8.2 Refraction, Lenses & Fibre Optics

Refraction, Lenses & Fibre Optics

Reflection keeps light in the same medium. Refraction sends light into a second medium at a new speed and usually a new direction. Lenses are carefully shaped interfaces that use refraction to form images. Optical fibres trap light by total internal reflection and carry it around bends — a technology now standard in modern avionics data buses and some lighting and sensing paths. This section finishes Appendix I 2.4 for B1/B2 level 2.

Refraction

When a ray travels from medium 1 into medium 2 (for example air into glass), part of the light may reflect, and part refracts into the second medium. The angle of refraction is the angle between the refracted ray and the normal in medium 2.

Physical cause: light’s phase speed changes with the medium. Absolute refractive index:

n = c / v

where c is the vacuum speed and v is the speed in the medium. Larger n means slower light. Approximate values: vacuum n = 1; air ≈ 1.0003 ≈ 1 for most work; water ≈ 1.33; typical crown glass ≈ 1.5; some plastics ≈ 1.4–1.6.

Rules of thumb (ray direction):

• Entering a medium with higher n (slower light): the ray bends toward the normal (θ₂ < θ₁ if θ is measured from the normal).
• Entering a medium with lower n (faster light): the ray bends away from the normal.
• Ray along the normal: no change of direction (but speed still changes).

Apparent depth of a pool, bending of a stick in water, and the aiming offset when looking through a canopy or thick acrylic window are everyday refraction effects. In hangars, looking into fluid sight glasses or through inspection ports, the image of a level or mark can appear shifted by refraction.

Snell’s Law (Concept)

Snell’s law quantifies refraction:

n₁ sin θ₁ = n₂ sin θ₂

• n₁, n₂ — refractive indices of the incident and refracting media
• θ₁ — angle of incidence (from the normal in medium 1)
• θ₂ — angle of refraction (from the normal in medium 2)

If n₁ sin θ₁ exceeds n₂ (possible only when light tries to leave a higher-n medium into a lower-n medium at a large enough angle), no real θ₂ exists: the ray cannot refract out and total internal reflection occurs (see fibre optics below). The limiting case defines the critical angle θ_c:

sin θ_c = n₂ / n₁ (for n₁ > n₂)

For glass (n ≈ 1.5) to air (n ≈ 1), θ_c ≈ arcsin(1/1.5) ≈ 42°. Any incidence greater than θ_c in the glass at a glass–air interface reflects entirely back into the glass (ideally, neglecting absorption and surface scatter).

Module 2 expects the concept and simple application: identify which medium is denser optically, predict bend toward/away from the normal, and recognise when total internal reflection is possible. Detailed multi-surface ray tracing belongs to specialist optics, not the AML physics paper.

Lenses

A lens is a transparent body bounded by two refracting surfaces (usually spherical). Thin-lens language is enough for Module 2.

Converging (convex) lenses

Thicker at the centre than at the edge (double-convex is the classic shape). Parallel rays near the axis converge to the principal focus after the lens. Focal length f is the distance from the optical centre to that focus. Converging lenses have positive f in the usual thin-lens sign convention used in many textbooks.

Image behaviour for a real object (qualitative, matching mirror logic):

• Object beyond 2F → real, inverted, diminished image between F and 2F on the other side.
• Object at 2F → real, inverted, same size, at 2F.
• Object between F and 2F → real, inverted, magnified, beyond 2F.
• Object at F → refracted rays parallel (image at infinity) — the camera “focused at infinity” idea.
• Object inside F → virtual, upright, magnified image on the same side as the object — the simple magnifying glass.

Real images again can be thrown onto a screen; virtual images cannot.

Diverging (concave) lenses

Thinner at the centre than at the edge. Parallel rays diverge after the lens as if they came from a virtual focus on the incident side. For any real object, a single diverging lens forms a virtual, upright, diminished image on the object side of the lens. Diverging elements are used to correct aberrations, widen fields of view, and in multi-lens camera and eyepiece designs.

Lens equation (awareness)

Many courses quote the thin-lens equation 1/f = 1/v + 1/u (with a consistent sign convention for object and image distances). You should recognise that focal length, object distance, and image distance are linked, and that magnification relates image size to object size. Exam questions more often ask qualitative image type (real/virtual, inverted/upright, magnified/diminished) than heavy algebra — but if numbers appear, apply the stated convention carefully.

