Newton's rings experiment

In a Newton&#39;s rings experiment, a long length lens of refractive index 1.3 is placed on a glass plate of refractive index 1.5. Circular interference finges are observed as light of wavelenght 589nm falls vertically onto the arrangement.

(a)Is the central spot bright or dark?Explain.

(b)State Two changes to the interference fringes when a liquid of refractive index 1.4 introduced into lens

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In a Newton's rings experiment, a long length lens of refractive index 1.3 is placed on a glass plate of refractive index 1.5. Circular interference finges are observed as light of wavelenght 589nm falls vertically onto the arrangement.

(a)Is the central spot bright or dark?Explain.

中央是暗斑(dark)。

因為光經透鏡(lens)反射時和入射光是同相。(由密介質經疏介質反射)

而光經玻璃(glass)反射時和入射光是反同。(由疏介質經密介質反射)

所以兩束光線一和入射線同相，另一為反相，所以兩束光是反相，波程差為零時為相消干涉。

(b)State Two changes to the interference fringes when a liquid of refractive index 1.4 introduced into lens

有兩項改變

1) 中央變為光斑。

2) 每一個環的距離變小。

1) 中央變為光斑。

因為光經透鏡(lens)反射時和入射光變成是反相。(由折射率1.3的透鏡進入1.4的液體中，由疏介質往密介質)

而光經玻璃(glass)反射時和入射光相仍為反相。(由疏介質經密介質反射)

所以兩束光線一和入射線同為反相，所以兩束光是同相，波程差為零時為相長干涉。

2) 每一個環的距離變小。

因為光在液體中的波長變少，所以環間的距離減少。