The discussion revolves around the challenges of observing interference patterns in thin films, specifically using two microscope slides separated by a sheet of paper. Participants explore various light sources and techniques to achieve visibility of fringes, addressing both theoretical and practical aspects of thin film interference.
Participants express various viewpoints on the effectiveness of different light sources and techniques for observing interference patterns. There is no consensus on the best approach, and multiple competing views remain regarding the factors influencing visibility of fringes.
Limitations include the potential for thickness variation in the materials used, the dependence on the quality of light sources, and unresolved questions about the optimal viewing conditions for observing interference patterns.
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?mariut said: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?
According to my caliper it is 0,05mm.Baluncore said:Welcome to PF.
How thick is the paper?
How many fringes would you expect to see?
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.Baluncore said: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.
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.mariut said:The fluorescent lamp is much longer than the slides
there would probably be serious stretch ridges on that so you'd need to raise the slide on just one side.mariut said:the thinnest is the stretched foil
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.sophiecentaur said: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.
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.sophiecentaur said:there would probably be serious stretch ridges on that so you'd need to raise the slide on just one side.
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 said: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.
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.mariut said:The fluorescent lamp is much longer than the slides
there would probably be serious stretch ridges on that so you'd need to raise the slide on just one side.mariut said:the thinnest is the stretched foil
@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.mariut said: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.
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.mariut said:I succeeded to see the fringes.