Describe a rectangular-block refraction experiment
Trace a narrow beam through a transparent rectangular block and reconstruct its entry and exit directions.
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 incident and refracted directions.
- Draw normals at boundary contact points.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What should be recorded before moving a transparent block?
What you will learn
- Describe an experiment showing refraction in a rectangular block.
- Explain how surface outlines and ray marks support angle measurements.
Trace an actual beam with the block in place
In a teacher-supervised ray-box experiment, place a transparent rectangular block on paper and trace its outline. Direct a narrow beam obliquely onto one flat face. Mark well-separated positions on the incoming and outgoing visible paths without moving the block. Use the approved low-power source and never direct beams into eyes.
Remove the block after the ray positions have been recorded. Join the incident marks to find the entry point and the emergent marks to find the exit point. Join those two points to reconstruct the internal path. Draw a normal at each contact and measure the corresponding angles with a protractor.
Repeat and explain the observations
Repeat for several incidence angles while retaining the same block and outside medium. A ray entering glass from air obliquely bends toward the normal; on returning to air it bends away. For parallel faces with the same outside region, the emergent ray is parallel to the incident ray but can be laterally displaced.
Use a narrow beam, a sharp outline and widely separated ray marks to reduce direction-reading uncertainty. A thick pencil line or moving the block before marking introduces a construction error. The method is a description of measurements to perform, not a claim that the displayed schematic is measured data or that written work proves practical competence.
Pause and explain
How is the internal straight ray path reconstructed for a uniform rectangular block?
Worked example
A traced rectangular block shows oblique entry toward the normal and exit away from it. What should be drawn after removing the block?
Show the worked solution
- Join the incoming marks to locate entry.
- Join the outgoing marks to locate exit.
- Connect entry to exit, then add each local normal and measure the angles.
Answer Reconstruct both outside paths and the internal entry-to-exit path with two local normals.
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Test this topic →From guided practice to a new situation
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.
Outline a rectangular-block ray-box method that permits i and r to be measured.
Give me a hint
Preserve apparatus geometry before removing the block.
Compare my reasoning
- Trace the block and mark two positions on the narrow incoming and outgoing paths.
- Remove it and reconstruct entry, exit and the internal line.
- Draw local normals and measure angles with a protractor.
Outline, ray marks, reconstructed paths, normals and angle measurements.
Look for these in your work
- Preserve contact points.
- State how measurements follow.
Give two changes improving the direction measurement from a traced beam.
Give me a hint
Avoid uncertain thick lines and closely spaced marks.
Compare my reasoning
- Use a narrow beam and fine pencil marks.
- Separate the marks sufficiently to define a direction.
- Keep the block fixed while recording the outline and rays.
Narrow/fine traces and well-separated marks with a fixed block.
Look for these in your work
- Address direction uncertainty.
- Retain the geometry.
A student moves the block before tracing its outline. Why can the calculated internal angle be unreliable?
Give me a hint
Ray marks and the recorded boundary may now belong to different positions.
Compare my reasoning
- Entry and exit are found by intersections with the outline.
- A moved outline shifts those intersections.
- The joined internal path and normal angle can therefore be wrong.
The boundary and ray marks no longer reliably reconstruct the same path.
Look for these in your work
- Identify the mismatched evidence.
- Explain its effect on angles.
Common mistakes
- Keep the block in position while recording ray marks.
- Measure angles from normals at each face.
- Parallel emergent direction does not mean zero lateral displacement.
What can you explain now?
Why mark two separated points on each visible outside path?
Compare with the explanation
They determine the path's direction.
A single point alone does not fix the line's orientation.
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Make it stick
Describe a reproducible rectangular-block method and separate observations from construction errors.
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.