Chapter 30 – Lenses

I. DEMO – Show lenses c lose and far away

II. Definitions

a. Optical axis – an imaginary line going perpendicular to lens or mirror and bisecting it

b. Focus – the point on the axis where parallel light crosses after passing through the lens or bouncing off the mirror. It is half the distance from the center of the curve forming the mirror or lens

c. Converge – to come together

d. Diverge – to spread apart

III. Do Ray Diagram Activity

IV. Ray Diagrams

a. Rules

i. Any ray traveling parallel to the axis will be refracted so that it crosses focus 2

ii. Any ray that crosses focus 1 will be refracted so that it is parallel to the optical axis

iii. Any ray that goes through the center of the lens will not be refracted.

b. Assume light bends in the CENTER of the lens, although this is not really true

c. Pick the top of the object to determine where the top of the object will end up on the other side, assume the rest of the object will follow

V. Convex lens

a. Bulges out

b. Called a CONVERGING lens because it makes the light bend together

c. image = upside down (inverted), real

d. Lens EQ: 1/di + 1/do = 1/f

i. Real = +, virtual = -

ii. Convex f = +, concave f = -

iii. Do = ALWAYS + (for us J)

e. Magnification = image size same, bigger (magnified), or smaller (Demagnified) than object

i. M = hi/ho = - di/do

1. - = inverted = real

2. + = upright = virtual

f. Sample Problem

i. G: do = _____, f = ______

ii. F: di = ? m =?

iii. R: 1/do + 1/di = 1/f, m = -di/do

iv. S: 1/di = 1/do – 1/f, m = -di/do

v. C: 1/di = 1/ ( ) – 1/( )

di = ______ (Check with diagram!!!)

m = -( )/ ( ) = -________

g. Practice more real diagrams

h. What happens if object is between focus and lens?

i. image is upright (erect)

ii. image is virtual

iii. image is magnified

iv. Check with lens equation

1. G: do = _____, f = _____

2. F: di = ?

3. R: 1/do +1/di = 1/f, m = -di/do

4. S: 1/di = 1/f – 1/do, m = -di/do

5. C: 1/di = 1/( ) – 1/( ) = -_____

Di = - _____

M = -(- )/( ) = ______

VI. Concave lenses

a. Thinner in the middle

b. Diverging lens – diverges light

c. Foci are reversed, f = -

d. Rules are the same

e. Example:

i. Image is upright (erect)

ii. Image is virtual

iii. Image is demagnified

iv. Check with EQ:

1. G: do = ______, f = - ______

2. F: di = ?

3. R: 1/do + 1/di = 1/f, m = -di/do

4. S: 1/di = 1/f – 1/do, m = -di/do

5. C: 1/di = 1/(- ____) – 1/( _____)

1/di = - _____

di = - ______

m = -(- _____)/(______) = ______

VII. Images

a. Real

i. Can be focus on a screen

ii. Always upside down

iii. Only can be made by convex/converging lenses

iv. Light really passes through

v. Used with cameras, overhead projectors, telescopes, microscopes

vi. +

b. Virtual

i. Cannot be focused on a screen

ii. Always right side up

iii. Made with both convex/converging and concave/diverging lenses

iv. No light passes through it

v. Used with magnifying lenses, microscopes, telescopes

vi. Concave = ALWAYs demagnified, Convex = ALWAYS magnified

vii. –

VIII. Mirrors

a. DEMO – pig in mirror, large mirrors

b. Same rules apply EXCEPT use the CENTER OF CURVATURE which is 2f and light BOUNCES back instead of going through (only ONE focus)

c. Concave mirror – caves inward

i. Image is real, upside down, in front of mirror, real light

ii. Can pass fingers through image

iii. Check with lens equation

d. Concave mirror, image in front of focus

i. image is virtual, erect, behind the mirror, and magnified

ii. check with lens equation

e. Convex mirror = bulges outward, focus is – and behind the mirror

i. Image is virtual, erect, behind the mirror, and demagnified

ii. Check with lens equation

f. Plane mirrors

i. No focus

ii. ALWAYS right side up = real

iii. ALWAYS same size

iv. ALWAYS same distance behind mirror as object is in front

g. Human Eye and Ear

i. The eye parts

1. L – Pupil: the dark opening in the center of the eye that allows light to enter

2. M – Optic nerve: send impulses from light receptor cells to brain.

3. A- Cornea: clear outer covering of the eye, this does most of the bending of the light. This is where contact lenses are placed

4. D – Iris: the colored part of the eye. This moves in and out to change the size of the pupil in response to light

5. J – Convex lens: This creates a real, upside down image. It is flexible

6. K – Eye muscles: these squeeze and expand the lens so as to change the focal length

7. E- Retina: this contains the receptor cells. It is the “Screen” for the eye. There are two types of receptor cells:

a. S = cones: only for bright color

b. R = rods: very sensitive, no color, images are coarse and undefined

ii. Eye focusing

1. Distant objects (more than 6m(2ft))- all rays are essentially parallel, less bending is necessary so cornea can do most of the bending

