Topic 4: Lenses and VisionTopic 4: Lenses and Vision

Essential Learning OutcomeEssential Learning Outcome

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

Types of LensesTypes of Lenses

LensLens – – a curved piece of a curved piece of transparent material such transparent material such as glass or plasticas glass or plastic

Concave lensConcave lens - - thinner and thinner and flatter in the middle than flatter in the middle than around the edges causes around the edges causes light to spread out, or light to spread out, or divergediverge

Convex lensConvex lens - - thicker in thicker in the middle than around the the middle than around the edges causes light to come edges causes light to come togethertogether, or converge , or converge sometimes cause images to sometimes cause images to be inverted (upside-down)be inverted (upside-down)

A double concave lens A double concave lens is is thinner and flatter thinner and flatter in the middle than the edges. in the middle than the edges. 

Light passing through the thicker more curved Light passing through the thicker more curved areas of the lens will bend more than light areas of the lens will bend more than light passing through the thinner areas, causing passing through the thinner areas, causing the light to spread out or diverge. the light to spread out or diverge.

A double convex lensA double convex lens is thicker in is thicker in the middle than around the edges. the middle than around the edges. 

This causes the light to come together This causes the light to come together at a focal point, or converge. at a focal point, or converge.

Lenses and MirrorsLenses and Mirrors

Lenses are useful optical devices.  Eyeglasses, have been Lenses are useful optical devices.  Eyeglasses, have been made from lenses since the thirteenth century.  made from lenses since the thirteenth century. 

A convex lens A convex lens refracts the light rays from an object so they refracts the light rays from an object so they can be focused.  can be focused. 

Different size lenses can Different size lenses can convergeconverge the light rays at different the light rays at different distances, enabling distances, enabling correctionscorrections to be made to to be made to focal pointsfocal points. .

Image ProjectionImage Projectionlight from the left portion of the object is light from the left portion of the object is

directed to the right and the light from the directed to the right and the light from the top is directed to the bottom.  This inverts top is directed to the bottom.  This inverts the image.  Overhead projectors and film the image.  Overhead projectors and film projectors do this projectors do this

• http://www.sumanasinc.com/webconthttp://www.sumanasinc.com/webcontent/animations/content/visualpathwaent/animations/content/visualpathways.htmlys.html

• http://www.bbc.co.uk/science/humanhttp://www.bbc.co.uk/science/humanbody/body/factfiles/sight/sight_animabody/body/factfiles/sight/sight_animation.shtmltion.shtml

• http://http://iknow.net/animation.htmliknow.net/animation.html

Human EyeHuman Eye

The lens in the human eye is a The lens in the human eye is a convex convex lenslens. It takes light rays and . It takes light rays and focuses them on the focuses them on the retinaretina,, which which allows us to see images.allows us to see images.

Far-sighted visionFar-sighted vision (short eye) (short eye) • image falls behind retina image falls behind retina • trouble seeing near objectstrouble seeing near objects• corrected with convex lenscorrected with convex lens

Near-sighted visionNear-sighted vision (long eye) (long eye) • image falls short of retinaimage falls short of retina• trouble seeing distant objectstrouble seeing distant objects• corrected with concave lenscorrected with concave lens

http://micro.magnet.fsu.edu/primer/java/scienceopticsu/eyeball/index.html

Nearsighted eyeNearsighted eye cannot focus on objects cannot focus on objects far awayfar away (further than the far point < (further than the far point < infinity)infinity)

If their eye is too long, the image forms If their eye is too long, the image forms in in frontfront of the retina - this is a condition called of the retina - this is a condition called

Myopic, or near-sightednessMyopic, or near-sightedness

Correcting NearsightednessCorrecting NearsightednessNearsighted vision can be corrected by using a diverging lens ( making the light less convergent).

Farsighted eyeFarsighted eye cannot focus on an object cannot focus on an object at the near point of a normal eye.at the near point of a normal eye.

If their eye is too short, the image forms If their eye is too short, the image forms behindbehind the retina, making object that are the retina, making object that are close to them difficult to see.  close to them difficult to see. 

This condition is called This condition is called far-sightednessfar-sightedness..

Correcting FarsightednessCorrecting FarsightednessThe light is made to converge more by using aconverging lens.

http://webphysics.davidson.edu/physlet_resources/dav_optics/examples/eye_demo.html

http://health.howstuffworks.com/eye.htm

Short eyeballShort eyeball Long eyeballLong eyeball

?-sighted?-sighted

Image Image focuses ____ focuses ____ the retinathe retina

Light must Light must _____ to reach _____ to reach the retinathe retina

Eyeglass lens Eyeglass lens used?used?

Far-sighted Near-sighted

behind In front

Converge Diverge

Convex Concave

EyeEyeThe 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 changingthe flatness of the lens, in a process called accommodation.

