Lecture 13 ASTR 111 – Section 002 Optics and Telescopes

Lecture 13

ASTR 111 – Section 002

Optics and Telescopes

• If it is before 9 am, please do not sit in the last two rows.

Outline

• Quiz Discussion

• Finish a few slides from last lecture

• Optics (Reading is Chapter 6)

Quiz Discussion• 75% Computing your grade – will not cover in class• 66% Photons through a hole – will cover in class

• Can we finish going over lecture 12 in class? • Are you going to post the answers to lecture 12? • When is the next exam scheduled? • What acts as a nature's prism to create a rainbow in the sky? • If enacted, will clickers be mandatory? • I think we should use iclickers. Wouldn't it be easier than texting? • How many pets do you really have? • Can you review fully before the next exam? • Does it bother you when people come 45 minutes late to lecture and

slam there stuff around and make a lot of noise? Because it really bothers me.

• Why is this class getting exponentially more difficult?

Next Exam

• Next Tuesday.

• Same format as before. I will review on Tuesday and you can take it on Tues. or Wed.

• There will be a quiz due on Tuesday at 9am. The quiz will cover material covered last lecture and today’s lecture (lecture 12 and lecture 13 slides).

Key Questions• Why are there so many telescopes in

Hawaii?• Why is our best most famous telescope

orbiting Earth and not in Hawaii?• What is the difference between optical and

digital magnification (zoom)?• How and when (but not why) does light (and

other forms of electromagnetic radiation) bend?

• How does a telescope work?• What is the difference between magnification

and light-gathering power?

side note: What is the difference between optical and digital zoom?

side note: What is the difference between optical and digital zoom?

T

Same amount of information if I just expand the original

Practical note: What is the difference between optical and digital zoom?

T

Much more information (detail)

• You can create a digital zoom effect by taking a digital picture and expanding it (with photoshop, etc.)

• You can’t squeeze out more detail from the image (that is, increase the optical resolution), contrary to what you see on TV

Therefore

• How much larger is a raw image of 800x800 pixels than one with 1600x1600 pixels?

• 1600x1600 = 2,560,000 versus

• 800x800 = 640,0004x

1600

800

• Which sounds better in an advertisement:– 1600 x 1600 or over two million megapixels?

Can explain lots about telescopes and other devices

with only three optics principles

Principle 1• Light rays from distant object are nearly

parallel

Principle 1• Light rays from distant object are nearly

parallel

Collector

Principle 2• Light reflects off a flat mirror in the same

way a basket ball would bounce on the floor (angle of incidence, i = angle of reflection, r)

Principle 3 prep

What happens, a, b, or c?

• As a beam of light passes from one transparent medium into another—say, from air into glass, or from glass back into air—the direction of the light can change

• This phenomenon, called refraction, is caused by the change in the speed of light

Axle and wheel from toy car or wagon

Sidewalk

Grass

What happens, a, b, or c?

• As a beam of light passes from one transparent medium into another—say, from air into glass, or from glass back into air—the direction of the light can change

• This phenomenon, called refraction, is caused by the change in the speed of light

Axle and wheel from toy car or wagon

Sidewalk

Grass

Principle 3• Light changes direction when it moves

from one media to another (refraction). Use wheel analogy to remember which direction normal

90o

Low index (e.g., air)

Higher index (e.g. water)

Principle 3a• Light changes direction when it moves

from one media to another (refraction). Use wheel analogy to remember which direction normal

90o



Principle 3b• Same principle applies when going in

opposite direction

normal

90o



• Which normal does it bend towards/away from?

• Draw un-bent path and then bend a little towards or away from the nearest normal line.

Principle 3c• At interface light diffracts and reflects

(you can see your reflection

in a lake and someone in lake

can see you)



These angles are equal

i r

What happens to each beam?A

B

C

A

B

C

A

B

C

What happens?

?

?

?

zoom box

zoom box contents nearly flat whenzoomed inzoom box contents

zoom box contents

norm

al

90o

zoom box contents

To figure out path, draw normal and un-bent path.

zoom box contents nearly flat whenzoomed in

norm

al

90o

zoom box contents

Bends toward the normal.

What happens?

?

?

?zoom box

zoom box contents

zoom box contents

90o

zoom box contents

90o

The Lines Converge

Input parallel lines converge to focal point

F

F

What happens to the beams here?

