P14 - Light

• Normal


  The line perpendicular to the mirror
• Angle of incidence


  The angle between the incident ray and the normal
• Angle of reflection


  The angle between the reflected ray and the normal

  
• For any light ray reflected by a plane mirror, the angle of incidence = the angle of reflection

  
• Image formation by a plane mirror

  1. Light rays from an object reflect off the mirror

  2. Image and object are at equal distances from the mirror

  
• Virtual image


  • Formed at a place where light rays appear to come from after they have been reflected (or refracted)
  • Can't be projected onto a screen

  
• Real image


  Formed by focusing light rays onto a screen

  
• Specular reflection


  Reflection from a smooth surface where parallel light rays are reflected in a single direction

  
• Diffuse reflection


  Reflection from a rough surface where parallel light rays are scattered in different directions

  
• • Reflection from a smooth surface is called specular reflection
  • Reflection from a rough surface is called diffuse reflection

  
• If the angle of incidence increases


  The angle between the incident ray and reflected ray increases

  
• The image formed by a plane mirror is virtual, upright, and laterally inverted
• When light travels from air into glass, its speed decreases

  
• If the angle of incidence is zero when light travels from air into glass, the angle of refraction is also zero

  
• If the angle of incidence is non-zero when light travels from air into glass, the angle of refraction is smaller than the angle of incidence

  
• Refraction of light through glass objects

  1. Light ray changes direction at the boundary between air and glass

  2. Angle of refraction is smaller than angle of incidence when entering glass

  3. Angle of refraction is larger than angle of incidence when exiting glass

  
• Light ray from bottom of swimming pool refracts at water surface


  Angle of incidence is 40 degrees, angle of refraction is 75 degrees
• The swimming pool appears shallower than it really is when viewed from above due to refraction
• Refraction of white light through a prism

  1. White light is split into the colours of the spectrum

  2. This happens because the speed of light in glass depends on wavelength and colour

  
• Blue light travels slower in glass than red light, so it is refracted more

  
• Refraction is the change in direction of waves when they travel across a boundary from one medium to another

  
• When light refracts from air into glass, the angle of refraction is less than the angle of incidence

  
• When light refracts from glass into air, the angle of refraction is more than the angle of incidence

  
• The colour of a surface depends on the chemicals called pigments in the surface materials and the range of wavelengths in the incident light
• Pigments absorb light of specific wavelengths and strongly reflect other wavelengths
• A white surface has no pigments, so it reflects light of any wavelength, either partially or totally
• A red surface absorbs all colours except red, which it reflects
• Red, green, and blue are the primary colours of light because they can be mixed to produce any other colour of light
• A translucent object allows some light to pass through but scatters the light in different directions

  
• A transparent object allows light to pass through without being scattered
• Pigments


  Chemicals in the surface materials that absorb light of specific wavelengths and strongly reflect other wavelengths
• White surface


  Has no pigments, so it reflects light of any wavelength, either partially or totally
• Red surface


  Has pigments that absorb all the colours of light except for red, which is reflected
• Investigating reflection of a narrow beam of light

  1. Place ray box on blank sheet of paper

  2. Use ray box to direct light ray at mirror

  3. Replace mirror with different coloured surfaces

  4. Compare reflection from shiny and rough surfaces

  
• Transparent object


  Transmits all the incident light that enters the object, no light is absorbed at its surface

  
• Translucent object


  Lets light pass through but scatters or refracts the light inside it

  
• Opaque object


  Absorbs all the light that reaches it, no light travels all the way through

  
• Book observed in daylight

  • Blue front cover with white title
• Book observed in red light

  Appears black
• Green dress in blue lighting

  • Appears black