B Interference in Thin Films

[mariut]

Two microscope slides separated by a sheet of paper at one end. I tried different sources of light, yellow LED, red LED, fluorescent lamp and i can't see any fringes. What am i doing wrong?

 

[Baluncore]

Welcome to PF.

How thick is the paper?
How many fringes would you expect to see?

 

[sophiecentaur]

Two microscope slides separated by a sheet of paper at one end. I tried different sources of light, yellow LED, red LED, fluorescent lamp and i can't see any fringes. What am i doing wrong?

Have you read anything about the effect? There's a clue in the fact that thin films of oil on water produce fringes. How 'thin' is a film of oil?
Wiki is often a good start.

 

[mariut]

Welcome to PF.

How thick is the paper?
How many fringes would you expect to see?

According to my caliper it is 0,05mm.
Well I would be happy to see at least one :)
I tried with many different things, the thinnest is the stretched foil which according to my not very professional caliper is below 0,01 mm. I know the equation so i know it should be about 200 fringes for 0,05mm and 40 for 0,01mm.

 

[Baluncore]

Your eyes are sensitive, so will see any scattered LED light, so you may not notice the dark zones of the fringes. What dark background are you using when viewing fringes? All the light will appear to you to be the same colour. How do you illuminate the wedge from one direction with parallel rays of light?

If you calculate 100 fringes, they will be close spaced. Your light source needs to be collimated and narrowband like the laser diode from a laser pointer, not like an LED that can have quite wide thermal broadening. I would only expect to see the separated fringes of an LED, at the narrowest end of the wedge.

Reflect your light source from a CD or DVD onto a white screen, to estimate how pure, or how narrow, the bandwidth is.

 

[mariut]

Your eyes are sensitive, so will see any scattered LED light, so you may not notice the dark zones of the fringes. What dark background are you using when viewing fringes? All the light will appear to you to be the same colour. How do you illuminate the wedge from one direction with parallel rays of light?

If you calculate 100 fringes, they will be close spaced. Your light source needs to be collimated and narrowband like the laser diode from a laser pointer, not like an LED that can have quite wide thermal broadening. I would only expect to see the separated fringes of an LED, at the narrowest end of the wedge.

Reflect your light source from a CD or DVD onto a white screen, to estimate how pure, or how narrow, the bandwidth is.

I tried with different backgrounds like a black plastic housing of the speaker, a non-reflective screen of my computer or a black cotton t-shirt.
"How do you illuminate the wedge from one direction with parallel rays of light?". Not sure if I know what you mean, I hold the source of light directly above the slides. The fluorescent lamp is much longer than the slides so it covers the whole surface of them. The fluorescent lamp was my prime choice, but when it didn't work I used different available light sources to see if there is any difference.
I don't think I'm capable of estimating the quality of my light source.

 

[sophiecentaur]

The fluorescent lamp is much longer than the slides

It strikes me that such a wide light source will have a big range of angles of incidence. Ideally you need a collimated beam of monochromatic light. A useful source can be a projector shone through a hole. Different colours of light are then available (although the RGB sources are definitely not monochromatic). Try a laser pointer.

the thinnest is the stretched foil

there would probably be serious stretch ridges on that so you'd need to raise the slide on just one side.

Have you searched for 'practical' advice elsewhere? I searched Google with "Thin film interference demonstration" with several useful hits. Fact is that the sort of thickness variation on most 'thin' materials is too great for good patterns - and we see very few in everyday life except for oil films on water and even then you need good lighting. Perhaps a tray of water with oil floating on top would be the most uniform situation. A drop of oil at a time could reliably change the fringe spacing as the thickness changes.. Black material at the bottom of the water could help.

 

[mariut]

It strikes me that such a wide light source will have a big range of angles of incidence. Ideally you need a collimated beam of monochromatic light. A useful source can be a projector shone through a hole. Different colours of light are then available (although the RGB sources are definitely not monochromatic). Try a laser pointer.

