1. Physics 102: Lecture 19
Lenses and your EYE
Ciliary Muscles 1

2. 3 Cases for Converging Lenses
Object
Image
Inverted
Reduced
Real
Past 2F
Image
Object
Inverted
Enlarged
Real
Between
F & 2F
Demo 71
Image
Object
Upright
Enlarged
Virtual
Inside F 40

3. Only 1 Case for Diverging Lenses
Example F
P.A.
Image
Object F
Image is always virtual, upright, and reduced. 45

4. Review of Lenses Preflight 18.8
Focal point determined by geometry and Snell’s Law: n1 sin(q1) = n2 sin(q2)
P.A.
n1<n2 F
Fat in middle = Converging
Thin in middle = Diverging
Larger n2/n1 = more bending, shorter focal length.
Smaller n2/n1 = less bending, longer focal length.
n1 = n2 => No Bending, f = infinity
Lens in water has larger focal length since n2/n1 is smaller! 5

5. 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. 8

6. Preflight 19.1 Still see entire image (but dimmer)! (37% got correct)
37%, 25%, 37%
“There are still rays from the image bouncing off the lower half of the lens that will project the image. ” 8

7. Same as mirror equation
Lens Equation
Same as mirror equation do F
P.A.
Image
Object f F
do = distance object is from lens:
Positive: object in front of lens
Negative: object behind lens di
Example
di = distance image is from lens:
Positive: real image (behind lens)
Negative: virtual image (in front of lens)
Just like for mirrors
f = focal length lens:
Positive: converging lens
Negative: diverging lens 11

8. Lens Equation Example If d0 larger, di is smaller, |m| smallerdo F
P.A.
Image
Object f F di
Example
Just like for mirrors.
Make d0 larger. Then di smaller, m smaller
Make d0 smaller. Then di larger, m larger
If d0 larger, di is smaller, |m| smaller
If d0 smaller, di is larger, |m| larger 11

9. Multiple Lenses Example Image from lens 1 becomes object for lens 2 1
Lens 1 creates a real, inverted and enlarged image of the object.
Lens 2 creates a real, inverted and reduced image of the image from lens 1.
The combination gives a real, upright, enlarged image of the object. 32

10. Multiple Lenses: Magnification
do = 15 cm 1 2 f1 f2
f1 = 10 cm
f2 = 5 cm
Example
di = 30 cm
First find image from lens 1. 35

11. Multiple Lenses: Magnification1 2
do = 15 cm
L = 42 cm
di = 8.6 cm f1 f2
f1 = 10 cm
f2 = 5 cm
Example
di = 30 cm
do=12 cm
Now find image from lens 2.
Notice that do could be negative for second lens! 38

12. Multiple Lenses: Magnification1 2
do = 15 cm
L = 42 cm
di = 8.6 cm f1 f2
f1 = 10 cm
f2 = 5 cm
Example
di = 30 cm
do=12 cm
Net magnification:
mnet = m1 m2 40

13. The Eye One of first organs to develop. ~100 million Receptors
~200,000 /mm2
Sensitive to single photon!
Candle from 12 miles
Ciliary Muscles
Numbers in red are for a digital camera
Macula/fovea is super sensitive part only sees about 15 degrees 15

14. ACT: Focusing and the Eye
Ciliary Muscles
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
Lens and cornea have similar shape, and index of refraction. Cornea has air/cornea interface 1.38/1, 70% of bending. Lens has Lens/Vitreous interface 1.4/1.33. Lens is important because it can change shape.
Laser eye surgery changes Cornea 17

15. Eye (Relaxed) Example 25 mm
Determine the focal length of your eye when looking at an object far away.
25 mm ~ 1 inch
Object is far away:
Want image at retina: 21

16. Eye (Tensed) Example 250 mm 25 mm
Determine the focal length of your eye when looking at an object up close (25 cm).
Tensed means more curvature, more bending, shorter focal length
Object is up close:
Want image at retina: 25

17. Preflight 19.3 Image from mirror becomes object for eye!
A person with normal vision (near point at 26 cm) is standing in front of a plane mirror.
What is the closest distance to the mirror where the person can stand and still see himself in focus?
26cm
13cm
51%
39%
10%
1) 13 cm
2) 26 cm
3) 52 cm
Image from mirror becomes object for eye! 28

18. Near Point, Far Point Eye’s lens changes shape (changes f )
Object at any do should 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)

19. If you are nearsighted... Example (far point is too close) do dfar
Too far for near-sighted eye to focus
dfar
Near-sighted eye can focus on this!
Contacts form virtual image at far point – becomes object for eye. do
flens = - dfar
Want to have (virtual) image of distant object, do = , at the far point, di = -dfar. 42

20. Refractive Power of Lens
Diopter = 1/f
where f is focal length of lens in meters.
Example:
My prescription reads diopters
flens = -1/6.5 = m = cm (a diverging lens)
dfar = 15.4 cm (!)
My eyes: 6.5 diopters, implies f=-0.15 m (my far point is 0.15 m, or 6”, or 15 cm)
Big # means smaller f, more bending (worse eyesight)
flens = - dfar 43

21. If you are farsighted... Example
(near point is too far)
Too close for far-sighted eye to focus
dnear
Far-sighted eye can focus on this! do
Contacts form virtual image at near point – becomes object for eye.
Example
My near point with far correction: 65 cm
Want near point at 25 cm
Need 1/f=1/.25-1/ /f=2.5 diopters
Want the near point to be at do.
When object is at do, lens must create an (virtual) image at -dnear. 45

22. Farsightedness Near point dnear > 25 cm
To correct, produce virtual image of object at d0 = 25 cm to the near point (di = dnear)
Example:
My near prescription reads +2.5 diopters
flens = +1/2.5 = 0.4 m = 40 cm
therefore dnear = 67 cm (with my far correction)

23. ACT/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?
Farsighted person’s glasses are converging – like magnifying glass!
nearsighted
farsighted
Does it help if you have a high prescription?

24. See you next Monday