In this experiment, you are asked to plan an experiment to determine the critical angle of a semicircular glass block and to investigate what happens to the...

(a) Labelled diagram of the arrangement: [3]

The diagram shows the semicircular block on paper with the flat surface on one side. [1] The ray enters through the curved surface and is aimed at the centre O of the flat surface. [1] The normal is drawn perpendicular to the flat surface at O. [1]

(b) The ray should enter through the curved surface because: [1]

When the ray enters perpendicular to the curved surface (along a radius), it does not refract at the curved boundary. It passes straight through to the flat surface without changing direction. This ensures that refraction or total internal reflection occurs only at the flat surface, making the angle measurement meaningful.

(c) Procedure for finding the critical angle: [3]

1. Direct the narrow ray of light from the ray box at point O on the flat surface. Start with a small angle of incidence. Observe the refracted ray emerging from the flat surface into the air. [1]
2. Slowly rotate the ray box (or the block) to increase the angle of incidence at the flat surface. Watch the refracted ray bend further from the normal. Keep increasing the angle until the refracted ray just disappears along the flat surface. [1]
3. The angle of incidence at which the refracted ray just disappears and travels along the surface is the critical angle. Mark the positions of the incident and reflected rays on the paper, draw the normal, and measure the critical angle using a protractor. [1]

(d) The measurement to record is: [1]

The angle of incidence at the flat surface when the refracted ray just disappears (this is the critical angle c). Repeat the measurement several times and take an average to improve reliability.

(e) Observations at different angles:

(i) Below the critical angle: The ray is partly refracted out of the glass block (bending away from the normal as it passes from glass to air) and partly reflected back inside the block. Both a refracted ray and a weaker reflected ray are visible. [1]

(ii) Equal to the critical angle: The refracted ray travels exactly along the surface of the glass (at 90° to the normal). The reflected ray inside the block becomes stronger. [1]

(iii) Above the critical angle: Total internal reflection occurs. All the light is reflected back inside the glass block, obeying the law of reflection (angle of reflection = angle of incidence). No refracted ray emerges from the flat surface. [1]