textbook sections 27-1 – 27-3

Ciliary Muscles Physics 1161: Lecture 23 Lenses and your EYE • textbook sections 27-1 – 27-3

Parts of the Eye

F Review of LensesPreflight 23.11 Focal point determined by geometry and Snell’s Law: n1 sin(q1) = n2 sin(q2) n1<n2 P.A. 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!

F Review of LensesPreflight 23.11 Focal point determined by geometry and Snell’s Law: n1 sin(q1) = n2 sin(q2) n1<n2 P.A. 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!

Preflight 23.7 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?

Preflight 23.7 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.

Preflight 23.7 Still see entire image (but dimmer)!

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

Ciliary Muscles Amazing Eye 10 million 6,500 /mm2 • One of first organs to develop. • 100 million Receptors • 200,000 /mm2 • Sensitive to single photons! • http://hyperphysics.phy-astr.gsu.edu/hbase/vision/retina.html#c2 Digital Camera

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

Ciliary Muscles Which part of the eye does most of the light bending? 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.

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? Preflight 23.9 1) 13 cm 2) 26 cm 3) 52 cm 32% 56% 12%

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 Preflight 23.9 1) 13 cm 2) 26 cm 3) 52 cm Image from mirror becomes object for eye!

Multiple Lenses Image from lens 1 becomes object for lens 2 1 2 Example f1 f2 Complete the Rays to locate the final image.

Multiple Lenses Image from lens 1 becomes object for lens 2 1 2 Example f1 f2

Net magnification: mnet = m1 m2 Multiple Lenses: Magnification 1 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

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)

Preflight 23.10 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%

Farsighted person’s glasses are converging – like magnifying glass! Preflight 23.10 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?

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

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

magnifying glass virtual image object hi ho do di Compare to unaided eye: : 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 angularmagnification M:

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

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

textbook sections 27-1 – 27-3