The phenomenon of interference in insulating glass units ? (the so-called rainbow on the glass).

Optical effect – We sometimes receive complaints about the appearance of a "rainbow" of colour patterns in insulating glass units. They are not caused by glass defects or dirt on the glass, but are merely an optical illusion caused by light waves interfering with each other as they pass through or are reflected from an insulating glass unit. There are usually two types of bands: Brewster’s fringes and Newton’s rings. Brewster’s fringes are more visible in units made of float glass than in units made of drawn glass. The reason for this is that float glass has optically almost flat surfaces and the glass thickness is more uniform. Typically, these patterns are only visible when the glass unit is viewed at an angle. In the case of Newton's rings, the patterns are seen only in the centre of the unit and are either circular or elliptical in shape. Their presence indicates a faulty unit that should be repaired or replaced.

What are Brewster’s fringes?

BREWSTER’S FRINGES – They can appear in a unit where the two panes are almost of the same thickness. The difference in thickness between the two panes must be very small and approximately the same as the average wavelength of visible light, i.e. 0.0005 mm. Millions of light waves pass through or are reflected from an insulating glass unit. The reason that Brewster’s fringes are observed more often in units made of float glass than of drawn glass is the fact that float glass, which is a much better quality product, is very flat and presents relatively small differences in thickness. However, as already mentioned, a necessary condition for the appearance of the Brewster’s fringes is the presence of two panes of almost identical thickness in the insulating glass unit. The difference in thickness should be approx. 0.00035 to 0.0007. This condition, although rare, is more often met by float glass than by drawn glass. In addition, drawn glass has defects that mask the Brewster fringes that may occur.

What are Newton’s rings?

This optical effect occurs only in defective insulating glass units, when two panes touch (one to the other) or almost touch at the centre of the insulating glass unit. This optical effect is a series of concentric colour rings with the centre being the point of contact or close contact between the two panes. The rings are approximately circular or elliptical in shape. Newton’s rings are caused by interference, in which light waves have different path lengths when passing through a defective thermal insulation unit or are reflected from it. In this case, however, it is not the nearly equal thickness of the panes that causes the waves to be phase-shifted; the interference occurs inside a thin, wedge-shaped air film adjacent to the contact – or near contact – point of two panes.

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