OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
(ii) techniques and procedures used to investigate refraction and total…
Full specification wording
Electromagnetic waves: (ii) techniques and procedures used to investigate refraction and total internal reflection of light using ray boxes, including transparent rectangular and semi-circular blocks
A ray box produces a narrow beam of light to investigate refraction and total internal reflection (TIR).
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 5 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A ray box produces a narrow beam of light to investigate refraction and total internal reflection (TIR). For a rectangular block, trace the incident, internal, and emergent rays, measuring angles of incidence and refraction from the normal to verify Snell's law. For a semi-circular block, direct the ray into the curved face along a radius so it hits the exact centre of the flat face. Because it enters along the normal to the curved surface, it passes undeviated. At the flat face, the ray refracts out. By rotating the block to increase the angle of incidence at the flat face, the refracted ray eventually grazes the boundary; this is the critical angle. Further rotation results in TIR. Repeat measurements to reduce random error.
What earns the marks
- Use a ray box to produce a narrow beam and trace rays entering and exiting a rectangular block to investigate refraction.
- Measure angles of incidence and refraction from the normal drawn at the boundary of the rectangular block.
- For a semi-circular block, direct the incident ray through the curved face towards the centre of the flat face so it enters undeviated.
- Observe refraction at the flat face and increase the angle of incidence until the emergent ray grazes the flat boundary to find the critical angle.
- Increase the angle of incidence beyond the critical angle to observe total internal reflection at the flat face.
Common mistakes and how to put them right
Directing the ray into the flat face of the semi-circular block to find the critical angle.
Put it right: The ray must enter the curved face undeviated and undergo TIR at the flat face.
Measuring angles from the surface of the block.
Put it right: Draw a normal line at the point of incidence and measure angles from the normal.
Failing to aim the ray exactly at the centre of the flat face of the semi-circular block.
Put it right: This causes unwanted refraction at the curved boundary, invalidating the angle measurements.
Exam tips
- Clearly describe aiming the ray at the centre of the flat face through the curved side when explaining the semi-circular block experiment.
- State that repeating the experiment for various angles of incidence and calculating a mean reduces random error.
- Remember to mention using a sharp pencil to mark the centre of the ray to improve the precision of angle measurements.
Try the lesson
The start of the Electromagnetic waves: (ii) techniques and procedures used to investigate refraction and total internal reflection of light using ray boxes, including transparent rectangular and semi-circular blocks lesson, made from checked teaching notes for this specification point.
Listen
A ray box produces a narrow beam of light to investigate refraction and total internal reflection (TIR). For a rectangular block, trace the incident, internal, and emergent rays, measuring angles of incidence and refraction from the normal to verify Snell's law. For a semi-circular block, direct the ray into the curved face along a radius so it hits the exact centre of the flat face. Because it enters along the normal to the curved surface, it passes undeviated. At the flat face, the ray refracts out. By rotating the block to increase the angle of incidence at the flat face, the refracted ray eventually grazes the boundary; this is the critical angle. Further rotation results in TIR. Repeat measurements to reduce random error. A strong answer earns marks for these points. Use a ray box to produce a narrow beam and trace rays entering and exiting a rectangular block to investigate refraction. Measure angles of incidence and refraction from the normal drawn at the boundary of the rectangular block. For a semi-circular block, direct the incident ray through the curved face towards the centre of the flat face so it enters undeviated. Observe refraction at the flat face and increase the angle of incidence until the emergent ray grazes the flat boundary to find the critical angle. Exam tip: Clearly describe aiming the ray at the centre of the flat face through the curved side when explaining the semi-circular block experiment.
Key words
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Ray box
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Refraction in rectangular blocks
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Total internal reflection in semi-circular blocks
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Critical angle
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See every definition
- Ray box
- Produces a narrow beam of light to investigate refraction and total internal reflection.
- Refraction in rectangular blocks
- Tracing the incident, internal and emergent rays and measuring angles of incidence and refraction from the normal to verify Snell's law.
- Total internal reflection in semi-circular blocks
- Directing the ray into the curved face along a radius so it hits the exact centre of the flat face, then increasing the angle of incidence beyond the critical angle.
- Critical angle
- The angle of incidence at the flat face at which the refracted ray eventually grazes the boundary.
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