Chapter 15.1 Announcements:

Homework 15.1: due Tuesday, April 27, in class (Nguyen)

Exercises: 1, 2, 3, 5, 7, 8, 10, 11, 12, 13, 15

Problems: 1, 2, 4, 5

- All grades will continue to be posted at: http://www.wfu.edu/~gutholdm/Physics110/phy110.htm

- Listed by last four digits of student ID

We’ll now cover only parts of each chapter (let me know if you want me to cover something that is not on the list and that interests you):

- 5.1 Balloons

- 7.1 Woodstoves

- 9.1 Clocks

- 9.2 Musical Instruments

- 10.3 Flashlights

- 11. Household Magnets & Electric Motor

- 11.2 Electric Power Distribution

- 15.1. Optics, Cameras, Lenses

- 16.1 Nuclear Weapons

Final exam (cumulative): May 4, 2-5 pmPractices test will be posted

Chapter 15.1: Light, optics, cameras, lenses

- light and lenses- camera- eye

- Light- lenses bend light- focal point- cameras - object distance- image distance - focal length- lens equation!- film - aperture- depth of focus

Demos and Objects Concepts

Light from an Object

• An illuminated object reflects or scatters light (sends out light)

• You see object via this reflected or scattered light

• The object’s light forms diffuse illumination

• You can’t tell what object looks like from this diffuse illumination

… but we see object

Lenses Types of lenses:

• Most important simple optical device

• Lenses bend light

• Lenses can form images of objects

• Used in glasses, cameras, telescopes, binoculars, microscopes, …

• Converging lenses: thicker in the middle than the outside

• Diverging lenses: Thinner in the middle than the outside

Parallel rays incident on converging lens

• Light rays get refracted by a lens, that is: light gets bent by a lens.

• If the rays come in parallel to the principal axis (object at infinity), they will be focused in the focal point.

• focal length f

• focal length is the same on both sides, even if lens is not symmetric. Parallel rays coming in,

focus on the focal plane

Converging lens

Parallel rays incident on converging and diverging lenses

• Any lens that is thicker in the center than at the edges will make parallel rays converge to a point and is called a converging lens.

• Any lenses that are thinner in the center are called diverging lenses, because they make parallel rays diverge.

• Focal point of diverging lens: Point were diverging rays seem to be coming from.

Converging Lenses

• A converging lens uses refraction to bend light rays• Light rays converge after passing a converging lens• Rays from a common point on an object converge to

a common point on far side of the converging lens

Real Images• An image forms in space on far side of lens

• The image is a pattern of light in space that exactly resembles the object, except for size and orientation

• The image is “real” – you can put your hand in it and you can capture it on a screen (eye, camera).

i-clicker-1

A magnifying glass held in sunlight forms a round spot that can burn paper. What spot(s) will form if you hold that lens in light from the lamps overhead?

A. A round spot

B. A square spot

C. Spots that look exactly like the overhead lamps

D. Round spots arranged like the overhead lamps

Lenses and FilmEyes and Retina

• Film records the pattern of light it’s exposed to

• If you put film in a real image, it will record a pattern of light resembling the object

• For a good photograph, the real image should be sharply focused on the film and have the right size

Film or retina

Capturing image and leading up to lens equation

Focusing

• Light reaching the lens from an object is diverging• The nearer the object, the more its light diverges• Converging lens has trouble with diverging light

– Real image of nearby object forms farther from lens

– Real image of distant object forms closer to lens

1 1 1Object distance Image distance Focal length

Capturing image and leading up to lens equation

Focal Length

• Focal length measures the lens’ converging ability– Long focal length: weak convergence, long image distance

– Short focal length: strong convergence, short image distance

• The larger the object distance, the bigger the image– Long focal length: big images

– Short focal length: small images

Capturing image and leading up to lens equation

Ray tracing for converging lens to find the image created by the lens

(a) Ray 1 leaves top point on object going parallel to the axis, then goes through focal point.

(b) Ray 2 passes through F’, therefore it is parallel to the axis beyond the lens

(c) Ray 3 passes straight through the center of the lens.

o i

f

ho

hi

fio

111

The lens equationCapturing image and lens equation

fio

111The lens equation:

1. Draw a ray diagram

2. Solve for unknowns in the lens equation and magnification. Remember reciprocals

3. The height of the image, hi is positive if the image is upright, and negative if the image is inverted relative to the object (ho is always positive).

Magnification:

o

i

h

hm

o

i

Lens equation and magnification

i-clicker-2; -3Image formed by a converging lens.

You place a 7.6 cm high flower 1.00 m from the lens of a camera (focal length 5.0 cm).

1. What is the position (image distance) if the image

2. What is the size of the image on the film of the camera.

o = 100 cm i = ?

ho =

7.6 cm

hi

f = 5 cm

1A. 5.26 cm1B. 6.25 cm1C 7.00 cm1D. 7.60 cm1E. 10.00 cm

1A. 1 mm1B. 2 mm1C 3 mm1D. 4 mm1E. 5 mm

Lens Diameter and focusing

• Larger lens– converges more light

– brighter image

– focus becomes more critical

– less depth of focus

• Smaller lens– dimmer image

– focus becomes less critical

– more depth of focus

i-clicker-4

• A disposable film camera has no focus adjustment. To produce sharp photographs that are properly exposed, it needs a

A. large diameter lens and low sensitivity film.

B. small diameter lens and low sensitivity film.

C. large diameter lens and high sensitivity film.

D. small diameter lens and high sensitivity film.

Aperture and f-number

• Aperture characterizes the diameter of the lens• F-number is lens focal length (say 35 mm) divided by

lens diameter (adjusted by aperture). • Large f-number (22 or so on lens).

– Dim image– Large depth of field/focus (focus is forgiving)

• Small f-number (3.5 or so on lens).– Bright image– Small depth of field/focus (focus is critical)

Clear focal plane, short focal depth, focus is crisp

No clear focal plane, very long focal depth, focus is not crisp

B

Which photograph was taken with a larger aperture? i-clicker-5

A