Physiology of Vision Physics of vision Light is both matter and waves- electromagnetic energy Travels in packets – photons at a velocity of 186,000 miles/sec. (300,000 km/sec) February 22, 2019 Physiology of Vision

Physiology of Vision V I B G Y O R The visible spectrum is between Wavelength 400 – 800nm V I B G Y O R February 22, 2019 Physiology of Vision

Physiology of Vision Refractive power of lens Rays of light that Strike a lens more than 20 ft (6m) away Considered parallel They are brought to focus At principal focus February 22, 2019 Physiology of Vision

Physiology of Vision At < 20 ft away Rays are considered divergent Are brought to focus further February 22, 2019 Physiology of Vision

Refractive Power of Lens The greater the curvature of the lens The greater the refractive power Refractive power of a lens Is measured in Diopters This is the reciprocal Of the principal focal distance in meters Diopter = 1/f February 22, 2019 Physiology of Vision

Refractive Power of Lens Principal focus f Focal length DIOPTER = 1/f (f in meters) February 22, 2019 Physiology of Vision

Refractive Powers of Lenses = 10 diopters Convex lens 0.1 m Refractive power = 1/-0.1 = -10 diopters 0.1 m Concave lens February 22, 2019 Physiology of Vision

Refractive Power of the Eye Most of the refraction occurs At the anterior surface of the cornea Air/corneal interface Total refractive power Of the eye is 59 – 66 diopters The lens provides Approx. 15 diopters But this can be altered February 22, 2019 Physiology of Vision

Refractive power of the eye The total refractive power of the eye is 59 – 60 Diopters at rest The crystalline lens provides 15 diopters. This can be altered February 22, 2019 Physiology of Vision

Accommodation Parallel rays of light Rays of light from Blurred images From >6m away Brought to focus on the retina Rays of light from < 6m away are divergent Brought to focus behind the retina Blurred images February 22, 2019 Physiology of Vision

Accommodation To correct this The refractive power of lens You have either to Increase lens – retina distance OR Increase refractive power The refractive power of lens Can be changed voluntarily February 22, 2019 Physiology of Vision

Accommodation (normal eye) Parallel rays are brought to focus on retina Divergent rays are brought to focus behind the retina Lens focuses them on the retina February 22, 2019 Physiology of Vision

Accommodation This involves The refractive power Changing the shape of the lens The refractive power Can be increased from 15 to 29 diopters February 22, 2019 Physiology of Vision

Accommodation Mechanisms of accommodation Crystalline lens is Enclosed in capsule Held by suspensory ligaments attached on the ciliary body Parasympathetic activity Causes contraction of ciliary muscle Relaxation of suspensory ligaments February 22, 2019 Physiology of Vision

Accommodation Elastic lens capsule Relaxation of ciliary muscle Then assumes a more spherical shape Refractive power increases Relaxation of ciliary muscle Stretches the ligaments Which flattens the lens Refractive power decreases February 22, 2019 Physiology of Vision

Stimulate ciliary muscles Contraction Ligaments loosen Suspensory ligaments Lens Parasympathetic Stimulate ciliary muscles Contraction Ligaments loosen Lens focuses for Near Vision Sympathetic Inhibition of the Muscle Relaxation Suspensory ligaments tighten Lens focuses for far vision February 22, 2019 Physiology of Vision

Presbyopia Presbyopia Lens looses its elastic nature Loss of accommodation Due to aging Lens looses its elastic nature Due to loss of water February 22, 2019 Physiology of Vision

Presbyopia  Ability of the lens to become spherical Young children lens has Refractive power of 14 diopters At 45 to 50 yrs it is only 2 diopters Near point At 10 yrs = 9 cm At 60 yrs = 83 cm February 22, 2019 Physiology of Vision

Parallel rays are brought to focus on retina Emmetropic Eye Parallel rays are brought to focus on retina Divergent rays are brought to focus behind the retina Lens focuses them on the retina February 22, 2019 Physiology of Vision

Errors of Refraction Hyperopia (hypermetropia, farsightedness) Eyeball shorter than normal Parallel rays of light are brought to focus behind the retina Accommodation can compensate for the defect Defect is corrected by use of glasses with convex lenses February 22, 2019 Physiology of Vision

Hyperopia (far sightedness) Lens too flat or eye ball too short Parallel rays are brought to focus behind the retina Partly corrected by accommodation mechanisms (prolonged muscular efforts – tiring, headaches, blurring of vision) February 22, 2019 Physiology of Vision

Hyperopia (far sightedness) Corrected by a convex lens Bends the rays before they strike the refractive surface of eye Which finally focuses them on the retina February 22, 2019 Physiology of Vision

Errors of Refraction Myopia (nearsightedness) The antero-posterior diameter of eyeball is too long Parallel rays of light are brought to focus in front of the retina Defect is corrected by use of glasses with concave lenses February 22, 2019 Physiology of Vision

Myopia (near sightedness) Lens too convex or eyeball too long Parallel rays are brought to focus in front of the retina Corrected by a concave lens. Diverts the rays before they strike the refractive surface of eye February 22, 2019 Physiology of Vision