TSH.4 Optical Defects and vision correction

Refractive Power

  • Definition: Measure of how a lens converges or diverges light.

  • Formula: ( P = \frac{1}{f} ), where:

    • ( f ) = focal length (m)

    • ( P ) = refractive power (dioptres, D).

  • Key Insight: Shorter focal lengths yield greater refractive power.

Power of the Eye

  • Lens equation: ( \frac{1}{f} = \frac{1}{u} + \frac{1}{v} )

    • Variables:

      • ( u ) = object distance

      • ( v ) = image distance

    • Normal eye: Approx. ( v = 2.5 ) cm.

Maximum Refractive Power

  • Occurs when focused on near objects (near point):

    • ( u = 25 ) cm, ( v = 2.5 ) cm.

    • Calculation: ( P_{max} = \frac{1}{0.25} + \frac{1}{0.025} = 44 ).

Minimum Refractive Power

  • Occurs when viewing distant objects (far point):

    • ( u = \infty ), ( v = 2.5 ) cm.

    • Calculation: ( P_{min} = \frac{1}{\infty} + \frac{1}{0.025} = 40 ).

Power of Accommodation

  • Defined as difference in refractive power:

    • ( A = P_{near} - P_{far} ).

  • Influenced by changes in near and far points.

Corrective Glasses

  • Prescriptive Rule:

    • Calculation: ( P_{corrective} = P_{normal} - P_{defective} ).

Myopia

  • Definition: Condition where light focuses in front of the retina.

  • Causes: Eyeball too long or cornea too curved.

  • Corrective Method: Diverging (concave) lens to disperse rays.

Myopia Calculation Example

  • Given: Far point at 1 m.

  • Required refractive power:

    • ( \frac{1}{1} + \frac{1}{0.025} = 41 )

    • Normal vision: ( \frac{1}{\infty} + \frac{1}{0.025} = 40 )

    • Correction: ( PC_{D} = 40 - 41 = -1 ).

Hypermetropia

  • Definition: Condition where light focuses behind the retina.

  • Symptoms: Difficulty focusing on close objects.

  • Corrective Method: Converging (convex) lens to aid focusing.

Hypermetropia Calculation Example

  • Given: Near point at 1 m.

  • Required power:

    • Calculate for near vision (25 cm target):

    • Result: ( P_{D} = 3 ) D required.

Presbyopia

  • Age-related loss of accommodation.

  • Affects near vision but not distance vision.

  • Corrective Method: Converging lenses for reading.

Astigmatism

  • Definition: Distorted vision due to uneven cornea curvature.

  • Correction: Cylindrical lenses with varied refractive power for vertical and horizontal planes.

Other Eye Problems

  • Beyond optical defects, may include:

    • Trauma, glaucoma, cataracts, requiring medical intervention.

Review & Summing Up

  • Understanding optical defects is crucial in vision science.

  • Calculating corrective lens strength is integral to the treatment of refractive errors.