Announcements 11/4/11 Prayer Slinkies! HW 28-4 part (d) is canceled Exam update Progress Report due a week from Saturday The three demos from last time

PearlsBeforeSwine

Thought question I look at my image in a shiny round Christmas

tree ornament. Which of the following will be true?

a. The image will be upright (not inverted) and real.

b. The image will be upright and virtual.c. The image will be inverted and real.d. The image will be inverted and virtual.e. Can’t say… it will depend on where you

(the object) are relative to the focus.

Reading Quiz A converging lens should be:

a. convexb. concave

Convex/converging Lens

parallel rays converge to the focal point

fif not parallel, they converge (image formed) at a different point

at each interface, Snell’s law is

satisfied

Where will image be formed?

focusfocus

The equation:

1 1 1

p q f

Ray diagram just like one for concave mirror, except rays continue to right instead of reflecting back!

positive q = right hand side of lens q

Mp

Magnification same:

Concave/diverging Lens

parallel rays seem to be coming from the focal point

f

Diverging lenses

virtual image!The equation:

1 1 1

p q f (negative f)

Negative q: means image on same side of lens as object (virtual)

Demos

Demo: “Real image or not?” Class poll: What kind of image is this?

a. Realb. Virtual

Sign conventions: summary Assuming that the light rays are coming from the

left:quantity positive

+negative

p if on the left of the

lens/mirrorif on the right of the

lens/mirror (can only happen in a compound problem)

q if on the right of the lens (left side of mirror), means a real image

if on the left of the lens (right side of mirror), means a virtual image

f if converging: convex lens/concave mirror

if diverging: concave lens/convex mirror

M if image is right-side up if image is upside-down

Reading Quiz When you look straight down at an object

under water, it appears to be ____________ than actuality.

a. closer to youb. farther from you

Thick lenses and surfaces

1 2 2 1n n n n

p q R

1 2

1 1 11n

f R R

Lens-makers’ eqn:

Careful with signs!!! (see table 36.2)

Thought question Can a converging lens ever produce a

virtual image?a. Yesb. No

Thought question What happens if I cover up the upper half of

lens?a. Nothingb. Upper half of image disappearsc. Lower half of image disappearsd. Intensity of image goes down by 50%

Thought question Can a diverging lens ever produce a real

image?a. Yesb. No

Multiple element problems Image of one becomes object of next Worked problem: Find qfinal, Mtot

60 cm 15 cm

f = 20 cmf = -60 cm

Answers: image is 20 cm to right of -60cm lens; M = -0.67