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?

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 = 640,000 4x

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

(an aside) Something is Fishy Look around in textbooks on physics and astronomy. You will find this analogy (or one with marching soldiers). What you won’t find is a physical explanation of how the wheels behave like light. This is an analogy that is useful for remembering which way light bends when going from one material to a denser material. It does not explain why it is so important that axle does not bend or give an explanation that uses the wave or particle picture of light

Principle 3 Light changes direction when it moves from one media to another (refraction). Use wheel analogy to remember which direction normal 90 o 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 90 o Low index (e.g., air) Higher index (e.g. water)

Principle 3b Same principle applies when going in opposite direction normal 90 o Low index (e.g., air) Higher index (e.g. water)

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) Low index (e.g., air) Higher index (e.g. water) These angles are equal i r

What happens to each beam? ABCABC ABCABC ABCABC

What happens? ? ? ? zoom box

zoom box contents nearly flat when zoomed in zoom box contents

normal 90 o zoom box contents To figure out path, draw normal and un- bent path.

zoom box contents nearly flat when zoomed in normal 90 o zoom box contents Bends toward the normal.

What happens? ? ? ? zoom box

zoom box contents

90 o

zoom box contents 90 o

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!?

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)

A B C

A normal Line A bends toward the normal Dashed line is path of line A if it does not bend When a light ray goes from air into glass (or water) it bends toward the normal

A normal Line A bends away from the normal Dashed line is path of line A if it does not bend When a light ray goes from air into glass (or water) it bends toward the normal. When a light ray goes from glass (or water) into air, it bends away from the normal.

A normal Line A bends away from the normal Dashed line is path of line A if it does not bend When a light ray goes from glass (or water) into air, it bends away the normal

What happens? ?

?

Now we can explain

… rainbow color ordering

Sunlight diffraction reflection 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 diffraction reflection diffraction Water droplet Sunlight

Now we can explain

… how an eye works

“Seeing” your optic nerve

… how an eye works Retina Info from distant object is concentrated on small area on retina Eye lens

… how an eye works Retina Eye lens Light from Sun Light from a distant lighthouse Sunlight lower than lighthouse light

… how an eye works Retina Eye lens Light from Sun Light from a distant lighthouse Sun 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 r 2 A1 is proportional to (1 cm) 2 = 1 cm 2 A2 is proportional to (4 cm) 2 = 16 cm 2

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 broken and 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 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 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 were both under water, what coins would the person see? Path of photon if air is replaced with water Path of photon that bends away from normal as it goes from water to air. Normal

Coin 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 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 Toilet paper roll

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 Toilet paper roll

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 Toilet paper roll

Coin Pupil Toilet paper roll Path of photon if no water 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? No. 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?

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? 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 coin Edge of coin

White is empty space Person sees at edges of moon at edges of tube. Light from edge of moon is just able to pass through tube and into pupil

Earth’s atmosphere White is empty space What does person see at edges of tube?