Physics IA · C · Wave behaviour · topic 17 of 30
Refraction: how closely does light obey Snell's law? · IB Physics IA idea: guide, sample graph and step-by-step plan
simple kit, lots to evaluate :)Every textbook says sin i / sin r is constant. Your job is to put numbers on how constant, and to find out what your blocks do when the rule gets stretched.
- ✓ you like careful, hands-on measuring
- ✓ you can draw straight lines with a sharp pencil
- ✓ you want a clean law to test
The big idea
← sines, not angles!Light bends at the boundary between two materials, and Snell's law says sin θ₁ / sin θ₂ is the same for every angle. So a graph of sin r against sin i should be a straight line through the origin whose gradient gives the refractive index. Your investigation is not "does it work?" but how well, for which materials, and what limits your accuracy. Reading angles off a real beam is hard, which is exactly why there is so much to evaluate.
Make it yours: pick one twist
Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.
Swipe for more twists →
Acrylic, glass, a water-filled block and a plastic you can source, same angles each time.
→ One graph with several lines; you compare gradients and their uncertainties, and can test whether n really is a property of the material.Send light out of the block into air and find the critical angle for each material.
→ A second, independent route to n (n = 1/sin C) that you can compare with the gradient method.Use filters or a laser of known wavelength, same block, same angles.
→ Tests dispersion: n depends slightly on wavelength, so your gradients differ by a small amount that your uncertainty must be able to detect.Photograph the rays from above and measure angles in software, or measure lengths and use trigonometry.
→ Different uncertainty budget; your evaluation can compare two methods of reading angles.Sugar or salt solutions in a thin-walled container, several concentrations.
→ Your independent variable becomes concentration, and the question is how n depends on it. Needs a careful method for the container walls.Link it to lenses, optical fibres or why a pool looks shallower.
→ Gives the introduction a purpose; the analysis stays the same.Your Physics IA research question: run the check
write it FIRST, then check itWrite your own question first. Then make sure it ticks every box:
- Names one independent variable and its range (which angles of incidence, in degrees, and how many).
- Names the dependent variable and how you will read it (which angle, measured how, from which line?).
- Says which materials and which light source stay the same, and what you will change between runs.
- Says what you will compare with: Snell's law, a data-book value of n, or another method.
- Is narrow enough for about 10 hours, with at least 6 angles and repeats for each material.
The set-up
A semicircular block is the easiest to get right: a ray aimed at the centre of the flat face leaves the curved face without bending. Draw your own version with your real equipment.
How your Physics IA could be laid out
The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.
Your question, where refraction matters (lenses, fibres, the eye) and what you expect for the line on your graph.
Snell's law in the form you will use, what refractive index means, and why the graph you choose should be a straight line.
Independent, dependent and controlled variables, with how each is set, measured or kept constant (block, wavelength, ray width).
How you set up the block, mark and draw the rays, find the normal, and measure both angles; what you did to reduce the width of the beam.
Short and specific: ray box lamp heat, laser class if you use one, broken glass.
A table of incident and refracted angles for each material, with units, resolution and repeats.
Converting angles to sines and carrying the uncertainty through; one worked example for each type of calculation.
Sine of one angle against sine of the other, error bars, best and worst lines, and what the gradient means.
Your value of n with its uncertainty, compared with a trustworthy reference; whether the data supports a constant ratio.
Strengths, weaknesses ranked by their effect on n, and improvements linked to each weakness.
Every source of theory and reference values.
What your Physics IA graph could look like
error bars on EVERY point ✓Made-up data, to show the form of a strong graph, not the answer. Yours will look different. example only
- Plotting sines, not angles, turns a curve into a straight line you can fit.
- Error bars in both directions: a ±1° error changes sin θ more at small angles than near 90°. Work out why, and size each bar separately.
- A second line for a second material lets the reader compare gradients, not just trust you.
- Check the intercept: does your line really pass through the origin? If not, think about a zero error in how you read angles.
- Another graph to consider: sin r against sin i for each colour of light on the same axes, to see whether the small differences in gradient are real.
Which reading matters most?
Rank by the effect on your result, here n worked out from one pair of angles. A quick table like this, with your numbers, shows where to improve first.
Your game plan
don't skip the trial run!Tick steps off as you go. We'll remember on this device.
0 of 10 done
- Read and predictWrite down Snell's law and what it predicts for your graph. Decide what you expect to be hardest to measure, and why.
- Trial run (1 lesson)Try one block and five angles. How wide is the beam where you read the angle? Can you find the normal reliably? Do the rays on paper agree with what you see?
- Narrow the beamTest a thin slit, a laser or marking points with pins. Decide which gives the smallest uncertainty and keep that for the whole investigation.
- Decide range and repeatsChoose at least six angles across a wide range, which materials, and how many times you repeat each. Write down your reason for each choice.
- Collect the dataRecord angles straight into a table with units and resolution. Note anything odd, such as a chipped block or a wandering beam.
- ProcessConvert to sines, find the uncertainty in each, and decide how you will show both uncertainties on the graph.
- Graph and fitPlot, add error bars, draw the best and worst lines, and find the gradient and its uncertainty. Check the intercept.
- CompareCompare n with a reference value and with other methods or materials. Is any difference larger than your uncertainty?
- EvaluateRank the sources of uncertainty by their effect on n. Make every improvement specific to a weakness you found.
- Check before your teacher's draftRun the checklist above, then get your RQ or draft checked. Your teacher reads one draft only, so make it count.
Physics you'll need
Traps to avoid in this Physics IA
- Reading angles from the edge of a thick beam instead of its centre line.
- Only two or three angles, or only small ones: you cannot tell a straight line from a coincidence.
- Measuring from the surface instead of from the normal.
- Using one material and calling it a "comparison of materials".
- A scratched or dirty block, or a rim that scatters light at the exit point.
- Ignoring a zero error in the protractor or in where the normal was drawn.
More Physics IA ideas like this
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This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.