Physics: Refraction

Physics: Refraction

1. Refraction - Waves Change Speed and Direction

   • The Speed of a wave is affected by the density of the substance or medium it is travelling in. e.g. Light waves travel more slowly in a denser medium (Usually).
   • So when a wave crosses a boundary between two substaces, it changes speed.
   • Since Wave Speed = Frequency X Wavelength, and the frequncy is fixed for each type of wave the if the speed changes the wavelength must also change.
   • The change in speed and wavelength can cause the wave to change direction too: This is refraction.
   • Converging or convex lenses use refraction to focus light wves to form an image of an object.
   • As a light ray hits the surface of the lens and passes from air to glass, it slows down. This causes the light ray to bend towards the 'normal'. (The line at right angles to the boundary at the pint where the ray enters or leaves.
   • When it hits the 'glass to air' boundary on the other side it speeds up and bends away from the normal.
   • The curvature of the lens means all the parallel rays hitting differenet parts of the lens are bent towards the same focal point, where an image is formed of whateevr the ligh is coming form.

2. A Triangular Prism can Refract Light to Form a Spectrum

   • Different wavelengths of light refract by different amounts. So white light disperses into its different colours as it enters a prism.
   • A Rectanglar prism has parallel boudnaires, so the rays bend ne way as they enter and bend back again by the same amount as they leave - the ligh leavign the glass is parlallel to the light entering it, so white lgiht emerges.
   • But with a trianglular psim, the boundaires aren;t parallel, which means the different wavelengths don't recombine, and you geta rainbow effect - a sepcturm.