Total Internal
Reflection
Purpose:
To Investigate the phenomenon of Total Internal
Reflection and to determine the refractive index of
Perspex.
Using a Light Box and a Perspex semi-circular slab, shine a single ray of light towards the
“centre” of the slab, along a radius.
Incident ray
Reflected Ray
Refracted Ray
Task 1
Keeping the incident ray pointing towards the centre of the slab, gradually increase the angle
of incidence, until the angle of refraction is 90. You will notice some colour flaring.
Using a pencil, draw around the slab and mark the direction of the incident ray, the refracted
ray and the reflected ray.
Task 2
At the critical angle, the angle of refraction is 90. Show that the sine of the critical angle ic is
given by sin ic 
1
where n is the refractive index of the Perspex. Measure the critical angle
n
of the Perspex and hence calculate the Refractive index of the Perspex. The actual value for
the absolute refractive index of Perspex is 1.495.
Task 3
Determine the % error of your measurement of the refractive index and suggest possible
reasons for any errors.