Vision Module 8 - HSC Biology This Lesson ● Structure of the eye ● Normal vision process ○ ○ ○ Refracting light Accomodation The Retina Syllabus Links IQ 8.5: How can technologies be used to assist people who experience disorders? ● explain a range of causes of disorders by investigating the structures and functions of the relevant organs, for example: – hearing loss – visual disorders – loss of kidney function ● investigate technologies that are used to assist with the effects of a disorder, including but not limited to: – hearing loss: cochlear implants, bone conduction implants, hearing aids – visual disorders: spectacles, laser surgery – loss of kidney function: dialysis ● evaluate the effectiveness of a technology that is used to manage and assist with the effects of a disorder Starter Questions Why is vision so important to animals, including humans? What are some disorders of the eye/vision? Extraocular Muscles Retina Choroid Ciliary Body Sclera Aqueous Humor Blood vessels Iris Blind spot Pupil Cornea Optic Nerve Lens https://www.youtube.com/watch?v=i3_n3Ibfn1c How does the eye allow vision? 1. Light Enters the Eye 2. Lens Focuses Light (accomodation) 3. Retina creates an electrochemical signal Light enters the eye through the Cornea, is refracted and then passes through the Aqueous Humor and Iris/Pupil The Lens ‘accommodates’ to focus the light/image on the retina When light/image hits the retina specific cells are activated (rods and cones). When these cells are stimulated they create an electrochemical signals that are sent via the optic nerve to the brain (occipital lobe) How does the eye work? Step 1: Light enters the eye But first, some physics revision… Refraction: • Refraction is the bending of light. It is due to a change in its speed of travel, because of a change in the density of the medium it is travelling through. • If it enters a medium where it travels more slowly (such as entering water, from air) then it bends towards the normal. How does the eye work? • When light is passed through a biconvex lens (rounded on both sides), the rays are refracted towards a center point known as the focal point • The rays then diverge (spread out) and cross over from that point • This is the basic principle for how the lens in the eye works Refractive Media ● To be refractive a medium must be transparent. ● The eye has four refractive media: the cornea, the aqueous humor, the lens and the vitreous humor. ● Of these, most refraction occurs when light enters the cornea. ○ ○ This is because this is where the greatest difference in speed of travel occurs. Because the cornea, aqueous humor, lens and vitreous humor are all similar in the speed at which they let light travel, there is little refraction between them. ● This information is particularly important when considering vision disorder treatments How Vision Works Step 2: Lens focuses light (accomodation) https://www.youtube.com/watch?v=o0DYP-u1rNM Accomodation ● Accommodation is the term used to describe the focusing of objects at different distances by changing the shape (convexity) of the lens ○ The result of this is to change the refractive power ● This change in shape is brought about by the ciliary muscles which affect the tension of the suspensory ligaments that hold the lens Accomodation in the Eye Activity: Draw a table to compare the shape of the lens; action of the ciliary muscles and refractive power for distant and nearby objects Accomodation in the Eye Distant Objects (distant vision) Close Objects (near vision) Lens shape Flat More curved or rounded (thinker/ fatter) Ciliary Muscles Relaxed Contracted/tight Refractive Power Low High How does the eye work? Step 3: Retina creates an electrochemical signal More physics! • Humans vision is able to see anything within the visible light part of the EM spectrum • Objects absorb some wavelengths and reflect others • White light is a mixture of all the colours in the EM spectrum • Object appear coloured because of the wavelengths they reflect The Retina ● The retina is the innermost layer of the eye and is a thin sheet of cells about 0.1mm thick ● The retina is made up of several layers, one of which contains the visual receptors ● These cells are called photoreceptor cells and are special types of neurons that are sensitive to light. ○ In a human eye there are two types of photoreceptors: ■ rods which contain the light sensitive pigment rhodopsin ■ cones which contain the light sensitive pigment photopsin The Retina The Retina ● Photoreceptor cells contain light sensitive pigments. ● These cells convert light into electrochemical signals that the brain can interpret. ● An electrochemical signal consists of a wave of sodium and potassium ions which move across the cell membrane of the neuron Rods ● Rods are responsible for vision in low light ● Rods have only one type of pigment (rhodopsin), thus they are not sensitive to different colours ● There are approximately 90 million rods in each retina, concentrated in the outer edges ● Vision involving rhodopsin allows humans to see in shapes of black, white and grey Cones ● Cone cells are responsible for colour vision and contain pigments of iodopsins ● There are approximate 6-7 million cones in the retina and these are concentrated in and near the fovea ● There are three types of cones found in the human eye which are all sensitive to different wavelengths of the visible spectrum. ○ Blue region: short wavelengths peak at 455nm ○ Green region: medium wavelengths, peak at 530nm ○ Red region: long wavelengths, peak at 625nm The Retina • The sensitivity of cone cells allow them to detect light on either side of the peak sensitivities, giving an overlap in some of the colours detected • Thus, a particular wavelength may stimulate more than one cone • By comparing the rate at which various receptors respond, as well as the overlap in colours, the brain is able to interpret these signals as intermediate colours, thus we can see a variety of colours Putting it all together Activity: Complete Blitzing Biology 21.1 ‘Structure and function of the human eye’