And parallel lines go out when source at focal point

F

But you said different colors bend different amounts!?

But you said different colors bend different amounts!?

This is chromatic aberration

How I remember red bends less

How my optometrist remembers

Red light bends only a little

Red light has little energy (compared to blue)

What happens?

?

?

Now we can explain

… rainbow color ordering

Sunlight diffractionreflection

Water droplet

Sunlight

Finish drawing. Red light bends less on refraction. All light reflects at same angle

Observer sees red higher in sky than blue

Sunlight

Sunlight diffractionreflection

diffraction

Water droplet

Sunlight

Now we can explain

… how an eye works

“Seeing” your optic nerve

• http://www.tedmontgomery.com/the_eye/optcnrve.html


Retina

Info from distant object is concentrated on small area on retina

Eye lens


RetinaEye lens

Light from Sun

Light from a distant lighthouse

Sunlight lower than lighthouse light

… how an eye worksRetina

Eye lens

Light from a distant lighthouseSun appears lower than lighthouse light

Now we can explain

… how telescopes work

• Magnification is ratio of how big object looks to naked eye (angular diameter) to how big it looks through telescope

Telescope principles

½ o

10 o

Magnification is 10/0.5 = 20x

• Although telescopes magnify, their primary purpose is to gather light

Telescope principles

Collector

• How much more energy does a 1 cm radius circular collector absorb than a 4 cm radius collector?– Same– 2x– 4x– 16x– Need more info

Collector

• How much more energy does a 1 cm radius circular collector absorb than a 4 cm radius collector?– Same– 2x– 4x– 16x– Need more info

Area of circle is proportional to r2 A1 is proportional to (1 cm)2 = 1 cm2

A2 is proportional to (4 cm)2 = 16 cm2

Reflecting telescope

• Previously I described a refracting telescope. The principles of reflection can be used to build a telescope too.

Problem: head blocks light!

Solutions

Twinkling stars

Flattened appearance. Whiter near top

Pencil looks brokenand thicker

Coin

Pupil

Three coins are placed on the bottom of an empty tray.1. Which coins can the person see?2. If the tray is filled with water, which coins can the

person see?3. If the person and tray are both under water, what

coins can the person see?

Coin

Pupil


person see?3. If the person and tray are both under water, what

coins can the person see?

Coin


person see?3. If the person and tray were both under water, what

coins would the person see?

Path of photon if air is replaced with waterPath of photon that bends

away from normal as it goes from water to air.

Normal

Coin


person see?3. If the person and tray were both under water, what

coins would the person see?

Continue process of drawing lines from different parts of coin to determine how much of it he can see.

Coin

1. Draw the path of photons emitted from the two positions shown on the coin towards the pupil.

2. Draw the path of photons emitted along the paths that you just drew if the tray is filled with water.

3. When the tray is filled with water, can you see the sides of the coin?

4. When the tray is filled with water, does the coin look the same size, larger, or smaller? Does it appear closer?

5. What would happen to your answers to (2)-(4) if the coin was red instead of green?

Pupil

Toiletpaperroll

Coin






Pupil

Toiletpaperroll

Coin






Pupil

Toiletpaperroll

Coin

Pupil

Toiletpaperroll

Path of photon if no water



3. When the tray is filled with water, can you see the sides of the coin? No.






4. When the tray is filled with water, does the coin look the same size, larger, or smaller? Does it appear closer? Yes.

5. What would happen to your answers to (2)-(4) if the coin was red instead of green? Hint: Red bends less.

Path of photon if no water

Path of photon due to refraction at air-water interface

Near the edge of the roll, he sees inner part of coin

The pupil sees the inner part of the coin at the outside rim of the roll. When there was no water, he could see the edge of the coin.

If there was no water, and the coin was lifted up, then the pupil would see the inner part of the coin at the rim.

Pupil concludes that coin looks closer or larger when there is water.

Inner part of coinEdge ofcoin

White is empty space

Person sees at edges of moon at edges of tube.

Light from edge of moon is just able topass through tube and into pupil

Earth’s atmosphere

White is empty space

What does person see at edges of tube?

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Lecture 13 ASTR 111 – Section 002 Optics and Telescopes