Physics of Diffraction

[cyeokpeng]

In a particular well-known experiment known as Newton's ring interference pattern, we get a dark fringe at the point of contact of the glass lens and the plane glass surface, followed by rings of bright fringes and dark fringes.

This seems to show that at that point of contact, the monochromatic light reflected from the glass surface is 180 degrees out of phase with the light reflected from the lens surface. How can that be possible?

[swansont]

You get a bright fringe at the center.

[cyeokpeng]

From the physics of optics, as light is reflected to the angle near the normal incidence, it will suffer a 180 degrees phase change if the refractive index of the reflecting material is larger than that of the surrounding material.

 

So, at the point of contact with the planoconvex lens and the plane surface, due to 180 degrees phase change, the two reflected waves should "destroy" each other. This should result in the centre point being dark followed by white bands, then dark lines. Any explanation for that?

[cyeokpeng]

From the physics of optics, as light is reflected to the angle near the normal incidence, it will suffer a 180 degrees phase change if the refractive index of the reflecting material is larger than that of the surrounding material.

 

So, at the point of contact with the planoconvex lens and the plane surface, due to 180 degrees phase change, the two reflected waves should "destroy" each other. This should result in the centre point being dark followed by white bands, then dark lines. Any explanation for that?

[swansont]

There is no phase change, since the reflecting light is within the lens. It doesn't see a higher index material. Light reflecting off the plane is in air and does see a higher index, so that light undergoes a ^{[math]\pi[/math]} phase change.

 

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[swansont]

There is no phase change, since the reflecting light is within the lens. It doesn't see a higher index material. Light reflecting off the plane is in air and does see a higher index, so that light undergoes a ^{[math]\pi[/math]} phase change.

 

More