# The light. Light (normally spreads out straight and into all directions. Refraction of light

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1 The light Light (normally spreads out straight and into all directions. Refraction of light But when a light ray passes from air into glas or water (or another transparent medium), it gets refracted (it looks like a broken stick): When the light ray comes out of the glass it gets refracted again. light ray normal normal light ray θ 1 air θ 1 air glas θ 2 θ 2 glas θ 3 The same happens to a single light ray passing through a lens: And accordingly many light rays passing as a parallel light beam through the lens:

2 Lenses Optical lens: If you cut the lens in half, from the side it looks like this: Convex (positive) lenses Convex lenses are thick in the center and thin at the ends. Positive lenses make things look larger. Optical power: +0.5; +1.0; +1.5;... ; +6 diopters Concave (negative) lenses Concave lenses are thin in the center and thick outside. Negative lenses make things look smaller. Optical power: -0.5; -1.0; -1.5;... ; -6 diopters lenses without optical power Optical power: 0.0 diopters With these lenses nothing changes. Addition: calculating with the optical power of lenses You can combine lenses to get different diopters: + = + = + = + = +2 + (-2) = (+2) = +2+2 = (+1) = -1

3 Light and Lenses When parallel sun rays pass through a positive lens, they meet all in one spot, called focal point F. The light is converged. In this point a paper would start to burn. The distance between lens and focal point is called focal lenght f. A beam of light passing through a negative lens is diverged (spread). The beam after passing through the lens, seems to come from a (not really existing) focal point in front of the lens. focal point F focal point F axis f f Positive lenses bring the light rays closer together. Negative lenses spread the light rays away from the axis.

4 The eye Ciliary Muscle Vitreous body Retina The ciliary muscle can change the thickness of the lens and thereby the refractive power. Lens Pupil Iris Choroid Sclera Fovea This is called accommodation. ciliary muscle zonular fibers Cornea Anterior Chamber Zonular fibers 24 mm Optic nerve The ciliary muscle is contracted, the zonula fibers are loose (thick lens, for near vision). How we can see a candle We consider only the top point of the flame: From here light rays go straight in all directions. Some of the light rays shine into the eye. They are refracted by the lens and then meet again in one image point on the retina. The ciliary muscle is decontracted, the zonula fibers are tight (thin lens the eye is relaxed, for far vision). The same thing happens to all other points of the candle. The image of the candle occurs on the retina. This image is upside down and reversed. Distance vision The light rays coming from remote objects are almost parallel. The lens of the eye is flat. The ciliary muscle is decontracted. flat lens Close-up vision In order to focus the light rays of a near object, the lens of the eye must accommodate strong (get round) to refract strong. round lens

5 Defective Vision - Myopia / Hyperopia Normally, the eyeball has a length of about 24 mm. When seeing far, almost parallel light rays fall on the lens, are refracted and focused on the retina. Light rays almost parallel: Myopia - shortsightedness The eyeball is too long. So the lens is not able to focus far objects on the retina but in front of it. Those people only see close things in focus. Image is focused in front of the retina. The eye is too long Image focused on retina. Shortsighted people need a negative lens for correction. Hyperopia - farsightedness The eyeball is too short. So the lens can t focus on near objects (the lens can t get round enough to focus the object on the retina.) The eye is too short In extreme cases those people even can t focus on far objects too! Farsighted people need a positive lens for correction.

6 Defective Vision - Presbyopia Presbyopia With increasing age, the lens and the ciliary muscles loose their ability to focus on nearby objects. The problem is the lens. Presbyopia usually starts between the age of 40 and 50 years. Little babys can focus at 5 cm in front of their eyes (this equals 20 diopters). A 25 year old adult can still focus at 10 cm (or 10 diopters). A person at age 60 or older can only see clearly at a minimum distance of about 1 meter (this equals 1 diopter). (The word presbyopia comes from the Greek word presbys, meaning old man ) The diopter - refractive power of the lens When the eye focusses from far to a near object, it must change its refractive power (= optical power). Looking at a near object, the ciliary muscle is contracted. The refractive power P of the eye is equal to the reciprocal of the focal length f (distance to the eye): f = 0.25m Refractive power of the lens: P = 1 / 0.25m = +4.0 diopters P = 1/f (in 1/m or diopter) Looking at high distance, the ciliary muscle is decontracted. Parallel light rays are focussed on the retina. The refractive power of the eye is 0 diopters. f = 100m P = 1/100m = 0.01 diopter = approx. 0 diopter Glasses of +3.0 for an old-aged person with accommodation of only +1.0 adds up to This means without glasses he/she can focus only on 1 meter, with glasses he/she can focus at 25 cm (1/4 = 0.25). With a lens of +2.0 you can look at an object at 0.5 m distance with your eye relaxed as if the object would be very far away.

