Light questions look friendly until the diagram appears. A glass block, a dotted line, an angle marked somewhere odd, and suddenly you are not sure whether to measure from the surface or the normal. The good news is that refraction and total internal reflection (TIR) questions in Combined Science Physics follow a very predictable structure. Once you have a fixed routine, these become some of the fastest marks in the paper.
Start Every Question by Drawing the Normal
Almost every mark lost in this topic traces back to one mistake: measuring the angle from the surface instead of from the normal.
Before you write anything, do this:
- Mark the point where the ray hits the boundary.
- Draw a dashed line perpendicular to the surface at that point. That is the normal.
- Label the angle of incidence between the incident ray and the normal.
- Label the angle of refraction between the refracted ray and the normal.
If a question gives you "the ray makes an angle of 30 degrees with the surface", the angle of incidence is 60 degrees, not 30. Examiners set this deliberately. Write the conversion down so you can still earn method marks even if you slip later.
Know the Two Rules Cold
Rule 1: Which way does the ray bend?
- Going from a less dense to a denser medium (air into glass or water): the ray bends towards the normal, so the angle of refraction is smaller than the angle of incidence.
- Going from denser to less dense (glass into air): the ray bends away from the normal.
- A ray hitting the surface along the normal (angle of incidence = 0) passes straight through with no bending.
Rule 2: The refractive index relationship
For light travelling from air into a medium:
n = sin i / sin r
Rearranged, sin r = sin i / n. Keep your calculator in degrees. Every year students lose marks because the calculator was sitting in radians from A Math practice.
For speed, n = speed of light in air / speed of light in the medium. The speed of light is always slower in the denser medium, and the frequency never changes during refraction. Only speed and wavelength change.
Total Internal Reflection: The Two Conditions
When a question asks "state the conditions necessary for total internal reflection", the mark scheme wants both of these, phrased precisely:
- Light must travel from an optically denser medium to a less dense medium.
- The angle of incidence must be greater than the critical angle.
Writing only "the angle must be big" gets zero. Writing "from glass to air" earns the mark only if you also generalise it. Use the standard wording.
The critical angle links to the refractive index by:
sin c = 1 / n
So for glass with n = 1.5, sin c = 0.667 and c = 41.8 degrees. Any ray inside the glass striking the boundary at more than about 42 degrees is totally internally reflected.
A Worked Example
Question: A ray of light travels inside a glass prism of refractive index 1.50 and strikes the glass to air boundary at an angle of incidence of 45 degrees. Explain what happens to the ray.
Working:
- Critical angle: sin c = 1 / 1.50 = 0.667, so c = 41.8 degrees.
- The ray is travelling from glass (denser) to air (less dense). Condition 1 is satisfied.
- 45 degrees is greater than 41.8 degrees. Condition 2 is satisfied.
- Therefore total internal reflection occurs. The ray is reflected back into the glass with the angle of reflection equal to the angle of incidence, 45 degrees.
Notice the structure: calculate, compare, conclude. That comparison line, "45 degrees > 41.8 degrees", is worth a mark on its own. Never skip it.
Common Mistakes Dojo Tutors See Every Week
- Forgetting the normal. Always draw it as a dashed line and label it.
- Using n = sin i / sin r backwards. Remember: i is always the angle in air.
- Saying light "speeds up because it bends". The bending happens because the speed changes, not the other way round.
- Claiming the frequency changes. It does not. Wavelength and speed change.
- Drawing a partial reflection at TIR. At total internal reflection, all the light is reflected. No refracted ray should appear.
- Sloppy ray diagrams. Use a ruler, add arrows to show direction, and keep the reflected angle visually equal to the incident angle.
Applications Worth Memorising
Combined Science regularly asks for real uses of TIR. Have two ready:
- Optical fibres in telecommunications and endoscopes. Light repeatedly undergoes total internal reflection along the fibre core, so signals travel with little loss even around bends.
- Prismatic reflectors in periscopes, binoculars and bicycle reflectors, where 45 degree prisms turn light through 90 or 180 degrees more sharply than a mirror.
Practise five past-paper light questions in one sitting. The pattern repeats far more than you would expect, and once you spot it, these become guaranteed marks under time pressure.
Want a tutor who sat these exact papers recently to walk you through a ray diagram live? Message us on WhatsApp to book a free trial class at wa.link/rj3pry.