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## Main Question or Discussion Point

Hi,

When there is reflection, we generally use the phase shift upon reflection to be [itex]\pi[/itex]. Where does this [itex]\pi[/itex] come from or is it arbitrary? I ask because I came across an optics book which describes beam splitters (a mirror is of course a beam splitter with reflectivity, R=1 and transmittivity, T=0) and as long as we have

[tex]e^{ikx} -> \sqrt{T}e^{ikx} + \sqrt{R}e^{i\theta}e^{iky}[/tex]

[tex]e^{iky} -> \sqrt{T}e^{iky} + \sqrt{R}e^{i\theta'}e^{iky}[/tex]

and [itex]\theta+\theta'=\pi[/itex]

it is perfectly valid to choose any phase shift for the reflected beam. So why is [itex]\pi[/itex] everywhere? In particular, if we talk about thin film interference, the fact that it is [itex]\pi[/itex] seems to be very important.

And on the same note, reflected light will experience a 180 degree phase change when it reflects from a medium of higher index of refraction and no phase change when it reflects from a medium of smaller index. Why is this so?

Thank you!

When there is reflection, we generally use the phase shift upon reflection to be [itex]\pi[/itex]. Where does this [itex]\pi[/itex] come from or is it arbitrary? I ask because I came across an optics book which describes beam splitters (a mirror is of course a beam splitter with reflectivity, R=1 and transmittivity, T=0) and as long as we have

[tex]e^{ikx} -> \sqrt{T}e^{ikx} + \sqrt{R}e^{i\theta}e^{iky}[/tex]

[tex]e^{iky} -> \sqrt{T}e^{iky} + \sqrt{R}e^{i\theta'}e^{iky}[/tex]

and [itex]\theta+\theta'=\pi[/itex]

it is perfectly valid to choose any phase shift for the reflected beam. So why is [itex]\pi[/itex] everywhere? In particular, if we talk about thin film interference, the fact that it is [itex]\pi[/itex] seems to be very important.

And on the same note, reflected light will experience a 180 degree phase change when it reflects from a medium of higher index of refraction and no phase change when it reflects from a medium of smaller index. Why is this so?

Thank you!