The angle of incidence is measured from the normal, the line perpendicular to the surface. If a diagram gives the angle between the incident ray and the surface, subtract it from 90° before comparing it with the critical angle.
The 2027 SEC G3 Physics syllabus (K323) includes critical angle and total internal reflection. Use the example below to check both the angle and the direction in which the light travels.
- Draw the normal where the ray meets the boundary.
- Convert any angle measured from the surface.
- Check the direction of travel and compare with the critical angle.
Worked example | A ray is 35° from the surface
A ray in glass strikes a glass–air boundary. Its angle to the surface is 35°, and the critical angle for glass to air is 42°. The question asks whether total internal reflection occurs.
The angle of incidence is 90° − 35° = 55°. Light travels from higher refractive index glass towards lower refractive index air. Since 55° exceeds 42°, total internal reflection occurs.
Comparing 35° directly with 42° would give the wrong prediction. The 35° label belongs between the surface and the ray. The 42° critical angle is measured from the normal.
When does the refracted ray run along the boundary?
At the critical angle, the angle of refraction is 90° from the normal. For this glass–air boundary, the incident ray would then make 90° − 42° = 48° with the surface. The critical angle is the threshold. Total internal reflection occurs above it.
If the incident ray makes 60° with the surface, it makes 30° with the normal. That is below the critical angle. A refracted ray can travel into the air, even though some light may also reflect at the boundary.
What if the ray starts in air?
Keep the angle of incidence at 55° but send the ray from air into the glass. The higher-to-lower refractive index condition fails. You cannot apply the glass-to-air critical angle to predict total internal reflection for this direction.
A study partner can point to the surface label and ask whether a larger angle to the surface means a larger angle to the normal. The two add to 90°, so one becomes smaller as the other grows.
Frequently asked questions
Is the critical angle measured from the surface?
Measure it from the normal on the incident side.
Can I use 90° as the angle of incidence when a refracted ray follows the surface?
At the threshold, 90° is the angle of refraction. The angle of incidence is the critical angle.
Official information
SEAB’s 2027 SEC G3 Physics syllabus (K323), Topic 12 Light. Checked 9 November 2026.