Misconception 1: "Angles are measured from the surface"
This is the most common mistake in optics questions. ALL angles in reflection and refraction are measured from the normal — the line perpendicular to the surface. If a ray hits a mirror at 30° to the mirror, the angle of incidence is 60° (not 30°). Always draw the normal first in diagrams.
Misconception 2: "Light always bends when crossing a boundary"
If light hits a boundary at exactly 90° (perpendicular, along the normal), it does NOT bend — it continues straight through. It does slow down and its wavelength changes, but there is no change in direction. Bending only happens when the angle of incidence is greater than 0°.
Misconception 3: "Total internal reflection means no light escapes at all"
TIR means no light is refracted (transmitted into the less dense medium). It still reflects entirely. For TIR to occur, BOTH conditions must be met: the light must be in the denser medium AND the angle must exceed the critical angle. If either condition is not met, some light is refracted out.
This common misconceptions covers Common Misconceptions within Reflection & Refraction for GCSE Physics. Revise Reflection & Refraction in Waves for GCSE Physics with 17 exam-style questions and 15 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 8 of 13 in this topic. Use this common misconceptions to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Reflection & Refraction
According to the law of reflection, the angle of incidence is:
Explain why a ray of light bends when it passes from air into water.