# Homework Help: Index of refraction from reflected light (polarised)

1. Oct 14, 2015

### Jon.G

1. The problem statement, all variables and given/known data
A beam of light is reflected from the surface of some unknown liquid, and the light is examined with a linear sheet polarizer. It is found that when the central axis of the polariser (that is, the perpendicular to the plane of the sheet) is tilted down from the vertical at an angle of 54.30◦ , the reflected light is completely passed, provided the transmission axis is parallel to the plane of the interface. From this information, compute the index of refraction of the liquid.

2. Relevant equations

3. The attempt at a solution
Honestly, I'm having some trouble even just picturing this. The wording about the polariser confuses me a little.

I'm not sure where to start. My first thought is to go to Brewster's angle as this involves reflected light getting polarised, but I thought a condition for this was that Incident (or reflected) angle + transmitted angle = 90.
But if the transmission axis is parallel to the interface this would mean the light is normally incident, and this doesn't feel right (and even if it was I wouldn't know where to go from here)

Any help or hints appreciated.

2. Oct 14, 2015

### ehild

The plane of the polarizer has to be perpendicular to the reflected beam, and its transmission axis is parallel to the interface, so perpendicular to the plane of incidence.
What do you know about the reflection of the parallel and perpendicular polarized light? Is it any angle of incidence when the whole reflected light is polarized perpendicularly to the plane of incidence, no matter of the polarization state of the incident light? The polarizer transmits the whole reflected intensity, if the reflected light is polarized parallel with the transmission axis of the polarizer. That happens at 54.3°angle of incidence.