REFRACTIVE INDEX & TOTAL INTERNAL REFLECTION

By sending a narrow beam of light through a perspex block, you can easily see the refraction (bending) of the light as it re-enters the air. This is called refraction. As the light speeds up as it emerges from the block, its wavelength increases and the angle with the normal also increases.








Now if we increase the angle of incidence, we begin to see a lot more reflection, as the light begins to strike the tiny irregularities on the perspex/air interface at angles greater than the critical angle.












Finally, if we increase the angle of incidence so that all the lght strikes the perspex/air interface at an angle greater than the critical angle. we see the phenomenon called
Total Internal Reflection

Students collect data for a large number of incident and refracted angles and use MSExcel spreadsheets to calculate the sines of these angles.

Angle in perspex=P

Angle in air=A

Because SineA/SineP is the refractive index of perspex relative to air, a graph of SineA vs SineP will have a gradient equal to the refractive index of the perspex.

Using a data-projector the resulting graph can be shown to other students.

Good experimental practice results in a good fit to the regression line!