The fluorescent lamp is a tube so I can easily cover most of it and leave only the length of the slides uncovered or as I saw on youtube, put the slides inside a cardboard box, make a hole of the size of slides on top of the box and put my lamp on it.
I actually tried a red laser pointer once, but I could see only the small red spot :)

 

[mariut]

there would probably be serious stretch ridges on that so you'd need to raise the slide on just one side.

Both ends of slides are pressed together tightly, so any ridges should be flatened out, anyway my caliper can't see any diffrence between the ends with the foil and the ends without it, so I expect the difference is less than 0,01mm.

 

[mariut]

Have you searched for 'practical' advice elsewhere? I searched Google with "Thin film interference demonstration" with several useful hits. Fact is that the sort of thickness variation on most 'thin' materials is too great for good patterns - and we see very few in everyday life except for oil films on water and even then you need good lighting. Perhaps a tray of water with oil floating on top would be the most uniform situation. A drop of oil at a time could reliably change the fringe spacing as the thickness changes.. Black material at the bottom of the water could help.

Yes, I have, but so far I have not found any useful hints, except one which I found on this forum, there was a thread over a decade ago, somemone had exaclty the same problem as me and actually it was you who suggested the head position "You have to get your head in the right place to view the fringes or the direct specular reflection can mask them.". Of course I tried to look at the slides from different angles but I'm just thinking I may not have paid enough attention to it because as far as I remember I could see only reflections of me or other stuff so tonight I'll try again with carton box if I find one.

 

[sophiecentaur]

The fluorescent lamp is much longer than the slides

It strikes me that such a wide light source will have a big range of angles of incidence. Ideally you need a collimated beam of monochromatic light. A useful source can be a projector shone through a hole. Different colours of light are then available (although the RGB sources are definitely not monochromatic). Try a laser pointer.

the thinnest is the stretched foil

there would probably be serious stretch ridges on that so you'd need to raise the slide on just one side.

Have you searched for 'practical' advice elsewhere? I searched Google with "Thin film interference demonstration" with several useful hits. Fact is that the sort of thickness variation on most 'thin' materials is too great for good patterns - and we see very few in everyday life except for oil films on water and even then you need good lighting. Perhaps a tray of water with oil floating on top would be the most uniform situation. A drop of oil at a time could reliably change the fringe spacing as the thickness changes.. Black material at the bottom of the water could help.

 

[hagopbul]

0.05 mm is thick film about 50 micrometer Shouldn't the thickness of the film or the gap between two slides measured in nano meter?

From the thread I understood the following
He had two transparent slides on a stack the width of the space between two slides is about the thickness of a sheet of paper

 

[Andy Resnick]

Yes, I have, but so far I have not found any useful hints, except one which I found on this forum, there was a thread over a decade ago, somemone had exaclty the same problem as me and actually it was you who suggested the head position "You have to get your head in the right place to view the fringes or the direct specular reflection can mask them.". Of course I tried to look at the slides from different angles but I'm just thinking I may not have paid enough attention to it because as far as I remember I could see only reflections of me or other stuff so tonight I'll try again with carton box if I find one.

@sophiecentaur is correct, you need to carefully prepare the illumination conditions in order to see fringes- thin film interference is a combination of temporal and spatial coherence.

What that means is that in order to produce 'good' fringes, your illumination needs to be monochromatic (single wavelength) and in the form of a plane wave. Using your LED, the easiest thing to do is place the LED a couple of meters (or more) away from your slides. Yes, the intensity will be low and you will need to be in the dark. If you know how to produce a collimated beam using your LED, you'll get brighter results.

 

[mariut]

I succeeded to see the fringes. I was looking everywhere except where I should have been. To see fringes I have to look exactly at the reflection of the fluorescent tube in the microscope slides, previously I avoided being blinded by it for all cost :). It probably wouldn't have happened if I have done it in daylight.
Thank you guys for your assistance.

 

[sophiecentaur]

I succeeded to see the fringes.

In the days of 35mm colour slides, you could get fringes on the screen when the slides were held / protected in glass slides. (referred to as Newton's rings) The wrinkles were a suitable depth. Only fussy people would spot them. When the slide warmed up, the fringes would move about top make themselves a bit more annoying.