TEST OF PRACTICAL KNOWLEDGE QUESTION Using the diagram above as a guide, carry out the following instructions: Fix a plain sheet of paper on the Celotex boa...

Answer Details

Test of practical knowledge (refraction through a glass prism/block)

Note: the traced rays, the measured angle \(\theta\), the table and the graph must come from your own ray-tracing work; the procedure and the fully determined theory parts are given below.

Method (summary): trace the block outline ABCD, draw the normal, direct a ray PQ making the incidence angle \(i\) with the normal, and locate the emergent ray with pins P₃ and P₄. For each \(i = 10^\circ, 15^\circ, 20^\circ, 25^\circ, 50^\circ\), measure the angle \(\theta\) between the emergent ray and the face, then compute \(\cos\theta\) and \(\sin i\). Tabulate, plot \(\cos\theta\) (vertical) against \(\sin i\) (horizontal), and find the slope \(s\).

Two precautions:

• Fix the pins upright (vertical) and well separated so the ray direction is accurate.
• Mark the outline of the block sharply and read all angles with the eye directly above the protractor to avoid parallax.

(b)(i) Laws of refraction of light

• First law: the incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane.
• Second law (Snell's law): the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given pair of media, \(\dfrac{\sin i}{\sin r} = n\).

(b)(ii) Meaning of refractive index 1.65

It means that when light passes from a vacuum (or air) into the material, the ratio \(\dfrac{\sin i}{\sin r} = 1.65\); equivalently, the speed of light in vacuum is 1.65 times its speed in the material \(\left(n = \dfrac{c}{v}\right)\), so light travels more slowly and bends more towards the normal in that material.