Edexcel IGCSE Physics · Spec 3.17
Refraction in Rectangular Blocks, Prisms and Semicircles
Covers trace rays through three glass shapes, Measure angles from the normal and Explain why semicircular blocks are useful.
Physics revision video
Refraction in Rectangular Blocks, Prisms and Semicircles
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Explained
Tracing rays through glass, and why the shape is chosen deliberately
Three shapes appear in this topic, and each is used because of what its surfaces do to the ray. Knowing why a semicircular block is chosen is worth as much as knowing how to trace the ray through it.
The rectangular block
Place the block on paper and draw round it. Send a narrow ray in at an angle, or line up two pins to define it, and mark where the ray enters and where it emerges.
At the first surface the light slows down and bends towards the normal. At the second it speeds up and bends away from the normal by the same amount. The two surfaces are parallel, so the emerging ray is parallel to the original ray, just shifted sideways. That sideways shift is called lateral displacement.
Remove the block and join up your marks to see the path inside the glass.
The semicircular block
This shape solves a specific problem. Aim the ray at the centre of the flat face and it enters through the curved surface first, along a radius, which means it meets that curved surface along its normal and is not refracted at all on the way in.
So the only refraction happens at the flat face, where you want to study it. That is why the semicircular block is the one used for finding the critical angle: you can increase the angle inside the glass without anything happening at the entry surface to complicate it.
The prism
A triangular prism has surfaces that are not parallel, so the two refractions do not cancel. The ray leaves in a new direction, which is why a prism deviates light.
It also splits white light into a spectrum, because each colour has a slightly different refractive index in glass. Violet slows most and bends most; red bends least. This is dispersion, and the second surface increases the separation the first one began.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme for the block practical lists the credited steps: a means of recording the position of the block, allowing drawing around it on paper; a means of recording the rays; marking the normal where the light enters or leaves the block, allowed if it is shown on the diagram; measuring the angles of incidence and refraction; and repeating for a range of angles of incidence.
For the analysis it credits using sine i divided by sine r to calculate the refractive index, and allows instead a graph of sine i against sine r with the gradient used to find it. Repeating and averaging is a separate mark.
On a ray drawing part, the same mark scheme wants the ray to emerge into the air and to refract correctly, bending away from the normal, allowing dashed construction lines and not requiring an arrow.
The examiner report on that paper shows how hard this proved. It records errors in measuring angles from the normal, notes that many candidates measured the wrong angle, and says that although most knew the ray would emerge into the air, only a quarter drew it refracting in the correct direction.
Practical care that earns marks
Draw the normal as a dashed line at right angles to the surface at the exact point the ray meets it, and draw it before you measure anything. Use a sharp pencil and a ruler, and mark the ray with two well separated points rather than one, because a small error in position becomes a large error in angle over a short line.
Take readings at several angles of incidence rather than one, and calculate the refractive index from each. If the values agree, your method was sound. If one is well out, you have found an anomaly worth mentioning in an evaluation.
Spec 3.17
What you need to know
- Trace rays through three glass shapes
- Measure angles from the normal
- Explain why semicircular blocks are useful
Active recall
Quick check
Answer each question before opening the answer.
From which line are angles measured?
The normal
Why is the prism ray deviated overall?
The prism faces are not parallel
Why does a radial ray not bend at the curved face?
It meets the surface normally, at zero incidence angle
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