Eye physiology- Optics

An image entering the eye is inverted:

Image formation in an optical system is described by Snell’s Law:

  • A light ray entering a denser medium (higher refractive index) is bent towards a line perpendicular to the surface.  In the eye, this provides a means of socusing and magnifying an image.

Parallel rays passing into a denser medium with a convex surface (e.g. lens) are brought into focus as a function of the:

  • radius of curvature
  • ratio of refractive indices of the media

In the eye, there are three surfaces which light has to pass through:

  1. the cornea: the aqueous humour that separates the cornea and the lens and the vitreous humour that fills the remainder of the eye have refractive indices of around 1.34
  2. the front surface of the lens: the lens has a refractive index of around 1.41
  3. the back surface of the lens

The total refracting power of the eye (once this is added up) is 60-64 dioptres (dioptre is a product of refractive index and curvature).  It should be noted that the cornea accounts for around 2/3 of this (lens is the remainder).

The focal length of the eye can also be equated to power (P=1/D).  The focal length of the eye looking at a distant object is 0.024m (distance from cornea to retina).  The power in this instance is just over 40 dioptres (note this is almost entirely due to the cornea).

Accommodation

Accommodation is the process by which the lens to becomes more rounded for near vision.

The lens is held between the Zonules of Zinn.

  • Far (distant/long) vision
    • Relaxing the ring of the circular ciliary muscles (by control of CN III)
    • Zonules are taught
    • Lens at miminum strength
  • Near vision
    • Contraction of the ciliary muscles
    • Zonules relax
    • Lens rounds for greater strenght

The maximum power of the lens is around 20D.  After this point the eye cannot focus on nearer objects (near point).  Similarly, there is a furthest point in which the eye can focus (far point).  It should be notes that any object that is closer than 9m (in the normal individual) requires some focus by altering the lens.

If the focussing power doesn’t match the size of the eye, you get refractive errors.

(see Hypermetropia, Myopia)

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