Comparing the Eye Comparing the Eye and the Cameraand the Camera

A camera controls the A camera controls the amount of light using:amount of light using:

diaphragmdiaphragm controls the controls the apertureaperture (the opening) of (the opening) of a lens or optical systema lens or optical system

a a shuttershutter limits the passage limits the passage of lightof light

The human eye controls the The human eye controls the amount of light using:amount of light using:

• irisiris functions like a functions like a diaphragm in dim lightdiaphragm in dim light, ,

• the iris the iris increases the size increases the size of the eye’s opening (the of the eye’s opening (the pupilpupil))

Iris reflex – the natural adjustment in the size of the pupil

CameraCameraLens refracts lightFilm (or light detector) near focal point of the lensFocusing by movement of the lens detector distance.Aperture changes the light intensity level of the image.

SimilaritiesSimilaritiesFunctionFunction Human EyeHuman Eye CameraCameraControls amount Controls amount of light enteringof light entering

IrisIris ShutterShutter

Opens to the Opens to the inside inside

PupilPupil ApertureAperture

Opens and Opens and closes to let in closes to let in lightlight

EyelidEyelid DiaphragmDiaphragm

Focus the imageFocus the image Cornea & lensCornea & lens LensLens

Light sensitive Light sensitive screen that screen that retains the retains the imageimage

RetinaRetina FilmFilm

DifferencesDifferencesEyeEye CameraCamera

Flexible eyeballFlexible eyeball

•Filled with fluidsFilled with fluidsRigid camera bodyRigid camera body

•Light proof boxLight proof box

Flexible lensFlexible lens

•Controlled by musclesControlled by musclesSolid lensSolid lens

•Not flexibleNot flexible

Rods & ConesRods & Cones

•Specialized cells which Specialized cells which send images to brainsend images to brain

Silver compound in filmSilver compound in film

•Retains imageRetains image

Seeing the ImageSeeing the Image

Cells in the retina detect light and Cells in the retina detect light and send electrical impulses to the brain send electrical impulses to the brain through the through the optic nerveoptic nerve. .

The point where the optic nerve enters The point where the optic nerve enters the retina is called the the retina is called the blind spotblind spot..

lens - a curved piece of transparent material; light refracts as it passes through a lens - a curved piece of transparent material; light refracts as it passes through a lens, causing the light to bendlens, causing the light to bend

concave lens - a lens that is thinner and flatter in the middle than around the concave lens - a lens that is thinner and flatter in the middle than around the edges; causes rays to spread out or edges; causes rays to spread out or divergediverge

convex lens -  a lens that is thicker in the middle than around the edges; causes convex lens -  a lens that is thicker in the middle than around the edges; causes rays to come together orrays to come together or converge converge

retina - light sensitive area at the back of the eye; the images you see are formed retina - light sensitive area at the back of the eye; the images you see are formed on the retinaon the retina

near-sighted - have trouble seeing distant objects; the eyes are too long causing near-sighted - have trouble seeing distant objects; the eyes are too long causing the image to form in front of the retinathe image to form in front of the retina

far-sighted - have trouble seeing near objects; the eyes are too short, causing the far-sighted - have trouble seeing near objects; the eyes are too short, causing the image to be formed behind the retinaimage to be formed behind the retina

accommodation - changing the shape of the lens to adjust for different distances of accommodation - changing the shape of the lens to adjust for different distances of objectsobjects

near point (of the eye) - the shorter distance at which an object is in focusnear point (of the eye) - the shorter distance at which an object is in focusfar point (of the eye) -  the longest distance at which the object is in focusfar point (of the eye) -  the longest distance at which the object is in focusdiaphragm - the device that controls the aperture of a cameradiaphragm - the device that controls the aperture of a cameraaperture - the opening of camera  aperture - the opening of camera  shutter - limits the passage of light into the camerashutter - limits the passage of light into the camerairis - The colored part of the eye which helps regulate the amount of light entering iris - The colored part of the eye which helps regulate the amount of light entering

the eye; it works like the diaphragm of a camerathe eye; it works like the diaphragm of a camerapupil - in the eye, the colored ring that adjusts size for the amount of light available; pupil - in the eye, the colored ring that adjusts size for the amount of light available;

like the aperture in the cameralike the aperture in the camerairis reflex - the natural adjustment in the size of the pupilsiris reflex - the natural adjustment in the size of the pupils

optic nerve - the nerve that connects the eye to the brainoptic nerve - the nerve that connects the eye to the brainblind spot - the point where the optic nerve enters the retina; there are no light blind spot - the point where the optic nerve enters the retina; there are no light sensing cells at this pointsensing cells at this point

http://http://www.exploratorium.edu/learning_stuwww.exploratorium.edu/learning_studio/cow_eye/doit.htmldio/cow_eye/doit.html