Physics waves 一問

Interference/superpostion 既 maxima 同 minima 係代表咩?

trough + trough 係 maxima 定 minima??


maxima 係 where "contructive interference" occurs

minima 係 where "destructive interference" occurs



05年AL Paper 1A 第2題 b)(ii)

b)(ii) Four positions of microphone B at which successive maxima occur are marked along the line PQ as shown in Fig. 2.3. Their corresponding distances from P are indicated in brackets.

P _________|__________|____________|_____________|_________ Q

---------(26.3cm)----- (58.2cm) -------- (89.3cm) ------ (120.8cm)

Calculate the speed of sound in air.



wavelength= (120.8-26.3)/3 = 31.5cm

Wave speed = 1053 x 0.315 = 332 m/s


如果maxima = constructive interference 既話, 一個wavelength裡唔係應該有2個maxima咩?


08年 AL Paper 1A 第3題b)(i)

In the first-order spectrum produced by the diffraction grating, an intensity minimum is observed at an angular position 33 degree.

Calculate the wavelength corresponding to this intensity minimum.

The answer is:



點解係(1)(lamda), 唔係(0.5)(lamda)?

如果intensity minimum係destructive interference既話, 唔係應該係0.5lamda咩?

我真係相當唔明!! 係米我concept有咩錯? 可唔可以解答下我?

上面D題目, 唔好意思, 我唔識成條連埋圖擺上黎.


2 個解答

  • 天同
    Lv 7
    9 年前

    Q: trough + trough 係 maxima 定 minima??

    A: It is maxima.

    Constructive interference indicates two waves exactly overlapping each other. That is, a crest overlaps a crest and a trough overlaps with a trough. In fact, when the crests overlaps, the troughs will automatically overlapping as the wavelengths of the two waves are the same.

    Destructive interference occurs when a crest overlaps with a trough.

    Q: 點解係除3,唔係除1.5?

    A: The (120.8cm) maximum is 3 orders higher than the (26.3 cm) maximum. This is how the 3 comes in. In other words, if the order of the (26.3cm) max is m, then the order of the (120.8cm) max is (m+3).

    Q: 如果maxima = constructive interference 既話, 一個wavelength裡唔係應該有2個maxima咩?

    A: No, you cannot say that. It is completely a wrong concept.

    By maximum or minimum, we refer to the intensity maximum or minumun. That is, a bright or dark fringe. You cannot isolate a wave. Be aware that at the point of maximium intensity, the wave is actually continuously passing through the point. Do not construe that a crest or trough is stationary at that point.

    Q: 點解係(1)(lamda), 唔係(0.5)(lamda)?

    A: In general, it should be (lamds)/2 for destructive interference to occur. As I do not have the complete question, I don't know whether there is other factors involved that might shift the interference pattern by half a wavelength.

  • 9 年前

    樓主放心 concept無錯 trough+trough的確係constructive interference樓主ge錯係錯係唔理解條題目je

    05果條係microphone A同microphone B收到ge wave ge interference

    假設係依刻microphone A收到ge wave係crest

    microphone B要一樣收到crest先會做到constructive interference

    所以mark低左ge position係 係果刻microphone B收到ge signal同為crest ge地方

    兩個crest之間ge距離咪即係一個wavelengh lor~

    08果條係問緊'IN the first-order spectrum'

    即係佢問緊係依個spectrum入面intensity最弱ge light ge wavelengh幾多

    原本如果就咁用white light射去個diffraction grating ge話

    係angular position 33度出現ge光應該係green light

    咁但係而家係果個位係minimum intensity

    原因係thin film個path difference令green light做左destructive interference


    所以樓主只要當佢係問緊係angular position 33度出現ge光ge wavelengh係幾多咁計就得ga la