Monday, 3 December 2007 - 6:19 PM EST

This is the case because the index of refraction of an air bubble is less then that that of air causing the interference to be constructive producing a light spot. However, the index of refraction for glass is greater then that of air therefore the rays will be out of phase producing a destructive interference producing a dark spot

Monday, 3 December 2007 - 7:13 PM EST

Interference fringes only occur with thin films because the film is only nanometers thick, allowing it to change the wavelength of light by only fractions of a wavelength.  This applies to most angles of light as well.  Even light traveling at steep angles is only altered by a a fraction of a waveleght.  A thick piece of glass, however, does not allow for this slight variation.  Light must travel a long distance and is out of phase by random amounts.  There is no consistent alteration of the wavelength, so no interference patterns are created. 

Monday, 3 December 2007 - 7:29 PM EST

Because thin films are so thin, light refracting through them is changing by portions of a wavelength.  After such a minimal change, they are then reflected back, but white light is divided into different colors because waves are slightly out of phase, but not enough to create destructive interference.  With glass, the thickness allows light to separate to degrees that do not allow for the slight variation needed to separate light into colors, having the effect of again showing white light

Monday, 3 December 2007 - 8:05 PM EST

Like Matt and Dave already said, the reason why the interference fringes occur in the soap bubble is that constructive interference is occuring.  This is because the surface of the bubble is thin enough to have the interfering waves differ by whole multiples.  In the glass, however, the wave lengths have further distances to travel, and since the index of refraction of the glass is higher than that of the air, the waves differ by 1/2 cycle, which creates destructive interference and therefore darkness.

Monday, 3 December 2007 - 8:06 PM EST

I agree with matt and dave. The slight variations in the thin film are not enough to produce constructive or destructive interference, only enough so that the light waves are slightly out of phase, causing white light to turn colors. The incredibly thin film is small enough to cause these slight variations.

Monday, 3 December 2007 - 10:08 PM EST

I agree with Matt and Joe. Since the soap bubble has aA thickness of only nanometers, the change of the wavelength of light only changes by a small amount which is why interference fringes occur with thin films over the thick glass. When light travels through a film it refracted much less compared to a thick piece of glass.

Tuesday, 4 December 2007 - 7:14 AM EST

The reason is obvious why interference fringes are noticible in thin films and not in a thick piece of glass. It is due to the difference in the thickness between the two. Since a thin film will be only nanometers thick then that change of going through the thin film will result in the wavelength of light only being changing very slightly. This means that no matter the angle that the light wave comes into contact with the bubble it will always produce constructive interference or a light spot. However, in the case of a thick piece of glass the change in the wavelength of the light will be more drastic then the thin film and combined with the fact that it will most likely have a higher index of refraction. This means that is will cause destructive interference or dark spots so, you will not be able to tell the interference fringes on a thick piece of glass.  

Tuesday, 4 December 2007 - 7:19 AM EST

Interference firnges are noticible only for a thin flike, like a soap bubble, because you have constructive interference.  The surface of the buble is very very thin and therefore this causes for waves to interfere and differe by a whole multiple.  On the other hand, with glass the waves have to travel a longer distance knowing that the index of refraction in glass is higer than the index refraction of air, therfore, the waves will differ by half units.  This conclusively will create a dark spot which comes from destructive interference.  I think that everyone agreed on this one.

Tuesday, 4 December 2007 - 8:37 AM EST

A thin film, such as a soap bubble, produces interference fringes because the light is only altered by a fraction of a wavelength, since the film is only nanometers thick.  However, for a thick piece of glass, the light separates by a much greater margin, greater than that necessary to observe interference fringes and no fringes are seen. 

Tuesday, 4 December 2007 - 8:41 AM EST

 I agree with Dave, Matt and Bob. In a thin film, the wavelength is slightly altered by portions of wavelengths which results in constructive interference. These slight differentiations usually result in colored light, the color depending on the length of the path from the surface of the film to the bottom of the film, then back to the top. In a thick piece of glass, the wavelength is altered by larger, random increments, making any reflection patterns non-discernable. The slight variations required for colors are not present and only white light is seen.