2 Anatomy and function of the human eye ConjunctivaCorneaScleraChoroidsRetinaIrisLensAqueous and vitreous humorCiliary bodyOptic nerve
3 Parts Structure Function Conjunctiva Thin, clear membrane covering the front of the eye and the inner eyelidsProtect the front part of the eyeagainst infectionProduce mucous that helps lubricate the eyeCorneaTransparent, dome-shaped window covering the front part of the eyeA powerful refracting surface providing 2/3 of the eye’s focusing powerBend light rays as they pass through and helps it to focusSclera“The white of the eye”Tough, white outerback part of they eyeballThe eye’s protective outer coatChoroidLies between the retina and scleraA dark pigmented layer containing many blood vesselsAbsorbs and prevents light scattering to prevent false images forming on the retinaRetinaFilm of the eyeThe innermost layer of the eye, contains the light sensitive cells or photoreceptors (rods and cones) and fibresContain foveaConverts light rays into electrical signals and sends them to the brain through the optic nerveIrisColoured part of the eyeA ring of muscle fibresContracts and expands, opening and closing the pupil in response to the brightness of surrounding lightLensA transparent, biconvex protein disc behind the pupilFocuses light rays onto the retinaRefractive media and has a role in accommodation
4 Aqueous humorA thin, watery fluid that fills the space between the cornea and the irisContinually produced by cilliary bodyNourished the cornea and the lens and gives the front of the eye its form and shapeRefractive mediaVitreous humorA thick, transparent jelly-like substance that fills the center of the eyeComposes mainly of water giving the eye its forma nd shapeBends rays of light as they pas throughRefractive mediaCilliary bodyConnects the choroids with the lens and contains the cilliary muscles and suspensory ligaments that hold the lens in positionAlters the shape of the lens - accommodationOptic nerveThe optic nerve connects the eye to the brainThe region where it leaves - blind spot because it has no photoreceptors and therefore cannot produce an imageTransmits electrical impulses form the retina to the brain
5 Electromagnetic specturm Energy from the sun reaches Earth as waves of electromagnetic radiation. The electromagnetic spectrum has a range of waves of different wavelengths. Light is one form of electromagnetic radiation and makes up a part of the spectrum. The shortest wavelengths are gamma rays and in order of increasing wavelengths, X-ray, UV, visible light, infra-red, microwaves, TV and radio waves.Only a limited range of these wavelengths can be detected by humans. This range is called the visible spectrum - from 380 to 750 nm. Different wavelengths correspond to different colours.UV and infra-red are outside the human range but other animals can detect them. For example, bees and many other insects can see well into the UV range and can navigate to pollen and nectar in flowers by following UV landing strips on petals.
6 Reasons for the differences in range of electromagnetic radiation detected AnimalRangeReasonsHumansnmActive during the day, colour vision important to distinguish food and to gain information about the environmentPit vipernmRelies on infra-red to locate prey in dark burrowsDeep sea fishnmLittle light penetrated to the depth at which they live, use bioluminescence to communicate: can only detect blue lightHoneybeenmSome flowers have ultraviolet markings on them which bees use to find pollen
7 Conditions under which refraction of light occurs Light travels in a straight line and is bent or refracted when it moves from one medium to another with different densities. Refraction occurs when the wave changes speed and direction. A ray of light moving into a more dense medium is refracted towards the normal: a ray of light moving into a less dense medium is refracted away from the normal. Light is not refracted if it hits the boundary of 90 degrees.
8 Refractive mediaRefraction is very important in the eye. As light passes into the eye it is refracted by four different transparent media. These are:The cornea, which causes most of the refractionThe aqueous humorThe lens, which fine focuses the image onto the retinaThe vitreous humorThey are called refractive media because light bends as it passes through them
9 AccommodationAccommodation is the ability to focus objects at different distances through a change in the curvature of the lens by the contraction of the ciliary muscles.It is the way our eye adjusts so that the light is always focused on the retina. It is important to allow clear vision.Without accommodation the eye would have a fixed focus and would not be able to change focus from distant to close objects, which will appear to be blurred.
10 AccommodationThe ciliary muscles control the thickness of the lens and attached to these muscles are suspensory ligaments. The ciliary muscles change the curvature according to the distance of the object to be focused.When the ciliary muscles contract, the ligaments loosen and the lens bulges outwards and becomes more rounded that is curvature increases.This focuses light from objects that are close. On the other hand, when the ciliary muscles relax, the ligaments tighten and the lens pulls inwards and flattens that is the curvature decreases. This focuses light coming from distant object
11 Change in the refractive power of the lens At rest, the ciliary muscles relax, the suspensory ligaments are taut and the lens is flattened. Vision would be focused on far objects and the refractory power would be at minimumAt maximum accommodation, the ciliary muscles contract, the suspensory ligaments that hold the lens are released and the lens become more rounded. This is fully accommodated and maximum refraction of light. Near object would be in focus.EyeShape of lensSuspensoryCiliaryFocusFocal lengthRefractive powerAt restFlattenedContractedRelaxedFar objectsLongLowFull accomodationRoundedNear objectsShorthigh
12 Myopia and hyperopiaSight defects are very common and can be caused by many situations. In normal vision the image is focused when it lands on the retina. If the light rays coming into the eye are not focused onto the retina then the image will be blurred.Two common refractive problems with eyes are conditions:Myopia (Short-sightedness) - can see close objects clearly but distant objects are out of focus - image is focused in front of the retinaHyperopia (Long-sightedness) - can see objects in the distance but close objects are out of focus - image is focused behind the retina
13 Technologies to correct myopia and hyperopia Several technologies used to correct these conditions include:Glasses and contact lensesThere are two types of lenses:concave lens - light passing through diverges (spreads out) - correct myopiaconvex lens - light passing through converges to a focal point - correct hyperopiaRefractive surgery can also be used to correct these conditions
14 Depth perceptionDepth perception is the ability to judge the distance between objects. Depth perception required the ability to see depth in our three dimensional world usually called binocular vision or sometimes even referred to as stereoscopic vision.Depth perception results from having forward facing eyes. This produces an overlap between the view from left and the view from the right eye. The images formed by both eyes are sorted in the brain that a three dimensional picture is formed.