WJEC GCSE Physics 1.6 The Total Internal Reflection of Waves Exam Questions

About WJEC GCSE Physics 1.6 The Total Internal Reflection of Waves

This short Unit 1 topic applies refraction to total internal reflection and its uses. You need to state the conditions needed for total internal reflection: light travelling from a more dense to a less dense medium, at an angle of incidence greater than the critical angle. You must explain how optical fibres carry signals by repeated total internal reflection, and compare optical fibres with satellites for long-distance communication. The topic finishes with medical imaging, comparing endoscopes, which use optical fibres, with CT scans. It builds directly on refraction in 1.5. Examiners commonly ask for ray diagrams and balanced comparisons of technologies.

What you need to know

• Conditions Total internal reflection happens when light travels from a more dense medium (e.g. glass) towards a less dense one (e.g. air), and the angle of incidence is greater than the critical angle. All the light is then reflected.
• Critical angle At the critical angle the refracted ray travels along the boundary. Below it, most light is refracted out with a little reflected; above it, all light is reflected.
• Optical fibres Thin glass fibres carry light or infrared signals by repeated total internal reflection along their length, with little energy loss, even round bends.
• Fibres versus satellites Optical fibres carry large amounts of data, are secure and have little delay, but cables are costly to lay. Satellites cover wide areas and remote places but are expensive to launch and introduce a time delay because signals travel long distances.
• Endoscopes An endoscope uses one bundle of fibres to carry light into the body and another to bring the image back, allowing doctors to see inside without major surgery.
• CT scans CT scans use X-rays to build detailed 3D images, but X-rays are ionising and increase cancer risk. Endoscopes use visible light so they are not ionising, but they are invasive and only view hollow organs.

Common mistakes in 1.6 The Total Internal Reflection of Waves

• Leaving out one of the two conditions for total internal reflection. You need both the direction (dense to less dense) and an angle greater than the critical angle.
• Measuring angles from the surface instead of from the normal. All angles of incidence and reflection are measured from the normal.
• Saying optical fibres use refraction to carry light. They rely on total internal reflection.
• Claiming CT scans are safer than endoscopes because they are non-invasive. CT scans use ionising X-rays, so there is a radiation risk, even though no tube is inserted.

Try three 1.6 The Total Internal Reflection of Waves exam-style questions