GCSE-readyTotal internal reflection and lens images
Light can be trapped inside glass by total internal reflection. Lenses form images that can be real or virtual.
Total internal reflection
At one angle, the critical angle, the refracted ray runs along the surface. For glass it is about 42°.
If the angle of incidence is bigger than the critical angle, all the light is reflected back: total internal reflection (TIR).
Uses: optical fibres (for communications and endoscopes to see inside the body).
Converging and diverging lenses
A diverging (concave) lens spreads parallel rays out, as if they came from a principal focus behind it.
The focal length is the distance from the lens to the principal focus. A more powerful lens bends light more, so it has a shorter focal length (and is usually more curved).
Real and virtual images
A virtual image is where rays only appear to come from; it cannot be shown on a screen. A converging lens used as a magnifying glass (object closer than the focal length) gives a virtual, upright, magnified image.
A diverging lens always gives a virtual, upright, smaller image.
Light in glass hits the surface at 50°. The critical angle of the glass is 42°. What happens?
- Light is going from glass (denser) to air.
- 50° is bigger than the critical angle of 42°.
- So no light escapes.
Answer: Total internal reflection: all the light reflects back into the glass.
Total internal reflection happens when light in a denser material hits the boundary at more than the critical angle (about 42° for glass), as in optical fibres. Converging lenses focus light; more powerful lenses have shorter focal lengths. Real images can be projected; virtual images cannot.
- Total internal reflection
- When light inside a denser material hits the boundary at more than the critical angle and is all reflected back.
- Critical angle
- The angle of incidence inside a denser material that gives an angle of refraction of 90°. sin c = 1 ÷ n.
- Normal
- A line drawn at 90° to a surface where a ray meets it. Angles of incidence and refraction are measured from it.
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Light inside water hits the surface at an angle smaller than the critical angle. What happens?
- Total internal reflection
- All the light is absorbed
- It bends towards the normal
- Most light refracts out into the air
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Answer: Most light refracts out into the air
Below the critical angle, light refracts out (with a little reflected).
Which of these devices works by total internal reflection?
- A magnifying glass
- A red filter
- A plane mirror
- Optical fibres carrying broadband signals
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Answer: Optical fibres carrying broadband signals
Light stays inside the fibre by repeated total internal reflection.
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Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.
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