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[Phy] optics_reflection問題求助

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optics_reflection問題求助

A strip of plane mirror 40 mm in length lies horizontally with one end resting on a fixed support and the other on top of a vertical rod.
A beam of light is reflected from the mirror and forms a spot on a screen 2.0 m above the mirror.

a)If the vertical rod is moved 1 mm upwards, how far does the spot of light on the screen move?       Answer: 100 mm

b)If the smallest movement of the spot of light that can be observed is 1 mm,what is the smallest movement of rod which can be recorded?     Answer:  0.01 mm


我有試下計架,計到佢rotate緊個mirror with angle theta = tan-1 (1/40) ,所以rotate 完, new incident angle 和 reflected angle 都會減theta?

但之後唔識計,有冇高人可以幫幫手指點下,thank you very much
   

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Ks 有無圖...無圖好難明...

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哎,佢question冇圖,我自己咁理解幅圖同咁計: http://imgur.com/F38Z9Xx

如果小弟冇解錯,rotating angle都計到tan-1 (1/40) ,但奇在個spot movement x+x' 就depends on 個incident angle會唔同, 所以唔知點解answer係constant 100mm, 救命,
希望高人路過比少少hints, thanks


question: http://imgur.com/fP6mABO
數字答案: http://imgur.com/N0F7cDm

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