Episode 317: Reflection and refraction of waves
Lesson for 16-19
- Activity time 145 minutes
- Level Advanced
This episode starts from the phenomenon of refraction and moves on to Snell’s law.
- Demonstration and discussion: Reflection and refraction with ripple tank (15 minutes)
- Discussion and student activity:
Marching soldiersmodel of refraction (10 minutes)
- Student experiment: Ray tracing through rectangular block (30 minutes)
- Discussion: Refractive index and Snell’s law (20 minutes)
- Worked example: Using refractive index (10 minutes)
- Student questions: Calculations involving refractive index (30 minutes)
- Discussion: Summary (10 minutes)
Discussion and demonstrations: Reflection and refraction with ripple tank
Show reflection of ripples at a straight barrier. Start with straight ripples striking a straight barrier, at an angle. Continue with a single straight ripple, then a curved ripple.
To show refraction with a ripple tank, you need to show how ripples change speed when travelling from deeper into shallower water (or vice versa). Submerge a sheet of glass in the water to provide an area of shallower water; the shallower, the better. Start with ripples arriving
head on to the boundary between deep and shallow water. You should be able to see that the separation of the ripples has decreased; this is because they are travelling more slowly.
Now alter the position of the glass so that the ripples enter the shallower area at an angle. It can help to concentrate on one ripple at a time; simply depress the vibrating bar and release it. You should see that the ripples change direction.
Now show diagrams to summarise these observations.
When the first part of the ripple touches the barrier a semicircular wave starts to travel away from the point of contact at the same speed as the incoming wave. This happens for every point on the ripple. The tangent to the new circles is the new wavefront. In the time taken for the end of the ripple farther away to reach the barrier, the reflected wave has travelled outwards the same distance so the equal angles can be seen.
Snell’s Law may have been covered previously but probably obtained experimentally by ray tracing with no reason given for the form of the law. Closer examination of the wave behaviour shows clearly the relationship between the two velocities and the sine of the angles.
Fermat’s principle of least time can be useful here – a ray of light, travelling between two points, takes the path of shortest time. An analogy of seeing someone in a river works well; to rescue them, you would run along the bank (greater speed) until you judged it sensible to swim (lower speed). The path taken is that followed by light.
Remind the students of the reversibility of light too. It often helps with problems.
Discussion and student activity: Marching soldiers’ model of refraction
Here is a practical way of explaining refraction. Students form a line, arms linked. They march forwards in step, so that they meet a boundary between, say, tarmac and grass obliquely. On the grass, they march more slowly. The outcome is that the line bends and changes direction.
A comparable effect is the skidding of a car if its wheels on one side leave the road and start slipping on a grassy verge. The car slews round.
Another analogy uses soldiers: As the soldiers walk onto the sand they slow down and so each line of soldiers bends. They end up moving in a different direction – this is just what happens when a beam of light hits a block of glass at an angle – it refracts.
Student experiment: Ray tracing through rectangular block
Students can gain practice in ray tracing through using a ray box and rectangular glass block. They can investigate Snell’s Law or make measurements of refractive index.
Discussion: Refractive index and Snell’s law
Explain the meaning of refractive index, and state Snell’s law. Points to mention concerning refraction:
Angles are measured from the normal (because this works if the interface is curved).
It is useful to remember which way the bending occurs (towards the normal when slowing down).
Rays only bend at points where their speed is changing (usually at interfaces between different media).
The refractive index of a material (i.e. the absolute refractive index) means relative to a vacuum, but this value is a good enough approximation where the other material is air.
Worked examples: Using refractive index
A useful equation derived from Snell’s Law using absolute refractive indices is:
n1sin( θ 1) = n2sin( θ 2)
A ray of light travelling from water to glass is incident at 50 ° to the normal. Calculate the angle of refraction in the glass.
( nwater = 1.33
nglass = 1.5 )
n1sin( θ 1) = n2sin( θ 2)
θ glass = 42.8 °
Student questions: Calculations involving refractive index
Discussion: A summary
Show diagrams of wavefronts going through a prism and both types of lens.
Remind students that this behaviour applies to other waves e.g. sound (a balloon filled with CO2 can act as a converging lens).
Summarise the ideas you have been looking at: reflection and refraction can both be explained with the idea of waves.