Optics

• Optotype

  The visual target used in an eye examination
• Calculating the height of the optotype


  1. tan5' = opp/6

  2. opp = 6tan5' = 6 x tan 0 ° ' '' 5 ° ' '' = 0 or 0.5
• Logarithm values

  • Log10 100000 = 5
  • Log10 1000 = 3
  • Log10 10 = 1
  • Log10 1 = 0
• 20/20

  Equivalent to 6/6, LogMAR 0.0, 30 cycles per degree
• Speed

  Wavelength x Frequency
• Refractive index (n)

  Speed in vacuo/speed in medium
• Law of reflection


  Angle of incidence (i) is equal to the angle of reflection (r)
• Relationship between i and i'


  sini/sini' = n'/n
• For a parallel sided block: i1' = i2
• For a non-parallel sided block (prism): i1' ≠ i2
• Calculating total deviation (dTOT)


  dTOT = i1 + i2' - a
• Deviation at 2nd surface (d2)


  d2= i2'-i2
• i1' + i2 = a (for 2nd surface/ angle of incidence)
• dtot = i1 +i2 - a = a(nprism-1)
• Fundamental paraxial equation for refraction at a spherical surface

  n'/ℓ' = n/ℓ + (n' - n)/r
• Vergence


  L' is the image vergence n'/l'

  L is the object vergence n/l

  FSURF is the focal power of the surface
• Real object


  Light diverges from it
• Real image


  Image vergence is converging
• First focal length


  Light from an object placed at the first focal length will image at optical infinity
• Second focal length


  Light from an object at optical infinity will form an image at the second focal length from the surface/lens
• Focal length (f')


  f'= n'/fsurf
• Magnification (M)


  M = h'/h
• Calculating magnification


  Tanω = o/a = h'/ℓ'-r

  Tanω = h/ℓ-r

  Tanω = h'/(ℓ' - r) = h/(ℓ - r)

  h' = h(ℓ'- r)/(ℓ - r) OR h'= hL/L'
• When light passes from one transparent material to another with a higher or lower refractive index, it bends (refracts) at an angle determined by Snell's law.
• The refractive index is the ratio of the speed of light in vacuum to its speed through the medium.
• Refraction occurs when light travels through different materials with varying refractive indices
• Snell’s Law states that when light travels across two different materials, the incident ray, normal, and refracted ray lie on the same plane.
• Refraction occurs because the velocity of light changes as it moves between media with different refractive indices.
• The whole of a 6/6 E subtends 5 minutes of arc at the nodal point of the eye.