1. Physics 1161: Lecture 19 Lenses and your EYE
textbook sections 27-1 – 27-3
Ciliary Muscles 1

2. Parts of the Eye

3. 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.
n1 = n2 => No Bending, f = infinity
Lens in water has _________ focal length!

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.
n1 = n2 => No Bending, f = infinity
Lens in water has larger focal length!

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.
48%
1. Only the lower half will show on screen
2. Only the upper half will show on screen
3. The whole object will still show on screen
40%
12%
How much of the image appears on the screen?

6. 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.

7. Preflight 19.1
Still see entire image (but dimmer)!

8. Two very thin converging lenses each with a focal length of 20 cm are are placed in contact. What is the focal length of this compound lens?
10 cm
20 cm
40 cm

9. Two very thin converging lenses each with a focal length of 20 cm are are placed in contact. What is the focal length of this compound lens?
10 cm
20 cm
40 cm

10. Amazing Eye One of first organs to develop. 100 million Receptors
200,000 /mm2
Sensitive to single photons!
Digital Camera
10 million
6,500 /mm2
Ciliary Muscles
Numbers in red are for a digital camera
Macula/fovea is super sensitive part only sees about 15 degrees

11. Which part of the eye does most of the light bending?
Ciliary Muscles
Cornea n= 1.38
Lens n = 1.4
Vitreous n = 1.33
Lens
Cornea
Retina
Cones

12. Which part of the eye does most of the light bending?
Ciliary Muscles
Cornea n = 1.38
Lens n = 1.4
Vitreous n = 1.33
Lens
Cornea
Retina
Cones
Laser eye surgery changes Cornea
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.

13. Preflight 19.3
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?
32%
1) 13 cm
2) 26 cm
3) 52 cm
56%
12%

14. 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
1) 13 cm
2) 26 cm
3) 52 cm
Image from mirror becomes object for eye!

15. Multiple Lenses Example Image from lens 1 becomes object for lens 2 1
Complete the Rays to locate the final image.

16. Multiple Lenses Example Image from lens 1 becomes object for lens 2 1

17. 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

18. 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)

19. 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 68%
Nearsighted 32%

20. 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!

21. Angular Size Preflight 19.6, 19.7
Both are same size, but nearer one looks bigger. q q q q
Angular size tells you how large the image is on your retina, and how big it appears to be.

22. Away from the film Towards the film
The focal length of the lens of a simple camera is 40 mm. In what direction must the lens be moved to “change the focus of the camera” from a person 25 m away to a person 4.0 m away? i.e. does the image distance increase or decrease?
Away from the film
Towards the film

23. Away from the film Towards the film
The focal length of the lens of a simple camera is 40 mm. In what direction must the lens be moved to change the focus of the camera from a person 25 m away to a person 4.0 m away? ? i.e. does the image distance increase or decrease?
Away from the film
Towards the film

24. Unaided Eye Bring object as close as possible (to near point N)
How big the object looks with unaided eye. q
object N h0
Bring object as close as possible (to near point N)
**If q is small and expressed in radians.

25. Magnifying Glass magnifying glass virtual image object hi ho do di
Magnifying glass produces virtual image behind object, allowing you to bring object to a closer do: and larger q’
Compare to unaided eye: :
Ratio of the two angles is the angular magnification M: