Announcements 11/2/12 Prayer Slinkies! (Seth, Ryan, William B, Clement) Progress Report due a week from Saturday Pearls Before Swine.

Announcements 11/2/12 Prayer Slinkies! (Seth, Ryan, William B, Clement) Progress Report due a week from Saturday

PearlsBeforeSwine

From warmup

Extra time on?a.(nothing in particular)

Other comments?a.Do astronomers have to worry about

the fact that air doesn't have an n of exactly 1?

Clicker question: 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:

From warmup

The mirror eqn and the thin lens eqn look identical. How exactly are they different?

a.The sign conventions are different, a positive focus means a focus that is "behind" the lens. The same is true of q.

b.(from my answer) Also--how f is calculated is different. For mirrors, it's simply related to the radius of curvature, but for lenses it relates to the curvature as well as the index of refraction of the lens glass.

Clicker quiz

Just out of curiosity… have I ever used your warmup answer?

a.yesb.no

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

“Lensmaker’s Eqn”

1 2

1 1 11n

f R R

Lens-makers’ eqn:

From warmup When you look straight

down at an object under water, will it appear to be closer to you or farther away than actuality? Why?

a. Because you are looking at a flat refracting surface the image of the object forms closer to the surface than the actual object is (figure 36.18). This means the object appears closer to you than it actually is.

Clicker Quiz Is that a real or a

virtual image?a. realb. virtual

Thick lenses and surfaces

1 2 2 1n n n n

p q R

Careful with signs!!!

(see table 36.2)

Clicker question: Can a converging lens ever produce a

virtual image?a. Yesb. No

Clicker 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%

Clicker 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