Seeing Newton's Rings: Why Magnification is Needed

[koustav]

why do we can't see Newton's ring through our naked eye?why do we need microscope? but interference pattern due to oil film or soap bubble can be seen through naked eye

 

[blue_leaf77]

Because the distance between fringes is small enough for our eyes to resolve with ease at a typical viewing distance.

 

[koustav]

Because the distance between fringes is small enough for our eyes to resolve with ease at a typical viewing distance.

so what about fringes formed due to soap bubble and oil layer on water?

 

[blue_leaf77]

It's not so easy to model the interference effect in the layer of soap or oil in one's daily encounter. In Newton's ring experiment, the "thin layer" is made out of lens, therefore the thickness varies monotonically as you further form the lens center. This will lead to the formation of many interference orders on the lens surface. On the other hand, thin layer made out of oil or soap varies in thickness in an undefined way, i.e. it's not monotonous. So, you can only expect to see a few orders of interference on the layer's surface. Imagine you can somehow form a bulk material shaped like a lens but made out of oil, since the refractive indices of these materials are on the order of a few units, you should observe that the Newton's rings formed will also be closely separated as it is in the ordinary lens.

 

[nasu]

There is nothing intrinsic to Newton's rings to require a microscope for viewing. See here for example:
You may need a microscope if you want to measure the spacing, for example. It is also possible that the spacing between rings is too small to see comfortably if the spacing between the lens and glass plate varies too fast. But you can arrange to see them with naked eye. Actually you can see a similar pattern between two microscope slides pressed together.

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