Describe refraction with a semicircular block
Use the curved-face radius to isolate refraction or internal reflection at the flat face.
Section 3.2.2 · Refraction of light
Core and Extended; Physics 0625, 2026–2028. Follow the selected Light section; ray diagrams use an ideal thin-lens or plane-boundary model. Written work is self-checked.
Before you begin
- Identify the radius as the circular-face normal.
- Trace narrow beams and boundary outlines.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Where is the centre of a semicircular block?
What you will learn
- Describe refraction experiments using a differently shaped transparent block.
- Explain why a centre-directed ray is normal to a circular face.
Use geometry to control one boundary
A semicircular block has a flat diameter face and a curved circular face. Trace the outline and mark the centre at the midpoint of its diameter. Direct a narrow ray through the curved face along a radius toward that centre. At the curved entry the ray follows the normal, so it enters without changing direction, although its speed changes.
The same ray can then reach the flat face obliquely. There the normal is perpendicular to the diameter, and the incidence angle is measured inside the block. Mark the outgoing direction if it refracts into air and any reflected direction inside. Vary the radial beam direction while retaining the centre contact at the flat face.
Compare shapes and limits
A rectangular block bends an oblique ray at two flat parallel faces. A centre-directed semicircular setup removes bending at the curved entry and makes the flat exit easier to study separately. It is the radial geometry that makes the first incidence normal, not a universal property that all curved faces cause no refraction.
If the incoming ray misses the centre, it is not generally along the curved-face normal and can bend on entering. Repeat with a correctly aligned narrow beam and record the actual paths. Teacher supervision and suitable optical apparatus remain required; a written procedure does not certify a performed experiment.
Pause and explain
A ray misses the circle centre on curved-face entry. Must it remain undeviated?
Worked example
Why does a ray aimed through the curved face toward a semicircle's centre enter without bending?
Show the worked solution
- The local normal is the radius through the entry point.
- The incoming ray follows that radius.
- Its incidence angle is 0°, so there is no direction change at that boundary.
Answer Radial entry gives normal incidence; the speed can still change.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe how to arrange normal entry at a semicircular block's curved face.
Give me a hint
Identify and aim toward the circle centre.
Compare my reasoning
- Trace the block and mark the diameter midpoint.
- Aim a narrow beam radially toward that point.
- The radius is the entry normal, so the direction stays unchanged there.
Radial centre-directed entry gives no bending at the curved face.
Look for these in your work
- Locate the centre.
- Use the local normal.
Explain why the ray may still bend at the flat face after radial entry.
Give me a hint
The two boundaries have different normals.
Compare my reasoning
- The radial ray reaches the diameter midpoint.
- The flat-face normal is perpendicular to the diameter.
- If the ray is oblique to that normal, transmission can change direction.
Normal entry at one boundary does not force normal exit at the other.
Look for these in your work
- Identify each normal separately.
- Retain the exit conditions.
A beam aimed away from the centre is used to claim the curved face has no refraction. What should be checked?
Give me a hint
The direction must be compared with the radius at the actual entry point.
Compare my reasoning
- Locate the entry and circle centre.
- Draw the radius as the local normal.
- Check the beam's alignment and repeat the intended centre-directed setup.
Check actual radial alignment; an arbitrary curved-face path is insufficient.
Look for these in your work
- Use the actual contact geometry.
- Do not infer a universal rule.
Common mistakes
- Aim through the circle centre, not any point on the flat face.
- A curved face is not always a no-bending boundary.
- Measure flat-face incidence inside the block.
What can you explain now?
Can radial entry change speed without changing direction?
Compare with the explanation
Yes.
Normal incidence removes bending, not the material's propagation-speed change.
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Make it stick
Compare rectangular and semicircular methods and explain the controlled radial entry.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.