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Topic 4: Lenses and Vision

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1 Topic 4: Lenses and Vision

2 Essential Learning Outcome
I can explain the use of concave and convex lenses in objects such as microscopes, eye glasses, cameras, binoculars and telescopes.

3 Types of Lenses Lens – a curved piece of transparent material such as glass or plastic Concave lens - thinner and flatter in the middle than around the edges causes light to spread out, or diverge Convex lens - thicker in the middle than around the edges causes light to come together, or converge sometimes cause images to be inverted (upside-down)

4 A double concave lens is thinner and flatter in the middle than the edges. 
Light passing through the thicker more curved areas of the lens will bend more than light passing through the thinner areas, causing the light to spread out or diverge.

5 A double convex lens is thicker in the middle than around the edges. 
This causes the light to come together at a focal point, or converge.

6 Lenses and Mirrors Lenses are useful optical devices.  Eyeglasses, have been made from lenses since the thirteenth century.  A convex lens refracts the light rays from an object so they can be focused.  Different size lenses can converge the light rays at different distances, enabling corrections to be made to focal points.

7 Image Projection light from the left portion of the object is directed to the right and the light from the top is directed to the bottom.  This inverts the image.  Overhead projectors and film projectors do this

8 http://www. sumanasinc

9 Human Eye The lens in the human eye is a convex lens. It takes light rays and focuses them on the retina, which allows us to see images. Far-sighted vision (short eye) image falls behind retina trouble seeing near objects corrected with convex lens Near-sighted vision (long eye) image falls short of retina trouble seeing distant objects corrected with concave lens

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11 Nearsighted eye cannot focus on objects far away (further than the far point < infinity) If their eye is too long, the image forms in front of the retina - this is a condition called Myopic, or near-sightedness

12 Correcting Nearsightedness
Nearsighted vision can be corrected by using a diverging lens ( making the light less convergent).

13 Farsighted eye cannot focus on an object at the near point of a normal eye. If their eye is too short, the image forms behind the retina, making object that are close to them difficult to see.  This condition is called far-sightedness.

14 Correcting Farsightedness
The light is made to converge more by using a converging lens.

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16 http://webphysics. davidson

17 Image focuses ____ the retina Light must _____ to reach the retina
Short eyeball Long eyeball ?-sighted Image focuses ____ the retina Light must _____ to reach the retina Eyeglass lens used? Far-sighted Near-sighted In front behind Converge Diverge Convex Concave

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19 Eye The human eye controls the amount of light using:
the iris functions like a diaphragm in dim light, the iris increases the size of the eye’s opening (the pupil) Iris reflex – the natural adjustment in the size of the pupil The eye is focused by changing the flatness of the lens, in a process called accommodation.

20 Comparing the Eye and the Camera
A camera controls the amount of light using: diaphragm controls the aperture (the opening) of a lens or optical system a shutter limits the passage of light The human eye controls the amount of light using: iris functions like a diaphragm in dim light, the iris increases the size of the eye’s opening (the pupil) Iris reflex – the natural adjustment in the size of the pupil

21 Camera Lens refracts light
Film (or light detector) near focal point of the lens Focusing by movement of the lens detector distance. Aperture changes the light intensity level of the image.

22 Similarities Function Human Eye Camera
Controls amount of light entering Iris Shutter Opens to the inside Pupil Aperture Opens and closes to let in light Eyelid Diaphragm Focus the image Cornea & lens Lens Light sensitive screen that retains the image Retina Film

23 Differences Eye Camera Flexible eyeball Filled with fluids
Rigid camera body Light proof box Flexible lens Controlled by muscles Solid lens Not flexible Rods & Cones Specialized cells which send images to brain Silver compound in film Retains image

24 Seeing the Image Cells in the retina detect light and send electrical impulses to the brain through the optic nerve. The point where the optic nerve enters the retina is called the blind spot.

25 lens - a curved piece of transparent material; light refracts as it passes through a lens, causing the light to bend concave lens - a lens that is thinner and flatter in the middle than around the edges; causes rays to spread out or diverge convex lens -  a lens that is thicker in the middle than around the edges; causes rays to come together or converge retina - light sensitive area at the back of the eye; the images you see are formed on the retina near-sighted - have trouble seeing distant objects; the eyes are too long causing the image to form in front of the retina far-sighted - have trouble seeing near objects; the eyes are too short, causing the image to be formed behind the retina accommodation - changing the shape of the lens to adjust for different distances of objects near point (of the eye) - the shorter distance at which an object is in focus far point (of the eye) -  the longest distance at which the object is in focus diaphragm - the device that controls the aperture of a camera aperture - the opening of camera  shutter - limits the passage of light into the camera iris - The colored part of the eye which helps regulate the amount of light entering the eye; it works like the diaphragm of a camera pupil - in the eye, the colored ring that adjusts size for the amount of light available; like the aperture in the camera iris reflex - the natural adjustment in the size of the pupils optic nerve - the nerve that connects the eye to the brain blind spot - the point where the optic nerve enters the retina; there are no light sensing cells at this point

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