3.10.4.3
Endoscopy and fibre optics
Medical Physics | AQA A-Level Physics
- Optical fibres transmit light using total internal reflectionWhen light travelling in a denser medium hits a boundary with a less dense medium at an angle greater than the critical angle, all light is reflected back. No refraction occurs. (TIR). Each fibre has a core (high refractive index $n_1$) surrounded by cladding (lower refractive index $n_2$). The critical angleThe minimum angle of incidence at the core-cladding boundary for total internal reflection to occur. Given by $\sin \theta_c = n_2 / n_1$. is given by $\sin \theta_c = \frac{n_2}{n_1}$.
- An endoscope contains two bundles of optical fibres:
- A coherent bundleA bundle of optical fibres where each fibre maintains the same relative position at both ends. This preserves the spatial arrangement of light, allowing an image to be transmitted. Fibres are very thin (about 10 micrometres diameter). carries the image. Fibres are arranged in exactly the same relative positions at both ends, so the spatial pattern is preserved. Individual fibres are very thin (roughly 10 micrometres diameter) to give high resolution.
- An incoherent bundleA bundle of optical fibres where the fibres are arranged randomly. It transmits light for illumination but cannot carry an image. Fibres are thicker (50-100 micrometres diameter). provides illumination. Fibres are arranged randomly (no spatial order needed), and are thicker (50-100 micrometres diameter).
- The endoscope also includes an aperture for focusing, and channels for air, water, and surgical instruments.
- Crucially, only the coherent bundle can transmit an image because the fibres must maintain their relative positions to preserve spatial information.