What it shows
Light going from air into glass bends toward the normal so that sin i / sin r is a constant, the refractive index. A graph of sin i against sin r for several angles has a slope equal to that index.
The science
Light travels more slowly in glass than in air, so a ray bends at the surface. Snell’s law states n = sin i / sin r, where i is the angle of incidence in air and r the angle of refraction in the glass, both measured from the normal. Since n is constant for a given material and colour of light, the graph of sin i against sin r is a straight line through the origin with slope n. The ray emerging from a parallel slab is parallel to the incident ray but shifted sideways.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A rectangular glass or acrylic slab (about 6 cm × 10 cm and 2 cm thick)
- A drawing board and white paper
- Four pins and drawing pins to fix the paper
- A protractor and a sharp pencil
- A ruler
- A calculator or a sine table
Safety
For this project: An adult must supervise the sharp pins and help with any tricky cutting.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fix the paper on the board, place the slab in the middle and trace its outline with a pencil, then remove it.
Draw a normal to the long side at a point N and draw a line making an angle i = 30 degrees to it; this is the incident ray.
Put the slab back on its outline, fix two pins P1 and P2 upright on the incident line, 5 cm apart.
Look through the opposite side and put two more pins P3 and P4 so that all four pins appear in one line.
Remove the slab and pins, join P3 and P4 and extend to the slab’s edge to meet it at E; join N and E to draw the path inside the glass.
Measure the angle of refraction r between NE and the normal, and the lateral shift of the emergent ray.
Repeat for i = 20, 40, 50, 60 and 70 degrees on fresh sheets or fresh lines.
Tabulate i, r, sin i, sin r and sin i / sin r, then plot sin i against sin r and measure the slope.
What you should see
The value sin i / sin r is nearly constant, about 1.5 for glass or about 1.49 for acrylic, and the graph is a straight line through the origin. The lateral shift grows with the angle of incidence.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1State Snell’s law.
For two media, sin i / sin r is a constant equal to the refractive index of the second with respect to the first.
2Why does light bend toward the normal on entering glass?
Light slows down in glass, which is the denser medium.
3Why does the emergent ray run parallel to the incident ray?
The two faces are parallel, so the bend on leaving cancels the bend on entering.
4What is the refractive index of glass in terms of speed?
n = speed of light in air divided by speed in glass.
Take it further
Repeat with a slab of different thickness and show that the lateral shift is greater for the thicker slab at the same angle.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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