They can investigate Snell’s Law or make measurements of refractive index.Įpisode 317-2: Measuring refractive index (Word, 28 KB) Discussion: Refractive index and Snell’s lawĮxplain the meaning of refractive index, and state Snell’s law. Students can gain practice in ray tracing through using a ray box and rectangular glass block. 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.Įpisode 317-1: Refraction: soldiers walking from tarmac onto sand (Word, 60 KB) Student experiment: Ray tracing through rectangular block Closer examination of the wave behaviour shows clearly the relationship between the two velocities and the sine of the angles.Īnother analogy uses soldiers: As the soldiers walk onto the sand they slow down and so each line of soldiers bends. 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. 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. The tangent to the new circles is the new wavefront. This happens for every point on the ripple. 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. Now show diagrams to summarise these observations. You should see that the ripples change direction. It can help to concentrate on one ripple at a time simply depress the vibrating bar and release it. Now alter the position of the glass so that the ripples enter the shallower area at an angle. You should be able to see that the separation of the ripples has decreased this is because they are travelling more slowly. Start with ripples arriving head on to the boundary between deep and shallow water. Submerge a sheet of glass in the water to provide an area of shallower water the shallower, the better. 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). Continue with a single straight ripple, then a curved ripple. Start with straight ripples striking a straight barrier, at an angle. Show reflection of ripples at a straight barrier.
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