Gr.12 Life Sciences The eye

The Human Eye

Others see the opposite. Most people see the red, Closer than the blue. Others see the opposite. How about you?

The Human Eye Your eyes are about to get a workout. Have you stretched your eye muscles yet? No? Then do that now!

Are you seeing spots?

Look at the cross for 10 seconds. What do you see?

Are these lines bent….?

…or straight?

What shapes do you see?

What is wrong with this sentence? Reading Aoccdrnig to rscheearch at Cmabrigde Uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit a porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.

The Eye The Structure and function Cornea: Responsible for the refraction (bending) of light rays from the object, to create the image on the retina Pupil: A circular opening in the iris which allows light into the eye The Structure and function Optic nerve: Carries nerve impulses from the retina to the brain Retina: Contains the light-sensitive receptor cells, i.e. the rods and cones Iris: The coloured part of the eye Sclera: The tough white outer coat, which protects the eye against damage Lens: Changes shape for near and distant (far) vision Choroid: A dark coloured layer which: • Reduces reflection • Is rich in blood vessels which supply the cells of the eye with nutrients and oxygen Vitreous humour: A jelly-like substance which gives shape to the eye

The Eye Functioning of the eye The eye performs the following functions: Binocular vision Formation of the image Accommodation Pupillary reflex

Binocular Vision Refers to the ability to focus on an object with both eyes, creating a single image During binocular vision the three pairs of muscles on the outside of each eye enable the eye to see three dimensionally. The ability to perceive depth is known as stereoscopy or stereoscopic vision Depth of perception enables us to walk down/up stairs.

Accommodation Refers to the ability of the eye to change the shape (convexity) of the lens to ensure a clear image is formed on the retina whether the image is near or distant to bend the light more to focus it Fat lens needed

Pupillary Mechanism Controlling light levels Your eye are very sensitive and can be damaged by harsh light. Your iris controls light allowed into the eye by changing the size of the pupil In bright light The circular muscles of the iris contract The radial muscles relax The pupil constricts The amount of light entering the eye is reduced Sclera Pupil Constricts (becomes Smaller) Radial muscles of Iris relax Circular muscles of iris contract

The radial muscles of the iris contracts The circular muscles relax Pupillary Mechanism In dim light The radial muscles of the iris contracts The circular muscles relax The pupil dilates The amount of light entering the eye is increased. Sclera Circular muscles of iris relax Pupil widens Radial muscles of of iris contract

Hypermetropia (long sightedness) Nature and treatment of visual defects Hypermetropia (long sightedness) Causes : Eyeball being too rounded Inability of the lens to become more convex common in the elderly Treatment Wear classes with convex lens

Myopia (short sightedness) Nature and treatment of visual defects Myopia (short sightedness) Causes : Eyeball being too long Inability of the lens of the eye to become less convex. Treatment Wear glasses with a concave lens.

Astigmatism Nature and treatment of visual defects The front surface of the cornea is curved more in one direction than in the other. Symptoms: Distortion or blurring of images at all distances Headache and fatigue squinting and eye discomfort and irritation Treatment Prescription glasses are required if the degree of astigmatism is great enough to cause eye strain and head ache, or distortion of vision.

Cataracts Nature and treatment of visual defects Refers to: The cloudy, opaque part of the lens not clear understanding of its causes. Treatment: Surgical removal of the lens replacing the lens with a synthetic lens.

Questions 1. Various possible options are provided as answers to the following questions. Choose the correct answer and write only the letter (A – D) next to the question number. Refer for to the diagram of the human eye below for 1.1 to 1.3 : 1.1 The parts that control the amount of light rays that enter the eye are….. A 1 and 2 B 1 and 3 C 2 and 3 D 3 and 4

Questions 1.2 The greatest convergence takes place when light rays pass through the part numbered….. A 4 B 5 C 6 D 11 13 The function of part numbered 9 is to: 1. reflect light 2. change the shape of the eyeball 3. refract light rays 4. prevent reflection 5. supply the retina with nutrients and oxygen 6. absorb excess light Choose the correct options for the function of part no.9 A 1, 2, 3 B 4, 5, 6 C 2, 4, 6 D 1, 3, 5

Questions 1.4 The following are part of the human eye: 1 cornea 2 lens 3 retina 4 iris 5 choroid Which parts are respectively concerned with/in: • Adjusting the focus? • Preventing internal reflection? • Recording changes in light intensity? A 2, 4 and 3 B 3, 4 and 1 C 2, 5 and 4 D 4, 1 and 5

Answers 1.1. C √ √ 1.2. C √ √ 1.3. B √ √ 1.4. C √ √

Questions 1.5 The diagram alongside that represents a section through part of the human eye. 1.5.1. Supply labels for the parts numbered 1, 3 and 5. (3) 1.5.2. Supply the number and name the part that controls the amount of light that enters the eye. (2) 1.5.3 Mention the changes that the part names in Question 3.5.2 will undergo when exposed to bright light. (4) 1.5.4. List one function for each of parts 4 and 7. (2)

1.5.5 Supply the number and the name of the part that is responsible for the accommodation of the eye. (2) 1.5.6. Explain the changes that will take place when the part named in Question 3.5.5 views an object closer that 6m.

1.5.1. 1 = cornea√ 3= pupil √ 5 = suspensory ligaments √ 1.5.2. Number 2 , the iris √ 1.5. 3. The radial muscles of the iris relax the circular muscles contract this will cause the pupil to constrict √ allowing less light to enter into the eye √ 1.5.4. Part 4 = the lens focusses the light rays onto the retina at the back of the eye √ . Part 7 = the vitreous body / vitreous humour is part of the refracting medium of the eye and helps to maintain the shape of the eyeball. √ 1.5.5 Number 6 = the ciliary muscles in the ciliary body √ √ 1.5.6. • The ciliary muscles contract √ • Releasing the tension on the suspensory ligaments so they become relaxed √ • The tension on the elastic lens becomes less √ • The lens becomes rounder/more convex √ • This increases the refraction of light through the lens so that the image focusses on the retina √ (18)

Practical work