Why does the phase of the reflected wave from a denser medium change?

[Malvina]

TL;DR

Why does the phase of the reflected wave from a denser medium change?

Why does the phase of the reflected wave from a denser medium change?

 

[berkeman]

Welcome to PF.

Can you show us your reading where you have seen this? Do you have experience working with the wave equation and the equations for reflection and transmission at the boundary between two media?

 

[Malvina]

https://en.m.wikipedia.org/wiki/Reflection_phase_change

​

Welcome to PF.

Can you show us your reading where you have seen this? Do you have experience working with the wave equation and the equations for reflection and transmission at the boundary between two media?

https://en.m.wikipedia.org/wiki/Reflection_phase_change​

Optics​

"Light waves change phase by 180° when they reflect from the surface of a medium with higher refractive index than that of the medium in which they are travelling.[1] A light wave travelling in air that is reflected by a glass barrier will undergo a 180° phase change, while light travelling in glass will not undergo a phase change if it is reflected by a boundary with air. For this reason, optical boundaries are normally specified as an ordered pair (air-glass, glass-air); indicating which material the light is moving out of, and in to, respectively"

but I can't understand why this is happening

 

[A.T.]

https://en.m.wikipedia.org/wiki/Reflection_phase_change

​

https://en.m.wikipedia.org/wiki/Reflection_phase_change​

Optics​

"Light waves change phase by 180° when they reflect from the surface of a medium with higher refractive index than that of the medium in which they are travelling.[1] A light wave travelling in air that is reflected by a glass barrier will undergo a 180° phase change, while light travelling in glass will not undergo a phase change if it is reflected by a boundary with air. For this reason, optical boundaries are normally specified as an ordered pair (air-glass, glass-air); indicating which material the light is moving out of, and in to, respectively"

but I can't understand why this is happening

Do you understand it for strings?
https://en.wikipedia.org/wiki/Reflection_phase_change#Strings

 

[A.T.]

See also this old thread:
https://www.physicsforums.com/threa...ransverse-waves-reflect-at-a-boundary.982142/

 

[Malvina]

I think I'm starting to understand. Thank you for helping me!

 

[sophiecentaur]

I think I'm starting to understand.

One way of looking at this involves continuity of quantities on either side of a boundary. In all waves, there are always two quantities that vary along the wave. Sound is a convenient wave to consider; the quantities wil be pressure and displacement. If the changing characteristics ('impedance') of the two materials across the boundary causes a reduction in amplitude (displacement) then the reflected wave will be negative such that the resulting amplitude of the emerging wave is less and there will be continuity at the boundary. If the impedance change is the other way round then the emerging wave will have greater amplitude so incident and reflected waves will add in phase. Note, this doesn't mean that there is an increase in energy flow across the boundary because the power flow is related to pressure and displacement. There has to be continuity; if there weren't continuity then there would be a step change at the boundary and where would that come from??

 

[Baluncore]

but I can't understand why this is happening

When you look in a mirror, why is your image reversed, with a LH helix becoming a RH helix ?

 

[sophiecentaur]

When you look in a mirror, why is your image reversed, with a LH helix becoming a RH helix ?

That is a matter of geometry and would apply to reflections from air into glass and glass into air. The phase change is a different issue.