2. Near objects – light more divergent, needs to be bent more, cornea + lens bend light

3. For camera, lens is moved in and out to change focus. For eye, the eye muscles squeeze and lengthen the lens to change focus

iii. Eye problems

1. Far sightedness

a. short eyeball

b. image is BEHIND retina

c. need CONVEX lens to correct

2. Near sightedness

a. Long eyeball

b. Image is IN FRONT of retina

c. Need CONCAVE lens to correct

3. Astigmatism

a. Unequal curvature of cornea (barrel shaped)

b. Causes distortion around edges of objects

c. Never quite in focus

d. Harder to correct with contact lenses because they sit on cornea

4. Color blindness

a. Can’t distinguish between read and green

b. Mainly found in boys = sex-linked, on XY chromosome – girls have 2 chances, boys only have 1

iv. Ear Parts

1. A – outer ear: directs sound waves inward

2. B – auditory canal: leads sound waves to ear drum

3. C – ear drum (Tympanic membrane): vibrates bondes inside ear as sound waves hit it

4. D – hammer: (one of three small bones) – vibrates first

5. E – anvil (one of three small bones) – vibrates second

6. F – stirrup (one of three small bones) – vibrates third

7. G – Eustachian tubes: equalizes pressure in ear drum to prevent damage when outside pressure changes. Usually collapsed. Swallowing = opens and air can go through making middle ear pressure = pressure of pharynx and outside of body

8. L – chochlea: inner ear, contains fluid for carrying sound waves

9. K – semi-circular ear canal : for balance, they are all perpendicular to each other and filled with fluid so that as fluid moves your body is aware of your position due to nerve stimulation

10. Z – auditory nerve: sends sound impulse to brain to interpret

h. Sound Intensity

i. Absolute intensity is W/m2…energy per area

ii. Normally rated to human hear in decibels = dB, a log scale

iii. Hearing threshold = 0 dB

iv. Rustling leaves = 10 dB

v. Normal conversation = 65 dB

vi. Thunder = 110 dB

vii. Rock concert = 110-120 dB

viii. Jet plane = 140-160 dB

ix. Pain and injury >120dB – can rupture ear drum, not repairable

Chapter 31 Diffraction and Interference

I. Diffraction

a. Huygen’s principle – all waves are made up of smaller waves and can combine or separate

b. DEMO – finger slit diffraction

c. Diffraction- the bending of waves as it goes THROUGH or AROUND a barrier

d. Barrier must be ALMOST = to wavelength or no diffraction will occur

i. This is why sound diffracts around corners but not light

ii. This is why AM waves travel further than FM =

1. AM = longer wavelength = more diffraction of hills

iii. DEMO – Sky

iv. This is why the sky is blue

1. Atoms in the sky diameter = blue wavelength = blue diffracted and scattered

2. Other colors are present but only see if all blue is scattered already

3. Orange/red = larger wavelength = more particles = rain

4. Violet = after other colors scattered

II. Interference

a. DEMO – overheads moving = interference pattern

b. These are all interference patterns

i. Dark = destructive = no light, waves subtract

ii. Bright = constructive = light, waves add

c. DEMO – computer demo – show constructive and destructive interference http://www.sciencejoywagon.com/physicszone/lesson/09waves.htm

d. Constructive = peak + peak or crest + crest = larger amplitude

e. Destructive = peak + crest = smaller or no amplitude

f. DEMO hair and laser beam diffraction

g. Laser light is diffracted and split into 2 beams which interfere making the interference pattern of dark and light

h. Young’s experiment – first person to show this interference – shows light is a wave – only works with monochromatic light

i. DEMO – DG with white light (works for each color separately based on wavelength)

j. DEMO – plates and polarizers

k. Thin film interference = constructive and destructive interference but different for EACH color (each wavelength) needs to match to see color, needs to exactly NOT match to not see color

l. Iridescence = color you see from this type of interference (oil, bubbles, seashells)

m. Interferometer uses this idea to measure very small thicknesses, as thickness change, different colors have max and this can be measured

III. Lasers

a. DEMO – Laser beam and flashlight

b. What is the difference between the two?

c. LASER = Light Amplification Stimulated Emission of Radiation

i. Monochromatic (one color)

ii. Coherent (all waves line up to make one STRONG wave through constructive interference

iii. Directional (virtually parallel)

iv.

Flash tube

Focusable

d. Ruby Red first ever made

i. Light is flashed

ii. Stimulates ruby atoms to emit photons

iii. Photons bounce back and forth to become larger (amplified)

iv. When large enough and together with enough energy can emerge as beam

e. Uses of lasers

i. Cutting

ii. Welding

iii. Surgery

iv. Hologams

v. Telecommunications

vi. Check out stands

vii. Weapons

viii. CD’s

IV. Holograms

a. Hologram is a 3-D virtual image

b. It is an INTERFERENCE pattern

c. Steps

i. Illuminate object with laser

ii. Place photographic films in line with reflect laser light and straight laser reference beam (combine to make interference pattern)

iii. Develop film

iv. Shine laser or parallel light back on film to re-create what cause the interference

v. Image is virtual and right side up and moves

Photography	Holography
2-D	3-D
Image forming device	No image forming device
Ordinary light	Laser light
Real image on film	Virtual image, interference pattern on film

d. Uses of holography

i. Diagnosis, viewing complex proteins, teaching aides, radar systems, satellite probes, counterfeiting deterrent, jewelry, decorations