Chapter 17.4- Thin-film interference Flashcards
What is thin film interference
The interference of light waves reflected from two boundaries
Why does the reflection of light lead to two reflections
- Reflected from the front surface of a sheet of glass
2. Reflected from the backsurface as it exits the glass into the air
When does light undergo a phase change
If it reflects from a boundary at which the index of refraction increases
When does light not undergo a phase change
There is no phase change at a boundary where the index of refraction decreases.
The two reflections we have also interfere with each other. What is the difference between when they’re in phase and out of phase
If the waves are in phase they will interfere constructively and cause a strong reflection.
•If they are out of phase they will interfere destructively and cause a weak reflection, or no reflection at all.
The path-length difference of the reflected waves is Δd = 2t. Why?
because the second wave travels through the film of thickness t twice.
The phase change that occurs when a light wave reflects from a boundary with a higher index of refraction is equivalent to adding an extra half-wavelength to the distance traveled.
If neither or both waves have a phase change due to reflection, then?
the net addition to the path-length difference is zero.
•The effective path-length difference is Δdeff = 2t.
If only one wave has a phase change due to the reflection, the effective path-length difference is?
is increased by one half-wavelength.
•The effective path-length difference is Δdeff = 2t + ½λ.
Does a film have to be a solid material?
No. It can also be air
A thin layer of air between two microscope slides pressed together Creates what? And what does the The varying air layer’s thickness cause?
Light and dark fringes
The varying air layer’s thickness Causes constructive and destructive interference at different points between the slides.
The bright colors of soap bubbles or oil slicks on water are caused by what?
They are due to thin-film interference of white light, or a mixture of all wavelengths, rather than just one wavelength of light.
For a soap bubble, what part of it undergoes a phase change?
The light reflecting at the front surface of the bubble (the air-water boundary) undergoes a phase change, but the back Reflection does not
What type of interference does light near the end of the visible spectrum undergo?
Destructive interference.
What type of interference does light undergo near the middle (green) of the visible spectrum undergo and what does it visually appear as?
Constructive interference and is strongly reflected.
Soap film of this thickness will therefore appear green
What is t here
The thickness
