Light On Glass

So you want a glass wall there? But what about the sun’s refection? Is the architect to blame?

Ah, another high-rise down-town building curtained with glass. Not even smart glass. Not even intelligent glass. Certainly not energy efficient glass. Perhaps glass with a blue tint as a nod to the largely misinformed public who think that a hint of a tint indicates energy efficiency or shade or . . . whatever.

Glass can be used both as a receptor – as in the case of glass-encased solar panels, or as a reflector – as in the case of the curved-mirror solar collectors that concentrate the reflected light to a heat point, thus generating energy. But glass has, anyway, a natural tendency to reflect light from certain angles and at times this can cause serious problems to both the environment and us citizens – as in this recent news item. Have a look at the video –

I have borrowed here from a website to offer a simplified explanation (reflection and refraction can get very, very complicated). The path of light in air incident on and transmitted through a glass plate is shown in the figure below. The angle of the incident ray to the normal is 45° and equals that of the reflected ray. The transmitted ray is refracted at an angle of 28° to the normal and exits the glass at an angle of 45° to the normal, an angle equal to that of the incident ray. This explains why, for example, the image we see through a flat-glass window pane is unchanged from that seen through an open window.

Light on GlassLight Incident on a Glass Plate

The reflected part of the ray is shown along with the light path for the refracted component.

Light incident normal to a glass plate does not change direction as the transmitted light continues normal to the surface (air/glass interface). The light is not refracted (that is, no change in angle) but the wavelength and velocity do change. Light does reflect as it encounters the air/glass interface (about 4% in this case).

Oh dear. And here, if you want to learn even more about this subject by carrying out some fascinating exercises:

And just remember the possible, unintended, consequences of your next glass interface eh sunshine!


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