If a beam of unpolarised light, travelling inside a block of glass of refractive index 1.5 undergoes total internal reflection at one of the internal surfaces, then:
A The refractive index outside the block must be less than 1.5.
B The refractive index outside the block must be greater than 1.5.
C The beam must be incident on the surface at the critical angle.
D The reflected beam is completely polarised.
E There is no reflected beam.


T.I.R occurs when light travels from high densely medium to less dense medium, so therefore would the option be A) outside the block, refractive index is less than 1.5?

Total internal reflection is discussed here:

Total internal reflection - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Total_internal_reflection)

\theta_c=arcsin(\frac {n_2}{n_1})

In your case, n1 is 1.5.

The critical angle is the minimum angle required to get total internal reflection. It doesn't have to be exactly the critical angle.

Since the range of the sine function is -1 to +1, n2 must be chosen to get a real (as opposed to an imaginary) angle. For example, if n1 is 1.5, and n2 is 2.0

\theta_c=arcsin(\frac {2.0}{1.5}) = arcsin(1.333)

and arcsin(1.333) is undefined.

Option (A) is correct