What is total internal reflection? State its conditions and obtain the relation between the critical angle and refractive index. Explain how an optical fibre works, with a labelled diagram.

1. 1 · Define in one line. Total internal reflection is the complete reflection of light back into a denser medium when it meets a rarer medium at more than the critical angle.
2. 2 · State the two conditions.
   • Light travels from an optically denser to a rarer medium.
   • The angle of incidence is greater than the critical angle ic.
3. 3 · Derive the relation.
   • Snell's law, denser medium to air: n sin i = 1 × sin r
   • At i = ic, r = 90°, so n sin ic = 1
   • So, sin ic = 1/n
4. 4 · Describe the optical fibre. A core of high refractive index is surrounded by a cladding of lower index. Light meets the boundary at more than ic and is totally reflected again and again.
5. 5 · Draw the fibre. [Long core and cladding, n(core) > n(cladding); zigzag ray reflecting at the boundary; a normal at one point with i > ic marked]
6. 6 · Give uses. Telephone and internet signals, and endoscopes that let doctors see inside the body.
7. 7 · Close in one line. Because almost no light escapes, fibres carry signals over long distances with very little loss.

Scoring tip: Write both conditions; one alone earns only half. Label the core and cladding with their relative refractive indices.