• Light rays bend when moving between mediums of different refractive index
  • Refractive index describes the speed of light through a material, relative to a vacuum
  • The refractive index of the cornea is 1.376, which means the light travels 1.376 times slower through the cornea than in a vacuum
• The angle of incidence (θ1) is the angle made between the incident ray and a line that is perpendicular to the surface at the point of incidence (dotted line)
• When a ray moves from a material of higher refractive index to a material of lower refractive index, the ray is refracted away from the normal (dotted line)

• The critical angle (θc) is the angle of incidence at which light is first reflected instead of refracted
• The critical angle for normal spectacles is 41°

• Total internal reflection is when the angle of incidence exceeds the critical angle
• Light is not refracted
• Light is reflected back into the medium with the higher refractive index

The anterior chamber angle is an important clinical structure in glaucoma. It cannot be seen directly because the light coming out of it is totally internally reflected by the corneal/air interface. A gonioscope changes the refractive index of this interface such that the angle becomes visible.

• A lens changes the vergence of light, whether it’s the crystalline lens, spectacles, contact lenses or IOL implants
• A plus lens converges light
• A minus lens diverges light

• U+D=V
• U → vergence of light entering the lens (object rays)
• D → vergence added to light by the lens (lens power)
• V → vergence of light leaving the lens (image rays)

Example

Light from an object 2m away is travelling to a +2D lens. At what distance is the image going to be formed?

- U= 1/2 = -0.5D   
    - Light coming from a natural object is divergent (minus)  
- D = +2D  
- V = -0.5+2 = +1.5D  
- Recall that D=1/f(m), therefore, 1/1.5 = 0.67m  
- The image will form 0.67m on the other side of the lens  

Where questions involve distance and diopters, take the reciprocal to switch between the units

• Most commonly used test in clinics
• Patients stand 6m away and read the letters on the chart.
• A score is given as a fraction where the numerator is the distance away from the chart
  • 20/200 = 6/60
    • This means that the patient can read at 6m, what the average person can read at 60m.
  • 20/20 = 6/6
    • This means that the patient can read at 6m what the average patient can read at 6m
• 6/12 (line 6) is the minimum driving requirement
  • This is crudely tested by asking the patient to read a number plate at 20m during the driving test

• More accurate than Snellen but mainly used in research settings.
  • There are 5 letters in each row at equidistance, this accounts for the crowding phenomenon, and makes LogMAR more accurate than snellen
• Patients stand 4m away
• A score between 1-0 is given where 0 is best.
  • 0 LogMAR = 6/6 snellen
  • 1 Logmar = 6/60 snellen
• Each letter is worth 0.02units, which are accumulated and subtracted from 1 to give the final score.

• The chart has black letters on either a red background or a green. The patient is asked to determine which background makes the letters clearer
  • No difference → no refractive error
  • Red background is clearer → hyperopia
  • Green background clearer → myopia
• The principle underlying this test is chromatic aberration
  • Red has a longer wavelength and focuses behind the retina
  • Green has a shorter wavelength and focuses in front of the retina

• A retinoscope is similar to an ophthalmoscope. The key difference is that the light source of the retinoscope can be quickly moved off the visual axis.
• Retinoscopy is an objective measure of visual acuity.
• The examiner shines the light from the retinoscope and aims to observe the red retinal reflex. Different lenses are added in front of the light to achieve the perfect perfect red reflex.
  • The power of these lenses tells you the amount of refractive error
  • The direction of the red reflex against the direction of the light from the retinoscope tells you the type of refractive error
    • Myopia → red reflex moves against the direction of light
    • Hyperopia → red reflex moves with the direction of light
      

      

In indirect ophthalmoscopy, the field of view is largest in cases of high myopia and smaller in hyperopia

• Wide beam of light diffusely illuminating the eye and external structures.
• Used for general examination

• This is the most commonly used type of illumination
• It focuses a beam of light directly on the part of the eye being examined

• Light is reflected back from structures to posteriorly illuminate structures in front. E.g from the iris to the cornea or the retina to the iris/lens
  

  

• Focuses the view on the corneal endothelium
• Useful in corneal endothelial diseases such as Fuchs’ endothelial dystrophy

• The beam of light from the slit lamp is projected onto the corneal limbus. This light is scattered throughout the cornea and illuminates corneal lesions