Quiz 4

Physics 7C Fall 2008Lecture 5: Ray Optics

Images

Lens Types & Ray Tracing

Overview of next few weeks:

Terms in Optics

Concepts: Object Image

Real Virtual Upright Inverted

Magnification Focus Reflection Refraction

Ways to locate Image: Thin Lens Equation

Image distance Object distance Sign of lens

Ray Tracing Principal Rays

Lens Types: Converging Diverging Convex Concave

What phenomenon most completely describes what you are seeing?

a) Refraction

b) Convergence

c) Divergence

d) Total Internal Reflection

e) Snell’s Law

Light passes from water (n=1.3) into an unknown substance

water

?

At larger angles, will there be total internal reflection?

a) Yes, past the critical angleb) No, not possiblec) Not enough information

Review: Light at an Interface

Activity 8.6.3 & FNT 1

Incoming Ray

Plastic prismn = 1.5

Which Ray?(a)

(b)

(c)Air

n = 1

(d)

Review: Light at an Interface

Activity 8.6.3 & FNT 1

Incoming Ray

Plastic prismn = 1.5

On leaving the prism, which way will the light bend?a) Up b) Down c) No bend

How will light emerge from the prism below?

Incoming Rays

a) Upb) Downc) Paralleld) Bent inward (converge)e) Bent outward (diverge)

How will light emerge from the prism below?

Incoming Rays

a) Upb) Downc) Paralleld) Bent inward (converge)e) Bent outward (diverge)

How will light emerge from the prism below?

Incoming Rays

a) Upb) Downc) Paralleld) Bent inward (converge)e) Bent outward (diverge)

Parallel rays are uncommon…how do lenses work on regular objects?

Diffuse reflection: rough surface reflects light in many directions.

Still obeys law of reflection, just scatters at many different angles.

Even flat surfaces like paper are not smooth to visible light. A flashlight beam shining on paper can be seen from most angles, not just the one complimentary to the incident.

Paper

Every part of the tree reflects light, scattering it all directions.

A ray diagram is used to simplify this picture.

What type of image does Ernie make?

a) Realb) Virtualc) Uprightd) Invertede) Biggerf) Smaller

c

Focal point, f

Obj

ect

plan

e

Imag

e pl

ane

o i

Imag

e pl

ane

c

Focal point, f

Object plane

o

iImage plane

Where will the image be?How big will the image be?

Thin-lens Equation

1 1 1

o id d f+ =

Magnification:

i

o

dm

d=−

You have a 2-lens system. You know only what the information shown below:

? f=-25cm

Image created by first lens

25cm

100cm

T/F: There is enough information to locate the image produced by the second lens.

You have a 2-lens system. You know only what the information shown below:

? f=-25cm

Image created by first lens

25cm

100cm

T/F: There is enough information to determine what type of lens the first lens must be.

You have a 2-lens system. You know only what the information shown below:

? f=-25cm

Image created by first lens

25cm

100cmT/F: There is enough information to determine what type of lens the first lens must be.

True: lens must be converging--make sure you know why!Extra question: what does the original object look like?

Suppose you replace the first lens with an identically shaped lens that has a higher index of refraction.

? f=-25cm

Image created by first lens

25cm

100cmHow will the blue arrow change:

Bigger or smaller?Upright or inverted?Closer or further?

Suppose you replace the first lens with an identically shaped lens that has a higher index of refraction.

? f=-25cm

Image created by first lens

25cm

100cmHow will the blue arrow change:

Bigger or smaller?Upright or inverted?Closer or further?

Stuck? A higher index of refraction means light will bend more…so a bigger or smaller focal length?

Suppose you replace the first lens with an identically shaped lens that has a higher index of refraction.

? f=-25cm

Image created by first lens

25cm

100cmHow will the blue arrow change:

Bigger or smaller?Upright or inverted?Closer or further?

No change: same orientation

Quiz 5: Lenses

You should bring a straight edge You may bring and index card with notes

(3 inch x 5 inch)

Ray Model: really short version

Hitting a boundary can lead to reflection or refraction, often both. We can calculate the angles for any outgoing ray-

always with respect to normal Lenses are constructed carefully to bend light

in particular ways to create images We can locate images with the thin lens equation

or a ray tracing, and describe them in many ways