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OPTICS and CLINICAL REFRACTION (including ophthalmic optics)

• How to easily remember the theoretical basis of light refraction
• Pretend that light is like a pair of wheels going on different terrain.  Once Tire B gets “stuck” in a denser material, e.g. glass, that wheel will grip onto that dense material, which causes the other wheel (Tire A) to spin more freely since it’s in a less dense material e.g. air.  It’s as if glass is like mud, where a wheel gets stuck, and the other wheel spins more freely.
• Therefore, you can see that light bends just like how these wheels behave.
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• How to remember that an increase in index of refraction results in increased reflectance
• You can remember this by thinking about how anti-reflective coating is essential for polycarbonate lenses – and hence polycarb is a higher index therefore has a higher tendency for reflections
• Prisms
• Need to remember how prism affects how you see?  Just remember, when you look at a pyramid (or any tall structure really), you tend to look at the apex.  Same thing with prisms – the prism makes the eye deviate in the direction of the apex
• When correcting eye deviations, point the APEX toward the direction of deviation
• Example below – use BD prism on the right eye if the patient has a right hyper (note that you can also use a BU prism on the left eye for the same effect):
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• An easy way to find negative cylinder power from the power cross
• To find the cyl power in the power cross, simply take the absolute value of the largest number (8.00) and subtract the smaller number (5.00) then tack on a negative.
• Remember, the Rx axis comes from the sphere axis – in this case, it’s 90
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• Lensometry
• An easy way to remember the sphere lines – “Sphere = Skinny Spaghetti”, thus the cyl lines are the three thicker lines.
• In cylindrical lenses, you can remember that the sphere lines must be read first because the front (F1) surface has the sphere part of the lens, which should come in focus first; the back surface (F2) has the cyl part which should come in focus 2nd.
• Classification of the axis of astigmatism and the shape of the eye
• Think of cyl in the cornea as a football – the curvature along one meridian changes faster than the meridian perpendicular to the latter
• With The Rule (WTR) astigmatism is like a football sitting “with the rule of gravity” – it is lying flat on the ground; the steeper/power meridian is along the 90 degree (vertical) meridian, and the flatter/axis meridian is along the 180 (horizontal) meridian.  This is why WTR corneas tend to have a roughly 180 degree axis.
• Against The Rule (ATR) astigmatism is like a football standing up “against the rule of gravity”; the steeper/power meridian is along the 180 degree (horizontal) meridian, and the flatter/axis meridian is along the 90 (vertical) meridian.  This is why ATR corneas tend to have a roughly 90 degree axis.
• An easy way to remember effective power with plus or minus lenses
• To increase the effective power of a minus lens, decrease the vertex.  Think of a myopic nerd (wears minus lenses) – they tend to push up on the bridge of the glasses, which increases effective power
• To increase the effective power of a plus lens, increase the vertex.  Think of a presbyopic grandma (wears plus lens readers) – you can imagine her pulling down on the bridge of the glasses, as if the frame is resting low toward the tip of the nose; this increases the effective power.
• An easy way to remember how to change sphere based on the retinoscopic reflex
• Turn the sphere dial down WITH the ground when you see WITH motion
• Turn the sphere dial up AGAINST the ground when you see AGAINST motion
• Convert Reduced Snellen (RS) to Metric to Printer’s Point
• Based on 1M = RS 20/50 = 8 point
• Simply use the reverse math operation to go against the direction of the arrow (e.g. divide instead of multiply or vice versa)