# Can glass reflect light?

## Main Question or Discussion Point

As light travels from air into glass, it travels from an optically less dense medium to a more dense medium. This means there is no chance that at any angle, the light will hit the glass surface to be totally internally reflected since one of the requirements for total internal reflection is for a ray to travel from a more dense to a less dense medium.

However, how come you see a reflection of yourself when you look into glass?

Im pretty sure there's an explanation for this but it has been puzzling me for a while.

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Simon Bridge
Homework Helper
You see your reflection in glass because glass reflects light. Ordinary window glass reflects about 4% of the light incedent upon it.
http://physics-animations.com/Physics/English/rays_txt.htm

You will notice that your reflection in a window is not as clear as the view beyond the window ... unless it is dark on the other side.
Technically, there are two reflections - one from each surface.

It is possible to treat glass with a thin film so that there is no reflection - or just manufacture the glass very carefully so that the reflection from the back surface destructively interferes with the reflection from the front adding to nothing.

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Any material reflects light.
As simple as that!
Try your rough cellphone cover, viewing at a really small angle with the surface. Doesn't it reflect the wall behind?

sophiecentaur
Gold Member
Whenever a wave encounters the transition between two media with different wave speeds then there will be some reflection (sound / light/ ocean / waves on strings etc. etc.).

DrDu
Total internal reflection is a special case of reflection. Or, to put it differently, not all reflections are total internal reflections.

"Total internal reflection" means what it says: total. If you don't have total reflection, you still have partial reflection. The reflectivity of dielectrics depends on angle of incidence, so that angles closer to grazing are more strongly reflected. This is described by the http://en.wikipedia.org/wiki/Fresnel_equations" [Broken]

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sophiecentaur
Gold Member
The thing about TIR is that it really is TOTAL. There is no mechanism for a wave to propagate energy onwards through the less dense medium. There is actually some energy on the other side of the boundary but the wave (evanescent wave) only hangs around against the surface and doesn't actually carry energy away.
TIR has to involve an oblique incidence, btw, it never works 'head on' to the surface.

Simon Bridge