable for sound than for light.
Okay thanks. any idea for question 2?
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Arbolus
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(Original post by hannah-ri0)
Okay thanks. any idea for question 2?
Same answer - the distance between the speakers is going to be close to the wavelength of the sound they emit.
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Stonebridge
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The sounds from the two speakers are coherent because they come from the same original source.
The light from the two light bulbs is not coherent because the light is emitted in random pulses with random phase differences from both. To make light coherent you need to take the light from a single source (one point on one bulb) and split it into two, as in the double slit experiment.

The phenomenon is called interference
To observe constructive or destructive interference the sources need to be coherent and, ideally, of the same amplitude.
.
Superposition is the name given to the way in which the resulting amplitude of a wave at a point, formed by the combining of two separate waves, is found by adding adding the amplitudes of the two waves at that point. This needs to take into account the fact that the amplitude can be positive or negative at that point.
So if the two combining waves have amplitude A, by superposition the resulting wave can have a maximum amplitude of 2A (=A+A) and a minimum of zero. (=A - A)

Zero amplitude is, strictly speaking, destructive interference. This arises from the original experiments where you obtained light and dark fringes with light. The dark fringes were called areas of destructive interference because no light was seen there. In fact, the intensity of the light in the fringe region is not on - off - on - off, like a zebra crossing, and the dark fringes are not uniformly dark. They appear that way because the naked eye is not sensitive enough to distinguish the subtle variation in the intensity of the light in the fringes.
So yes, interference can produce, by superposition, any intensity of light (or sound) from zero to a value equal to the sum of the amplitudes of the the two waves.
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hannah-ri0
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(Original post by Stonebridge)
The sounds from the two speakers are coherent because they come from the same original source.
The light from the two light bulbs is not coherent because the light is emitted in random pulses with random phase differences from both. To make light coherent you need to take the light from a single source (one point on one bulb) and split it into two, as in the double slit experiment.

The phenomenon is called interference
To observe constructive or destructive interference the sources need to be coherent and, ideally, of the same amplitude.
.
Superposition is the name given to the way in which the resulting amplitude of a wave at a point, formed by the combining of two separate waves, is found by adding adding the amplitudes of the two waves at that point. This needs to take into account the fact that the amplitude can be positive or negative at that point.
So if the two combining waves have amplitude A, by superposition the resulting wave can have a maximum amplitude of 2A (=A+A) and a minimum of zero. (=A - A)

Zero amplitude is, strictly speaking, destructive interference. This arises from the original experiments where you obtained light and dark fringes with light. The dark fringes were called areas of destructive interference because no light was seen there. In fact, the intensity of the light in the fringe region is not on - off - on - off, like a zebra crossing, and the dark fringes are not uniformly dark. They appear that way because the naked eye is not sensitive enough to distinguish the subtle variation in the intensity of the light in the fringes.
So yes, interference can produce, by superposition, any intensity of light (or sound) from zero to a value equal to the sum of the amplitudes of the the two waves.
thank you so much this is so helpful.
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