Depending on viewing angle, the apparent distance between tracks It is the gap between tracks that reflects the most light (or passes it if you remove the backing). A CD is pressed with tracks (either with bit pits, or just a groove for photosensitive dye), and the aluminized layer is placed over that. On a CD, the important distance is that between the 'data tracks', not the thickness of the disk or the spacing between 'bits' (a blank CD-R already diffracts). Reflection and diffraction merely refer to the two sides of the obstacles: reflected light bounces off the obstacles and diffracted light goes through the gaps. Other colors don't appear there because their wavelengths are not exact divisors of the multiple path lengths, so they can cancel out. For any colored spot that you see, there are several paths whose lengths differ only by a multiple of the wavelength. When the apparent distances between obstacles match the wavelength (color) of light, that color is reinforced (not cancelled, because the cancelling photons went through the gaps between the obstacles). An individual photon can go in any direction after a disturbance (at least we can't predict it), but paths whose lengths are much longer than the straight line path will cancel because the phase at the detector (eye) is affected by the path length. We often think that light goes in straight lines, and reflection angle equals incidence angle, but that is only a statistical property of bulk light. I mean reflection is there because of reflective surface, and even because of some thickness of aluminum I see colors (which is like thin films)?Īnd now comes the bumps/pits on cd? Because of them I also see colors or? For a reflection grating, this means that the diffracted beam is back-reflected into the direction of the incident beam (blue beam in picture). Is this like combination of reflection and diffraction? The Littrow configuration is a special geometry in which the blaze angle is chosen such that diffraction angle and incidence angle are identical. So my, my main concern is are colors over there because light travels different distances when in comes to the surface of CD because of bumps/pits?Ĭan I interpret diffraction colors on CD same as color on thin films?īecause although one part of CD is quite reflective surface the other is almost fully transparent. So when I shine a light on CD, I see different colors and they are changing I move the disc, which is because I change the angle $\theta$ between my eyes and disc which is followed by diffraction condition: I know that CD has tiny pits/bumps (order of size is micrometers). What is bothering me is what is the reason of why do we see the colors on compact disc? So, we all know if you shine some light on CD (compact disc) or DVD, you can see all the colors from red to violet.
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