A student is investigating the refraction of light through a rectangular glass block using the optical pin method. The student pins a sheet of paper to a drawing board, traces the outline of the glass block, and draws an incident ray towards one face of the block.
To trace the incident ray, the student places two pins, P1 P_1\,P1 and P2P_2P2, on the line representing the ray. The distance d d\,d between P1 P_1\,P1 and P2 P_2\,P2 is measured to be 2.5 cm.
State whether this distance d=2.5 cmd = 2.5\text{ cm}d=2.5 cm is suitable for accurate ray tracing. Explain your reasoning and recommend an appropriate minimum distance.
Explain how a larger separation between pins P1 P_1\,P1 and P2 P_2\,P2 leads to a more accurate determination of the direction of the ray.
Suggest two practical precautions the student should take when placing and viewing the pins to ensure that the rays are traced as accurately as possible.
Suggest one reason why different students, all carrying out this experiment carefully with the same apparatus, may still obtain slightly different values for the measured angle of refraction.
216 exam-style questions on CIE IGCSE Physics 3.2 Light, covering 3.2.1a Reflection of light, 3.2.1b Images in a plane mirror, 3.2.2 Refraction of light, 3.2.2a Refraction of light, 3.2.2b Total internal reflection, 3.2.2c Refractive index and critical angle, 3.2.2d Optical fibres, 3.2.3a Converging and diverging lenses, 3.2.3b Ray diagrams and the nature of images, 3.2.3c Virtual images and the magnifying glass, 3.2.3d Correcting long-sightedness and short-sightedness, and 3.2.4 Dispersion of light. Each one has a worked solution and a mark scheme showing where the marks go.