Waves #2

Waves #2

1. Reflection of Waves

   • Visible light is a transverse wave
     • KEY
       • angle of i - angle of incident
         • angle of r - angle of refraction
           • c - critical angle
   • Light bounces off objects into eyes
   • When light reflects from an even surface, it's reflected at the same angle
   • Law of reflection
     • Angle of incidence = Angle of reflection
     • Normal line is perpendicular to the surface at the point of incidence
     • Angle of incidence = angle between incoming wave and the normal
       • Angle of reflection = angle between reflected wave and the normal

2. Refraction of Waves

   • Sound waves travel faster in denser substances
     • Glass -> Air = speed changes
   • Ray diagrams
     • If the second material is DENSER than the first, the refracted ray bends TOWARDS the normal
     • If the second material is LESS DENSE, the refracted ray speeds AWAY from the normal
     • If the second material is less dense, the angle of refraction is larger than the angle of incidence
   • Rays passing through a glass block
     • Light entering a less dense medium - speeds up and bends away from normal
       • Light entering a more dense medium - slows down and bends towards normal

3. Refractive Index

   • refractive index = speed of light in a vacuum/speed of light in that material
     • so n = c/v
     • Light slows down in glass - refractive index is high
     • Refractive index of water is low - doesn't slow down as much as in glass
   • Snell's Law
     • Refractive index: n = sin i/sin r

4. Experiment: Refractive Index of glass

   • 1. Draw around a glass block on paper
     • 2. Direct a ray of light through it at an angle. Trace the incident and emergent ray
       • 3. Remove the block and draw in the refracted ray
         • 4. Draw in the normal line at the point where the ray enters the block
           • 5. Measure the angle of incidence and angle of refraction
             • 6. Calculate the refractive index using Snell's Law

5. Critical Angles

   • Total Internal Reflection
     • If you keep increasing the angle of i, the angle of r gets close to 90 degrees
       • Eventually i reaches a critical angle
         • Above this critical angle, you get a total internal reflection
     • Semi-circular block
       • If angle of i is less than C
         • Most light passes out, little bit is internally reflected
       • If angle of i is equal to C
         • Emerging ray comes out along the surface, lots of internal reflection
       • If angle of i is greater than C
         • No light comes out, it's ALL internally reflected
   • Optical Fibres
     • Made of plastic or glass
       • Light signals passing through it always hit the boundary at angles higher than c
         • So, the light is always totally internally reflected
   • Critical angle, c
     • To find: sin C = 1/n