7 Defective Vision - Astigmatism Besides myopia, hyperopia and presbyopia, there can be another common refractive error too. Astigmatism People suffering from myopia, hyperopia and presbyopia can be helped by normal spheric glasses. (Spheric means, the lenses are shaped like a part of a sphere). People suffering from astigmatism have a cornea which is deformed cylindrically. Normally, light coming from a round circle falling through a positive spheric lens, is focussed in a single point. sphere cylinder point The same light falling through a cylindric lens is focussed on a line (which can be vertical, horizontal or at any other angle). 2 cylindric diopters are as disturbing as 1 normal spheric diopter. line At night, people with astigmatism don t see the stars like points but like ellipses. Stars: seen with normal eye Stars: seen with astigmatism There are +cylinders and -cylinders. Such a cylindric deformation must be compensated with the opposite cylinder in the glasses. The positive cylinder of the eye can be compensated by a negative cylinder in the glasses (and vice versa). People with a strong astigmatism must be sent to the professional optician. -cylinder + cylinder

8 The glasses - technical terms lens notches shrink tubing bridge temple loop Measures of the lens rim 45mm bead 28mm u-wire spiral hinge for folding temple the size of the bridge bridge b Small Medium large size of bridge Important : Measure the size of the bridge inside between the wire! 13mm 19mm 24mm 54<DP< <DP<66 66<DP<71 PD: Pupil Distance The right inclination of the temples OK! NOT OK! : 2 = : 3 = 7.5

9 Pupil distance R PD (pupil distance) L Typical pupil distances: Small: 57.5 mm Medium: 63.5 mm Adults large: 68.5 mm (yellow point frame) (red point frame) (blue point frame) Important: Both eyes must look through the center of the lenses! If the glasses are too small or too big, the eyes are forced to look angular. The patient gets a headache. How to measure the PD Glasses are too small! Glasses are too small! R patient (1) L You (the optician) look face to face to the patient. You stand near in front of him. You close your left eye. The patient looks with both eyes to your right eye. With your right eye you look to the left of the patient. G optician R Here, the left eye is in the small area (S). R patient (1) L Don t move your heads now! Now the patient looks to your left eye. In this example the right eye is in the small area (S) too. This patient needs a small (yellow point) frame. L optician R patient (2) Here, the pupil is in the medium (M) area. This patient needs a medium (red point) frame. patient (3) Here, the pupil is in the medium (L) area. This patient needs a large (blue point) frame.

10 Pupil distance - test glasses Take a lens of 0 diopters, a needle and a ruler. Scratch a horizontal line through the middle of the lens. horzontal line Now scratch a mark 21 mm from the left side into the lens. mark 21 Now scratch a second mark 24 mm from the left side into the lens. mark 24 Now scratch two vertical lines into the lens. Write S, M and L on the lens and do all the same with the left lens. Now take a medium (red) frame and adapt the bridge, so that the distance between S left and S right amounts to exactly 60.5 mm. 60.5mm change the bridge

12 Testing the eye (2) Start with one eye Now cover the left eye with the opaque lens. If he has a problem to read the row 4, 5, 6 or 7 then start testing with -0.5 (if he has a problem with the first rows, start with -1.0 or even -2.0). Retinoscopy bar 1. Case: He can see better Continue with -1.0, -1.5, -2.0 etc. If two negative diopters are the same, take always the lower one! (Example: -2.5 and -3.0 are the same => take -2.5) not good not good good good 2. Case: He can see worse Continue with +0.5, +1.0, +1.5 etc. When the patient says it is very good, then continue measuring! (Example: +2.0 is very good, but you continue with +2.5, +3.0 etc.) Continue until the patient says now it is worse. Then the lens before was the best. (Example +2.0 is very good, +2.5 is very good, +3.0 is very good, but +3.5 is worse. So you take +3.0)! good good good not good 3. Case: -0.5, -1.0, -1.5 etc. is not better and +0.5, +1.0, +1.5 etc. is not better Now try the pinhole occluder. => If the patient can see better, than try again your lenses, especially the strong ones (-6.0/+6.0) If you can t help, send him to an optician with professional equipment. (He needs either very strong glasses or cylindric glasses because of astigmatism.) pinhole occluder => If the patient can t see better with the pinhole occluder, than most probably no optician can help him. Send him to an ophthalmologist (eye doctor). (The patient probably has a cataract, diabetic retinopathy, glaucoma etc.)

