LIGHT

LIGHT

1. Similarities/ Differences Between Light and Sound

   • Similarites
     • Both can be...
       • reflected
         • reflection - when waves bounce of a surface
       • refracted
         • refraction - when wave changes speed /direction when entering a denser or less dense material
       • diffracted
         • diffraction - the spreading of waves as they pass through a gap
   • Differences
     • light waves are transverse
       • sound waves are longitudinal
     • light can travel through a vacuum
       • sound cannot - can only travel through solid/ liquid/ gas

2. Law of Reflection

   • angle of incidence = angle of reflection
     • the incidence ray is the ray going in to the mirror (or surface), reflected ray is coming out of mirror
   • on a smooth surface i.e mirror, all rays are reflected at the same angle uniformly - this is why they appear shiny
   • the image in a plane mirror is...
     • virtual
       • cannot be touched or projected onto a screen - it looks like it's really there but it actually isn't
       • light rays that appear to be coming from behind the mirror (virtual images) are drawn with dashed lines
     • upright
       • looks the right way up
     • laterally inverted
       • if you stand in front of a mirror, your left side seems to be on the right in the reflection

3. Refraction

   • sound & light waves change speed when they pass across the boundary between 2 substances with different densities
     • EG air and glass
     • this causes the waves to change direction, this effect is called REFRACTION
       • as the angle of incidence increases so does the angle of refraction
         • angle of incidence= angle between normal and incident ray
       • an object is only visible if it reflects or refracts light
   • REFRACTIVE INDEX
     • the extent to which light is refracted when it enters a medium
       • n= sin i / sin r
         • n is the refractive index
         • i is the angle of incidence
         • r is the angle of refraction
     • Snell's Law

4. TOTAL INTERNAL REFLECTION

   • beyond a certain angle called the CRITICAL ANGLE all waves reflect back into the glass
     • we say they're totally internally reflected
       • uses of TIR
         • binoculars
     • Light travels from glass to air. Angle of refraction is greater than angle of incidence.
     • Light travels from glass to air. Angle of refraction is now 90°
     • Light is reflected back into glass. Angle of reflection equals angle of incidence.

5. CONVEX & CONCAVE MIRRORS

   • concave
     • reflect light inwards to one focal point
     • used to focus light
       • headlights
       • torches
     • law of reflection still true for concave mirrors
   • convex
     • reflect light outwards - so are not used to focus light
     • uses include
       • traffic safety mirrors
       • ceiling dome mirrors - shop surveillance

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