When light travels from a dense medium like glass toward a less dense one like air, it bends away from the normal. If the angle inside is increased past a certain value, the critical angle, the light can no longer escape and is entirely reflected back into the glass. This is total internal reflection.
An optical fibre is a thin glass thread designed so that light entering one end always strikes the sides at an angle greater than the critical angle. The light bounces down the fibre by repeated total internal reflection, losing almost no energy, and comes out clearly at the far end even around gentle bends.
This carries telephone and internet signals across the country as pulses of light, and lets doctors see inside the body with an endoscope.
In SPM you should link total internal reflection to the critical angle and explain fibre optics and similar applications.
Common misconceptions
- Light is guided by a mirror coating inside the fibre -> It is guided by total internal reflection at the glass surface, no mirror is needed.
- Total internal reflection happens at any angle -> It only occurs when the angle inside exceeds the critical angle.
- Light can be totally internally reflected going from air into glass -> It only happens travelling from the denser medium toward the less dense one.
The physics behind it
Total internal reflection is governed by the critical angle, the angle of incidence inside the denser medium at which the refracted ray would just graze along the boundary at 90°. The critical angle C is fixed by the refractive index n of the material through sin C = 1/n.
Refractive index is a ratio with no unit, while the critical angle is measured in degrees.
For a typical glass fibre with n = 1.50, sin C = 1 ÷ 1.50 = 0.667, so C = 41.8°. Any ray that strikes the wall of the fibre at an angle greater than 41.8° is reflected entirely back inside, because refraction out of the glass is then impossible.
The two conditions are that the light must be travelling in the optically denser medium and the angle of incidence must exceed C. A denser core raises n and lowers C, so a wider range of ray angles stays trapped, which is why the glass core of a fibre is made denser than the cladding around it.
Seeing it at a night market
You can watch total internal reflection at a pasar malam without any laboratory kit. A decorative fibre-optic lamp is a bundle of fine plastic threads with a small coloured bulb at the base; light fed in at the bottom bounces along each thread and only escapes at the cut tips, so the ends glow like tiny stars while the sides stay dark.
A simple kitchen version uses a steady stream of water pouring from a bottle with a torch shining into it from behind: the light follows the curving water and lights up the point where the stream lands in the sink, because it keeps reflecting off the inner surface of the water. The same effect makes a cut glass or a clear plastic ruler light up brightly along a polished edge when a lamp shines into one end.
In each case the light is not held in by any silvering; it is simply striking the surface too steeply to leave, exactly as it does inside a communications fibre buried along a Malaysian highway.
How this comes up in SPM
In Paper 2 this idea sits within the Light and Optics chapter and is examined with command words such as define, state, explain, describe and relate. You may be asked to define the critical angle and total internal reflection, to state the two conditions needed, or to relate the critical angle to the refractive index using sin C = 1/n.
Explain and describe questions often ask how a length of optical fibre carries a signal or how an endoscope lets a doctor see inside the body, so your answer should trace the repeated reflections along the core.
The topic connects directly to the neighbouring subtopics in the same chapter: refraction of light and the meaning of refractive index, which you need before critical angle makes sense, and the study of image formation by lenses that follows. It also links to natural phenomena covered here, such as the mirage seen shimmering on a hot Malaysian road, explained by light bending in air of changing density.
Practise resolving these ideas together rather than in isolation.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)