13 Testing the eye (3) Reading Glasses First you have tested the eyes of the patient at the long distance with the optical chart. If he needs reading glasses, you look at this table: Now add the value to the result of the far distance test. age age age 60 (or older) +3.0 Table: Age and diopters Example I: A 48(+1.5)-years-old woman needs for far distances L -2.5 and R -3.0 You give her Left ( ) = -1.0 Right ( ) = -1.5 Example II: A 70(+3.0)-years-old man needs for far distances L +1.5 and R 0.0 You give him Left ( ) = +4.5 Right ( ) = +3.0 Now put both lenses into a frame and let the patient try to read. Important: The patient should read in a relaxed position, the book on his knees or a little bit above his knees (this is normally a distance of cm). => If he can read it s OK. => If he can t see clearly, increase the power on both eyes by +0.5 (or even +1.0), then check again. Measure the PD (Pupil Distance) When you have found the right lenses, then measure the PD. Then adapt the length of the temples to the face/ears of the patient. At the end write the data in your book Prescription L... / R..., PD (Pupil Distance) Which frames you have sold (simple ones / folding temples), selling prize, signature of patient

14 Testing the eye (4) Common Problems Big difference between left and right eye If the difference between the left lens and the right lens is 2 diopters or more (e.g. left eye -0.5, right eye -3.0), the patient should test the glasses for some hours. If he gets problems like a headache, then change the stronger lens in order to decrease the difference (e.g. take -2.0 instead of -3.0). G D High difference between left and right eye. G D Reduced lens power. Some weeks later he can try the stronger lens again. Patient has problems with high diopters If a patient has glasses with high diopters for the first time in his life (e.g. L-4.5 and R-4.0) it might happen that he has problems with it. In this case you could try to correct him first with a reduced power (e.g. L-2.5 and R-2.0). Then you wait some weeks and then try the regular power again. G D These lenses cause problems. G D Reduced lens power. G D Some weecks later you try the regular power again.

15 Test (1) 1. What happens with the light ray? Complete the drawing.?? 2. What happens with the light beam? Complete the drawing. Draw in the focal point F and the focal lenght f.?? 3. What is the difference between a convex and a concave lens? How do they change objects, when looking through? 4. You have a convex lens but you don t know the power of the lens. What can you do? (You can use negative lenses from -0.5 to -6.0 diopters) 5. Draw the picture of an eye and explain its parts. 6. How does the accommodation of the eye work? Draw an eye a) reading and b) looking into the distance (with ziliary muscle, lens and zonular fibers) 7. How does the image of the candle occur on the retina? Complete the drawing. 8. What is Myopia and how can you correct it? 9. What is Hyperopia and how can you correct it? 10. What is Presbyopia and how can you correct it? 11. You look at an object which is 50 cm in front of you eye. What is the refractive power of your eye now?

16 Test (2) 12. How do people with astigmatism see the stars? 13. Describe the different parts of the glasses What are the measures of the lens? 4? ? What are the different sizes of the bridges? Small Medium Large??? 16. How many degrees is the inclination of the temples? 17. How do you measure the pupil distance of a patient? 18. Describe the different steps of testing the eyes. 19. A 65-year-old woman wants to have reading glasses. For looking into the distance she needs L+1.0 and R+1.5. What kind of reading glasses would you give her? 20. A 50 years old man gets glasses for the first time in his life. He needs L +4.0 and R+4.5. He complains about headache after he has tested the glasses for half an hour. What can you do? 21. A young man needs glasses for looking into the distance. For his left eye -3.0 is as good as What lens will you give him? 22. A young girl needs glasses for looking into the distance. For her right eye +3.0 is as good as +3.5 but +4.0 is worse. What lens will you give her?

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