Trace light through a parallel-sided transparent block
Explain entry toward the normal, exit away and parallel emergence with a lateral shift.
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028. Follow the selected Light section; ray diagrams use an ideal thin-lens or plane-boundary model. Written work is self-checked. Human teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Draw a separate normal at each contact.
- Know that glass slows light relative to air.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
When an oblique ray enters slower glass from air, which way does it bend?
What you will learn
- Describe passage at the two boundaries of a transparent block.
- Distinguish final direction from lateral displacement.
Follow the two boundaries in order
An oblique ray travelling from air into ordinary glass changes speed and bends toward the normal. Within a uniform block it travels straight. On passing from glass back into air it speeds up and bends away from the second normal, provided transmission is possible. Label the incident region at each face instead of applying one entry rule everywhere.
For parallel faces and the same outside medium on both sides, the two normals are parallel and the emergent direction is parallel to the incident direction. The block can move the ray sideways: the emergent line usually does not lie on the undeviated straight extension of the incoming line.
Do not generalise a parallel-face result
The same incidence path at a nonparallel pair of faces, such as a prism, need not emerge parallel. Surface orientation determines the second incidence normal and therefore the exit direction. State the geometry and the two surrounding regions when describing the light path.
Refraction need not remove all reflection. A simple school path diagram follows the main transmitted ray while omitting weaker reflected paths for clarity. The omitted rays do not mean all incident energy is transmitted; neither the angle drawing nor a direction change supplies an exact energy percentage.
Pause and explain
Which condition supports parallel emergence in this block model?
Worked example
Describe a main transmitted ray through an air–glass–air parallel-sided block at oblique incidence.
Show the worked solution
- Air to glass: it bends toward the normal.
- Inside uniform glass: it follows a straight path.
- Glass to air: it bends away and emerges parallel but laterally shifted.
Answer Toward on entry, away on exit; parallel emergence with lateral displacement.
Try a topic test
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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.
Describe both bends for oblique passage through an air–glass–air parallel-sided block.
Give me a hint
Treat the two incident regions separately.
Compare my reasoning
- The first incident region is air, so entry is toward the normal.
- The second incident region is glass, so transmitted exit is away.
- Parallel faces restore the initial direction with a possible shift.
Toward, then away; parallel emergence with possible lateral displacement.
Look for these in your work
- Name the region changes.
- Retain the face geometry.
Explain why a parallel emergent ray need not follow the incoming ray's undeviated line.
Give me a hint
Direction and position are different properties of a line.
Compare my reasoning
- The internal segment changes direction.
- The exit restores direction at a different contact point.
- The resulting outside path is parallel but can lie to one side.
The internal path changes the exit position, giving a lateral shift.
Look for these in your work
- Separate direction from position.
- Use both boundary contacts.
May a prism's two nonparallel faces be assumed to restore the incoming direction?
Give me a hint
Examine whether the second normal is parallel to the first.
Compare my reasoning
- Prism faces are nonparallel.
- Their normals are therefore different in orientation.
- The two bends need not cancel, so a net deviation can occur.
No; nonparallel faces can produce net deviation.
Look for these in your work
- Identify the changed geometry.
- Do not transfer the parallel-block rule blindly.
Common mistakes
- Apply the correct region change at each face.
- Parallel direction does not mean the same line.
- A prism does not have the parallel-face cancellation.
What can you explain now?
Can an emergent ray be parallel to the incident ray and still displaced?
Compare with the explanation
Yes.
Parallel lines can be separated sideways.
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Make it stick
Trace both boundaries with their local normals and distinguish emergence direction from displacement.
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.