Optics of vision and Refractive errors Physiology Lecture Slideshow

Learning Outcomes

By the end of lecture students should be able to:

• Explain refraction.
• Explain concept of convergence and divergence.
• Define focal length, focal point and power of lens.
• Comment on physiological anatomy of eye.
• Differentiate between emmetropia, myopia, hyperopia,
  astigmatism, presbyopia
• Summarize their treatment.

Special Senses

• Sensory receptors are grouped into special organs known as
  special senses

1. Eyes– Vision
2. Ear– Hearing
3. Taste Buds– Taste
4. Olfactory membrane– Smell

Vision

• Eyes are composed of
  •OPTICAL COMPONENTS
• Multiple interfaces and media
  •NEURAL COMPONENTS
• Receptor
• Visual Pathway
• Visual Cortex

Introduction

• Light travel through air at velocity of 300,000km/sec
• Travel slower in dense medium
• Refractive index– ratio of velocity of light in air to velocity of
  light in the substance

300,000:200,000

1:0.50

• All the rays pass through a single point– Focal point

Refraction

• Light passes through angulated surface- light rays bend because of
  change of media
• Bending of light at an angulated surface– Refraction
• Refraction depends upon

1. Refractive indices of two media
2. Degree of angulation

• Mechanism that helps to focus light on retina

Components of the Eye’s Optical System

• Cornea
• The anterior chamber
• The iris and pupil
• The crystalline lens
• The retina

Lenses

• 2 types

1. Spherical lenses
   Converge or diverge light in both the planes(vertical and
   horizontal)
2. Cylindrical lenses
   Converge or diverge light in one planes

•Convex lens

• Convergence of light on to a single focal point
• Light rays passing through the center do not refract
• Light rays towards outer surfaces bend more– converge
• Stronger the lens more will be convergence

•Concave lens

• Divergence of light on to a single focal point
• Light rays passing through the center do not refract
• Light rays towards outer surfaces bend more– diverge
• Stronger the lens more will be divergence

Focal Point

• Light rays that pass through the spherical lens are refracted
  at all edges of the lens (in both planes) toward the central
  ray, and all the rays come to a focal point.

Diopter

• Refractive power is measured in diopter
• Reciprocal of focal length (1/f)
• More a lens bend light rays more is the power of lens
• Power of lens expressed in
• Convex lens– +
• Concave lens— –

Cylindrical lens

• A cylindrical lens is a lens which focuses light into a line
  instead of a point, as a spherical lens would.
• Light from two different sides bend in one plane and form a
  focal line

Physiological Anatomy of Eye

Errors of Refraction

Emmetropia

• Emmetropic eye—normal
• Equal measure– no refractive error– desirable optical system
• When parallel light rays from distant objects are in sharp
  focus on the retina when the ciliary muscle is completely
  relaxed
• Emmetropic eye can see all distant objects clearly when
  ciliary muscles are relaxed
• For focusing near object ciliary muscles must contract for
  clear vision– accommodation

Ametropia

• Parallel rays do not fall on retina, when ciliary muscles are
  relaxed
• Includes
• Far sightedness (Hyperopia)
• Near sightedness (Myopia)
• Astigmatism
• Presbyopia

Hyperopia

• Far sightedness
• Focusing parallel rays of light behind retina, when ciliary
  muscle is fully relaxed
• Clear distant vision
• Distant objects will be focused on retina by contraction of
  ciliary muscles
• ERROR
• Defective near vision

• Causes
• Short eyeball
• Weak lens system
• Presentation: Headache
• Due to continuous contraction of ciliary muscles
• Correction:
• Convex lens

Myopia

• Near sightedness
• Focusing parallel rays of light in front of retina, when ciliary
  muscle is fully relaxed
• Clear near vision
• No mechanism to reduce lens power
• ERROR
• Defective far vision

• Causes
• Excessive long globe (axial myopia) : more common
• Excessive refractive power (refractive myopia)
• Increase in the curvature of the cornea or the surfaces of
  the crystalline lens
• Near objects will be focused on retina easily
• When the far object is brought closer accommodation bring
  about the clarity of image
• Correction
• Concave lens

Astigmatism

• Focusing of light rays along two different lines
• Cornea or lens don’t have same refractive power in both planes
• Eye’s lens system
• Combination of two cylindrical lenses of unequal powers placed at
  right planes

Causes

• Abnormal curvature of cornea and lens in one or both the
  planes
• Treatment

1. Cylindrical lens (problem in one planes)
2. Cylindrical and spherical lens (problem in both planes)

Presbyopia

• Physiologic loss of accommodation in advancing age
• Deposit of insoluble proteins in lens in advancing age
• Elasticity of lens progressively decrease—decrease in
  accommodation
• Around 45 years of age , accommodation become less than
  3D reading is possible at 40-50 cm
• Difficultly reading fine print , headache , visual fatigue

• Treatment
• Convex lenses in near vision
• Reading glasses
• Bifocal glasses
• Progressive power glasses