🩺 MedAtlax

Optics refractive errors

3 min read

Optics

Refractive Errors

Dr. Waleed AlRashed

Optics

Reflection vs Refraction

Reflection

  • The pathway of light described as straight-line rays
  • Reflection off a flat surface follows a simple rule:
    • Angle in (incidence) equals angle out (reflection)
    • Angles measured from surface normal (perpendicular)

Usage of Flat Mirrors

  • Regular mirrors
  • Periscope

Convex Mirrors

  • Curves outward

  • Reduces images

  • Virtual images

  • Use: Rear view mirrors, store security…

CAUTION! Objects are closer than they appear!

Usage of Concave Mirrors

Refraction

  • Light also goes through some transparent media
    • Glass, water, eyeball vitreous jell, lens, air
  • The light slowing factor is called the index of refraction
    • Glass has n = 1.52, meaning that light travels about 1.5 times slower in glass than in vacuum
    • Water has n = 1.33
    • Air has n = 1.00028
    • Vacuum is n = 1.00000 (speed of light at full capacity)

Refraction at a Plane Surface

  • Light bends at interface between refractive indices
    • Bends more the larger the difference in refractive index

Convex Lenses

  • Thicker in the center than edges.
    • Lens that converges (brings together) light rays.
    • Forms real images

The Magnifier

Concave Lenses

  • Lenses that are thicker at the edges and thinner in the center.
    • Diverges light rays
    • All images are erect and reduced.

The De-Magnifier

The Refraction of the Eye

  • Anterior corneal surface produces most of the eye’s refracting power ≈ 40 Diopter. 65%

  • Aqueous, lens + vitreous provide the remaining ≈ 20 Diopter. 35%

Ophthalmology Diagram

  • 40 Diopters
  • 20 Diopters

Eye Conditions and Corrections

Clinical Features

  • Myopic patients usually have problems seeing at distance.
  • The vision at near is normal.

Visual Acuity (V.A.)

  • V.A. improves with pinhole in patients with refractive errors.
  • Myopes squeeze their eyes to see clearly.

ACCOMMODATION

  • Flexible mechanism by which the eye can change its ref. power.
  • Contracture of ciliary muscle relaxation of the zonules more biconvex state of the lens.
  • Loss or reduction in this mechanism result in Presbyopia

A 50 y- old man started to have difficulty in reading. VA 20 /20 for distance.

PRESBYOPIA.

Slit lamp

Phoropter

The visual acuity test measures the smallest letters that patient can read on a standardized chart at a distance of 20 feet or 6 meters.

  • Snellen chart :
    • 20/ 20
    • 20/ 40
    • 20/100
  • V.A which improve with pinhole refractive error

Tumbling E Chart Optotype

Snellen Chart Optotype

Landolt ring Chart Optotype

Picture Chart Optotype

Spectacle prescription

  • VA (visual acuity)

  • OD (oculus dexter):right eye

  • OS (oculus sinister):left eye

  • OU (oculus uterque):both eyes

  • IPD=PD(inter-pupillary distance =pupillary distance)

Glass or spectacle prescription

Graph View

Backlinks