Aviation uses of lenses

• Cockpit displays, HUDs, and camera systems use multi-element lenses to form and collimate images.
• Borescopes and videoscopes for internal engine and structure inspection depend on lens trains (and often fibre or digital image guides).
• Smoke detectors, optical ice detectors, and some proximity sensors use lenses or shaped optics to send and collect light beams.
• Landing lights and LED assemblies may use lenses (or combined lens–reflector optics) to shape the beam — refraction partnering with the reflection ideas of the previous section.

Fibre Optics and Total Internal Reflection

An optical fibre is a thin, flexible strand (usually glass or plastic) that guides light from one end to the other by repeated total internal reflection inside a high-index core clad by a lower-index cladding.

Structure

• Core — central region, refractive index n_core (higher).
• Cladding — surrounding layer, n_clad < n_core.
• Buffer/jacket — mechanical protection, not part of the optical design.

Light launched into the core at a sufficiently shallow angle to the core–cladding interface (angle of incidence from the normal large enough, i.e. grazing enough relative to the wall) exceeds the critical angle for the core–cladding pair and reflects back into the core. That process repeats along the fibre, including around gentle bends, so energy stays trapped until it exits the far end (or is lost to absorption, scattering, or bend radii that are too tight).

Why aviation uses fibre

Compared with copper for data:

• High bandwidth — many gigabits per second on modern links; supports dense avionics networking.
• Low mass and small diameter — weight and space savings on aircraft.
• Electromagnetic interference (EMI) immunity — light is not affected by the RF and electrical noise that plague long copper runs near generators, transmitters, and power feeders.
• Electrical isolation — no conductive path between boxes (helps with ground loops and some lightning/EMI design goals).
• Security — harder to tap without detection than some copper lines.

Challenges for maintenance: fibres need careful bend radius control, clean connectors, protection from crush and chafe, and correct handling (no tight knots, correct strain relief). A broken fibre or contaminated ferrule drops optical power and can drop a bus. Troubleshooting is optical power, continuity, and connector condition — not voltage and continuity in the copper sense.

Lighting and other optical paths

Besides data buses, aviation uses light guides and fibre bundles for:

• Instrument and panel lighting remote from the lamp (legacy and some modern designs).
• Status indicators bringing light to a visible surface from a remote LED.
• Sensing — some fire/smoke and foreign-object or ice detection concepts use optical paths; engines and APU bays may use optical flame detectors.

In every case the physical principle is the same: confine light by total internal reflection (or by reflective walls in a light pipe) and deliver photons where the system needs them.

Linking Reflection, Refraction, and Images

A short integration for revision:

1. Mirrors — reflection, i = r; plane → virtual same-size; concave → can be real or virtual; convex → always virtual diminished for real objects.
2. Interfaces — refraction, Snell’s law; bend toward normal into higher n.
3. Lenses — two (or more) refracting surfaces shaped to converge or diverge; same real/virtual vocabulary as mirrors.
4. Fibres — high-n core, low-n cladding, total internal reflection for guided data and light.

If you can sketch a normal, mark i and r or θ₁ and θ₂, and state whether an image is real or virtual, you have the core of Module 2.4. Category A candidates are not examined on this block, but B1 and B2 need level-2 understanding — enough to explain, not only to recite definitions.

Practical Safety and Maintenance Notes

• Laser and high-intensity sources in test equipment and some avionics can damage eyes; follow PPE and manufacturer lockout procedures.
• Fibre ends can be sharp; treat broken glass fibre as a puncture/contaminant hazard.
• Cleaning optical connectors requires approved methods and materials; abrasives and wrong solvents ruin end-faces.
• Canopy and window optics affect pilot vision; crazing, delamination, and improper polishing change refraction and scatter and are airworthiness issues, not only cosmetics.

Optics in Module 2 is deliberately applied: understand light’s speed, bounce (reflection), bend (refraction), focus (lenses/mirrors), and guide (fibres) so later modules on instruments, avionics, and cabin systems rest on solid physics.