• Lab 4: Optics this week.

• HW 9: Read through before lab. Lab will help you better understand creating images and how lenses work.

• Topics: This week– Finish cameras, Digital Cameras– Optical communication

We want to have a special expert guest lecturer on Friday, April 23rd, 3pm. How eye works, visualization, science behind optical illusions, etc… This class would be substitute for class on Thursday, April 22nd. Class points as usual for attending Friday lecture. If you can’t make it, we will provide alternative means (equal time) for earning those points: a. I can make it on Friday’s at 3 pm. b. I cannot make it on Friday’s at 3 pm.

Exam 2: 30 possible

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7

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12

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A and B’s

C’s

D’s

The Basic Idea

ObjectDistance

ImageDistance

ObjectLens

With focal length f

Shutter/IrisFilm

Dark Box

Put film, Retina here!

Lens bends the light.

Focal length (f)

Send in parallel rays from distance light spot: Distance to focus is focal length

Focal length (f)

More curved lens is… More bending powerShorter focal length

Focal length: the shorter the focal length, the more bending power

Lens equation:

Distance to object

Focal length111

Distance to image

of111 i

ff

Object Distance (o)

Image Distance (i)

f

f

compound lenses- same basic physics, but 1) hard to make simple piece of glass to perfectly match Lens eq. (specially for different colors). Easier to stick two pieces together. 2) Multiple lenses allow more adjustments- bigger Mag.,same object and image distance (zoom lens), really short f. l., big M (microscope)

(o) (i)

f

f

What will happen to image if we increase focal length of lens?a. Image is same size, same placeb. Image is same size and further from lensc. Image is bigger and further from lensd. Image is smaller and closer to lens

iof

111

Lens equation:

(o)

(i)

f

f

Answer is c: Like Zoom Lens.Extra-special lens w/ variable f, increase f to zoom in, decrease to zoom outIncrease f Less bending power Further distance to image Bigger image.

(o) (i)

f

f

What will happens to brightness of image?a. Less brightb. More brightc. Same brightness

iof

111

Lens equation:

(o)

(i)

f

f

Answer is a: Same amount of light captured by lens, but now spread over larger area… less light per area… dimmer. How can we compensate for less light per area of film?

(o) (i)

f

f

iof

111

Lens equation:

(o)

(i)

f

f

How can we compensate for less light? 1. More light sensitive film2. Larger lens / Open Iris all the way3. Longer exposure time4. Use Flash

Two objects different distances from lens

f

f

Screen at Focus of 2.

1

2

2

1

What will happen at screen if close down iris in front of lens? a. Images brighter and object 1 more in focusb. Images dimmer and object 1 more in focusc. Images brighter and object 1 more out of focusd. Images dimmer and object 1 more out of focus

Depth of Focus (a demo) Two objects different distances from lens

f

f1

2

2

1

Light Rays from “1” diverging behind focus point.

Rays through outer part of lens diverging strongly.Closing down iris blocks rays that are diverging most, Improves depth of focus. (squinting – improves vision. Nighttime – iris open, poor vision)

Screen at Focus of 2.

Answer is b.

Digital vs Film CamerasMechanical Electrical Shutter

Mechanical Iris

ObjectDistance

ImageDistance

ObjectLens

With focal length fFilm

Electronic Light Sensor

(semiconductor device)

Dark Box

Battery Required!

You have already seen same physics principles in other applications: Lens/Optics, Color/Light, Resolution/Pixels, Semiconductors, P-N junctions, capacitors

Film vs Electronic Sensor

ObjectDistance

ImageDistance

Object

24 mm

36 mm

FILMElectronic Light Detector (MUCH SMALLER)

6.6 mm

4.4 mm

With 35 mm camera, stand 15 ft away from tree, whole tree just fills film. How should the lens system be set up in the digital camera so that when you are standing 15 ft away from tree, the whole tree just fills the digital camera’s detector:

a. the lens and lens-detector distance must be identical to that of the 35mm

b. the lens curved more and the lens-detector distance shorter

c. the lens curved less and the lens-detector distance shorter

d. the lens should curved more and the lens-detector distance longer

Film vs Electronic Sensor

How should the lens system be set up in the digital camera so that the view captured is the same as the 35 mm camera when I am about the same distance away from the subject:

Answer is b. The lens curved more and the lens-detector distance shorter. Need more bending power (curvature). To focus entire image in smaller area.

(o) (i)

f

f

(o) (i)

f

f Digital Camera: Lens has more curvature More bending power Rays converge to focus faster smaller image

Digital Camera: Lens has more curvature More bending power Rays converge to focus faster smaller image

Film Camera: Lens has less curvature Less bending power (refraction) Rays converge to focus slower larger image

Semiconductor Device (converts light to electrical charge)

Most digital cameras use: charge-coupled device (CCD Array)

The Electronic Detector

4.4 mm(1200 pixels)

6.6 mm (1600 pixels)

Lots of tiny light detectors (each 1 pixel)The more light photons that hit, the more electrical charge builds up on the pixelCannot discriminate between colors… all colors treated equally

To capture information about color: put colored filters in front of each pixel

Transmit 100% of Orangeish-yellow

Transmit 25% of Reddish-orange

To capture information about color: put colored filters in front of each pixel

Question:If the camera is taking a picture of an orange, light will be detected by: a. blue pixelsb. red pixelsc. green pixelsd. both b and ce. both b and a

Answer is d. Both green pixelsand red pixels. Red filters will pass larger fraction of orange light than green filters will. Ratio of light detected/recorded is indication that color was orange.

Interpreting color

Computer determines true color at center pixel is determined by looking at amount of light collected at surrounding pixels… that way you have info on red, green, and blue.

Resolution

24 mm~”2000” pixels

36 mm (~3000 “pixels”)

FILM

6.6 mm (1600 pixels)

4.4 mm(1200 pixels)

Traditional film (~200 iso) has effective resolution of ~3000 x 2000 pixels ….Captures more detail than most CCD arrays.

Question: To improve the amount of detail captured by the digital camera, you need to:

a. Increase number of pixels on the CCD array

b. Increase the physical size of the CCD array, keeping the total number of pixels the same

c. Both a and b.2.1 million pixels 2.1 Megapixels

Answer is a.

actually chunks of chemical

Optical Zoom vs Digital Zoom

Optical Zoom: RealImage is larger on array

Digital Zoom: RealImage is same size on array, uses subset of pixels and then guess at detail…. NO EXTRA INFORMATION.

A pixel of the CCD (Charged-coupled Device) Array

En

erg

y

---------

+++++++

Thin Insulator

Thin Conductor

P-Typesemiconductor

N-Type

Excess negative charges

Excess positive charges

Depletion Region

Question: What will happen if apply a positive voltage at A?

a. Nothing, no electrons can moveb. Electrons in N-type will flow to

the right and pile up at insulator until charges balance

c. Electrons will flow from P-type to N-type and pile up at insulator until charges balance

d. Electrons will flow continuously from P to N to A.

A