lect19 handout

Slide1: 

Today’s Lecture will cover textbook sections 23.9, 24.1, 3-4, 6 Physics 102: Lecture 19 Lenses and your EYE

Review of LensesPreflight 18.8: 

Review of Lenses Preflight 18.8 P.A. Focal point determined by geometry and Snell’s Law: n1 sin(q1) = n2 sin(q2) Fat in middle = Converging Thin in middle = Diverging Larger n2/n1 = more bending, shorter focal length. n1 = n2 => No Bending, f = infinity Lens in water has _________ focal length! n1<n2

Slide3: 

1) Rays parallel to principal axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays through F emerge parallel to principal axis. Review: Converging Lens Principal Rays F F Object P.A. Image is real, inverted and enlarged

Preflight 19.1: 

Preflight 19.1 A converging lens is used to project a real image onto a screen. A piece of black tape is then placed over the upper half of the lens. How much of the image appears on the screen?

Slide5: 

Lens Equation F F Object P.A. Image do = distance object is from lens: Positive: object __________ lens Negative: object __________ lens di = distance image is from lens: Positive: ________ image (behind lens) Negative: ________ image (in front of lens) f = focal length lens: Positive: ___________ lens Negative: ___________ lens di = m =

Amazing Eye: 

Amazing Eye One of first organs to develop. 100 million Receptors 4 million 200,000 /mm2 2,500 /mm2 Sensitive to single photon! Candle from 12 miles

ACT: Focusing and the Eye: 

ACT: Focusing and the Eye Cornea n= 1.38 Lens n = 1.4 Vitreous n = 1.33 Which part of the eye does most of the light bending? 1) Lens 2) Cornea 3) Retina 4) Cones

Eye (Relaxed): 

Eye (Relaxed) 25 mm Determine the focal length of your eye when looking at an object far away.

Eye (Tensed): 

Eye (Tensed) 25 mm Determine the focal length of your eye when looking at an object up close (25 cm). 250 mm

Preflight 19.3: 

1) 13 cm 2) 26 cm 3) 52 cm Preflight 19.3

Multiple Lenses: 

Multiple Lenses Image from lens 1 becomes object for lens 2 1 f1 f2 2 Complete the Rays!!

Multiple Lenses: Magnification: 

Multiple Lenses: Magnification f1 f2 do = 15 cm f1 = 10 cm di = 30 cm f2 = 5 cm L = 42 cm do=12 cm di = 8.6 cm 1 2

Near Point, Far Point: 

Near Point, Far Point Eye’s lens changes shape (changes f ) Object at any do can have image be at retina (di = approx. 25 mm) Can only change shape so much “Near Point” Closest do where image can be at retina Normally, ~25 cm (if far-sighted then further) “Far Point” Furthest do where image can be at retina Normally, infinity (if near-sighted then closer)

Slide14: 

If you are nearsighted... Want to have (virtual) image of distant object, do = , at the far point, di = -dfar. (far point is too close) flens =

Refractive Power of Lens: 

Refractive Power of Lens Diopter = 1/f where f is focal length of lens in meters. Person with far point of 5 meters, would need contacts with focal length –5 meters. Doctor’s prescription reads: 1/(-5m) = –0.20 Diopters

Slide16: 

If you are farsighted... When object is at do, lens must create an (virtual) image at -dnear. Want the near point to be at do. (near point is too far) flens =

Preflight 19.4: 

Preflight 19.4 Two people who wear glasses are camping. One of them is nearsighted and the other is farsighted. Which person’s glasses will be useful in starting a fire with the sun’s rays?

Angular SizePreflight 19.6, 19.7: 

Angular Size Preflight 19.6, 19.7 Angular size tells you how large the image is on your retina, and how big it appears to be. How small of font can you read? Highwire Caramel Apples Rabbits Kindergarten Hello Arboretum Halloween Amazing Both are same size, but nearer one looks bigger.

Unaided Eye: 

Unaided Eye Bring object as close as possible (to near point N) How big the object looks with unaided eye. **If q is small and expressed in radians.

Magnifying Glass: 

Magnifying glass produces virtual image behind object, allowing you to bring object to a closer do: and larger q’ Ratio of the two angles is the angular magnification M: Magnifying Glass

Angular Magnification M=N/do: 

Angular Magnification M=N/do M = di For max. magnification, need image at N, so set di = -N:

See you next class!: 

See you next class! Read Sections 25.1, 3-4