Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 09/10/2012 03:30:01

Title: Why would a pattern only be visible in a reflection?
Post by: thedoc on 09/10/2012 03:30:01
Richard Almand  asked the Naked Scientists:
   Hi Guys,

My question is:

The other day I was looking out of the window of the train. I noticed I could see a reflection of the window behind me. There appeared to be a pattern clearly visible on the glazing behind me that was not visible when looking at it directly, even on the same angle. The pattern was large spots with fuzzy edges in a checker board type layout. It looked far too regular to be a natural occurrence, I put it down to something in the tempering or laminating process of the glass, but why is it only visible when viewed in a reflection of the same type of glass?

Richard Almand (From Wellington, New Zealand)
What do you think?
Title: Re: Why would a pattern only be visible in a reflection?
Post by: RD on 09/10/2012 04:47:19
... I noticed I could see a reflection of the window behind me. There appeared to be a pattern clearly visible on the glazing behind me that was not visible when looking at it directly, even on the same angle. The pattern was large spots with fuzzy edges in a checker board type layout ...

A polarising filter can reveal the stress (https://en.wikipedia.org/wiki/Photoelasticity) trapped in laminated (safety) glass ...

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... why is it only visible when viewed in a reflection ...

When viewed at an angle, glass can polarise reflected light ...

(https://upload.wikimedia.org/wikipedia/commons/6/65/Brewsters-angle.svg)
https://en.wikipedia.org/wiki/Brewster%27s_angle

i.e. act like a polarizing filter

Title: Re: Why would a pattern only be visible in a reflection?
Post by: Soul Surfer on 12/10/2012 08:50:58
The light from the sky in particular a clear blue sky is in fact polarised and as the previous answer says this shows in the reflection from the windscreen so you do not need any other polariser.