State three observable phenomena where a particle behaves like waves. State the scientific principle underlying the operation of fibre optics. (b) Explain e...

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Three observable phenomena in which particles behave like waves.

• Electron diffraction: a beam of electrons passed through a thin crystal (or graphite film) produces diffraction rings, just as X-rays do (Davisson-Germer experiment).
• Interference of electrons: electrons passed through a double slit build up an interference pattern of bright and dark fringes.
• The electron microscope: electrons are focused and form images because they behave like short-wavelength waves; neutron diffraction by crystals is a further example.

Scientific principle underlying fibre optics. Fibre optics works on the principle of total internal reflection of light. When light inside the denser core strikes the core-cladding boundary at an angle greater than the critical angle, it is totally reflected back into the core, so light is guided along the fibre with very little loss even when the fibre bends.

(b) Terms used in fibre optics.

• (i) Core: the central, thin strand of very pure glass (or plastic) of higher refractive index along which the light signal actually travels by repeated total internal reflection.
• (ii) Cladding: the outer layer of glass of lower refractive index surrounding the core. Because it is optically less dense than the core, it ensures that light striking the boundary is totally internally reflected and kept inside the core.