When we shine a laser beam to the both side of the mirror (silver coated and uncoated), I am not getting fringes. How can we recognise both the surfaces? Is there any problem with my technique ?
In case of usual mirror, reflective material (in your case, silver) is deposited on a glass plate (substrate). Ravi is asking for the material of the plate.
If you are using a mirror with a transparent substrate (plate) in the laser wavelength, you can identify coated and uncoated surface by measuring reflected laser spots.
When you shoot your laser to uncoated surface in oblique incidence condition, you will get two reflected laser spots.
In this case, you have the fresnel reflection from the uncoated surface and mirror reflection from the coated surface.
What about Brewster's Angle? The intensity of the light reflected in the dielectric surface should be =0 for p-polarized incident light.
Best method of determination of surface type has been proposed by Juan Marcos Sanz. His proposition includes the explanation about reason why interference of reflected beam is not observed (at least by eye): very low contrast of interference resuls from huge difference of two reflected beams.
using Brewster angle you can find it. and your laser is H or V polarized??
Both methods suggested by Muhammad and Juan are applicable. What you can also do is a simple straight forward measurement using power meter to measure the reflected beam power of He-Ne low power laser (say 5mw) from both surfaces. Or even use a Thorlab's Det 10 or 100 and get the signal's value on a scope . The coated surface should show higher value.
Best,
Rad
If the silver (conductor) is on glass (insulator), then shouldn't a simple resistivity test determine which side the silver is on? A "multi-meter" from "Home Depot" would do the trick, wouldn't it? :-)
Am I missing something here?
https://www.flickr.com/photos/85210325@N04/14348488010/in/album-72157645018820696/
It seems the mirror u have is polished at its back side.First of all just by looking both side of the mirror using naked eyes u can conclude. The side which does not show the depth of the substrate is the coated side.
Now using laser, when u will see the reflection from the non-coated side, at long distance u will find two laser spot while one when reflected from coated side.
Look at the mirror edge-ways at an acute angle . The unsilvered part shows a double image, for example of a point of light. The silvered part shows one sharp image. Looking sideways is also a test of the averall flatness of a mirror because uneven surface shows a warped image.
The unequivocal precise method for determining film characteristics (thickness & flatness) is X-Ray Reflectivity (XRR) which is a similar concept as Vladimir suggests.
https://www.researchgate.net/post/Would_anyone_have_a_clue_as_to_the_origin_of_unexpected_spots_in_the_real_time_2D_XRR_signal_from_thin_film_samples_on_substrates_Yuneda_Spots?_tpcectx=profile_questions
In some case, I do a quick test by looking to the reflection of one laser beam on unknown mirror. On the screen, if the higher reflection is closer to the incident light, so the coated surface is closer to the screen and vice versa. Just assume that the thickness of the substrate is large enough to separate 2 reflections.
Why do want to use a laser to do the test? There are other simpler methods that you could try!
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