Lesson overview
Refraction and total internal reflection: Why does a straw look bent in a glass of water?
Shine a ray box at a glass block at 20°, 40° and 60°, and along the normal, the line at 90° to the surface. Predict which way the ray bends, then read both angles on the angle scale. Supplement: compare sin i ÷ sin r. Then turn a ray box around a half-disc of glass, watch the ray that leaves and the ray reflected inside, and find the critical angle, beyond which no light gets out. Last, send light around the bend of a light pipe.
Suggested sequence
- Lesson 1 · Refraction of light. The normal, the angle of incidence and the angle of refraction; a ray through a rectangular glass block measured with a ray box and an angle scale: towards the normal going in, away from it coming out; Supplement: refractive index as a ratio of speeds and n = sin i ÷ sin r. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Total internal reflection. Internal reflection, the critical angle and total internal reflection found with a semicircular block; reflecting prisms, diamonds and a light pipe; Supplement: n = 1 ÷ sin c and optical fibres in telecommunications. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Refraction of light
- Define and use the terms normal, angle of incidence and angle of refraction (3.2.2.1)
- Describe an experiment to show refraction of light by transparent blocks of different shapes; describe the passage of light through a transparent material (limited to the boundaries between two mediums only) (3.2.2.2, 3.2.2.3)
- Supplement: Define refractive index, n, as the ratio of the speeds of a wave in two different regions; recall and use the equation n = sin i / sin r (3.2.2.6, 3.2.2.7)
Part 2: Total internal reflection
- Supplement: Define refractive index, n, as the ratio of the speeds of a wave in two different regions; recall and use the equation n = sin i / sin r (3.2.2.6, 3.2.2.7)
- State the meaning of critical angle; describe internal reflection and total internal reflection using both experimental and everyday examples (3.2.2.4, 3.2.2.5)
- Supplement: Recall and use the equation n = 1 / sin c; describe the use of optical fibres, particularly in telecommunications (3.2.2.8, 3.2.2.9)
Misconceptions to listen for
- “Line 4 is the reflected ray.” — Line 4 goes on into the glass, so it is the refracted ray. A reflected ray would come back into the air.
- “Line 1 is the normal, because it is at 90° to line 2.” — The normal is the dashed construction line. Line 1, with the glass on one side of it, is the boundary.
- “Line 3 is the reflected ray, because it is in the air.” — Line 3 points towards the boundary, so it is the light arriving: the incident ray. A reflected ray would point away from the boundary, back into the air.
- “z, because it is the angle between the ray in the glass and the surface.” — The angles of incidence and refraction are measured from the normal, not from the surface.
- “x, because it is next to the normal.” — x is between the normal and the incident ray: it is the angle of incidence.
- “w, because it is the angle the light makes with the glass.” — w is between the incident ray and the surface, in the air. Both angles are measured from the normal, and the angle of refraction is in the glass.
- “The angle of incidence is 22°.” — The 22° is measured from the surface. The angle of incidence is measured from the normal: 90° − 22° = 68°.
- “The angle of refraction is bigger than the angle of incidence, 51° against 22°.” — Those are angles to the surface. From the normal, the ray in the glass is at 39°, smaller than 68°.
- “Light bends towards the normal only when it comes from air.” — What matters is the change of speed. Light is slower in glass than in water, so entering glass from water it slows down and bends towards the normal.
- “Along the normal the speed stays the same, because the direction does.” — The speed changes at every boundary. Along the normal, only the direction stays the same.
Scope
The normal, the angle of incidence and the angle of refraction (3.2.2.1); refraction by a rectangular and a semicircular glass block (3.2.2.2); the passage of light through a transparent material, at a boundary between two mediums (3.2.2.3); the critical angle (3.2.2.4); internal reflection and total internal reflection, with experimental and everyday examples (3.2.2.5); Supplement: refractive index as the ratio of the speeds of light (3.2.2.6), n = sin i ÷ sin r (3.2.2.7), n = 1 ÷ sin c (3.2.2.8) and optical fibres, particularly in telecommunications (3.2.2.9). Refraction of water waves is in Waves in a ripple tank, lenses in Lenses and images, and dispersion in Light and the electromagnetic spectrum.