This difference in depth perception can be used to calculate refractive index. The formula is: Refractive index
n = Real depth
_________________________ Apparent depth
Since real depth is greater than or equal to apparent depth, n is greater than or equal to 1. QUESTIONS AND ANSWERS
4. A block of Perspex is on top of a picture. When the picture is viewed through the Perspex, it appears to be 4 cm away. However, the thickness of the Perspex is measured as 5.96 cm. From these fi gures, determine the refractive index of Perspex.
You may now complete Exercise 14B (page 152).
You may now proceed with the mandatory experiment: Measurement of the refractive index of a liquid (page 430).
Total internal reflection
As light exits a denser medium, the ray refracts away from the normal. We can extend this fact to say that eventually the ray will bend so far away from the
normal that it begins to reflect back again. This phenomenon is total internal reflection (TIR) (Fig. 14.10). air (rarer medium)
r C i glass (denser medium) 12 3
total internal reflection
FIG. 14.10 Total internal reflection occurs when a ray bends so far away from the normal that it begins to reflect back again
90° (runs along block interface)
i > C
The critical angle is the angle of incidence in the denser medium that results in an angle of refraction of 90° in the rarer medium.
Total internal reflection occurs when a light ray, travelling from a denser to a rarer medium at an incident angle greater than the critical angle, is completely reflected.
