IGCSE Physics / Light waves
Practical: refraction in blocks and prisms
Edexcel International GCSE Physics (4PH1), chapter 12: Light waves, specification point 3.17. Free lesson.
Read the revision notes · IGCSE Physics past papers
Refraction practicals show how to trace light through glass and measure its change of direction. A semicircular block isolates refraction at one face, while a prism separates white light into a spectrum.
How to trace a ray through a glass block
Use a ray box with a narrow slit to produce a thin ray. Marking the ray at two points gives a more accurate line, because you can join the crosses with a ruler after removing the block. The ray bends towards the normal as it enters the block and away from the normal as it leaves.
Why a semicircular block bends light at one face
A semicircular block lets you study light leaving glass at just one face. Aim the ray through the curved side towards the centre of the flat face. At the curved side, the ray travels along the normal, so it does not bend there. At the flat face, it bends away from the normal as it leaves the glass. In the example, the angle of incidence inside the glass is 30°. The ray leaves at a larger angle of about 49°.
How a prism makes a spectrum
When white light enters a triangular glass prism, it spreads out because each colour is refracted by a different amount. The colours bend again as they leave the prism, then form a band of colours called a spectrum on a screen. Red is bent the least and violet the most. The splitting of white light into colours is called dispersion.
How to improve ray tracing
Improvement questions need a specific problem and a matching fix. These changes make the ray path and the outline easier to mark accurately. A wide ray makes its middle unclear, so use a narrower slit. Crosses that are too close together make the ray line less accurate, so space them far apart. A thick pencil line can hide small angles, so use a sharp pencil. If the block moves, the rays may no longer meet its outline. Draw round it first and check that it has not shifted.