lecture 13 astr 111 – section 002 optics and telescopes
TRANSCRIPT
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
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)
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
Low index (e.g., air)
Higher index (e.g. water)
Principle 3b• Same principle applies when going in
opposite direction
normal
90o
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
How my optometrist remembers
Red light bends only a little
Red light has little energy (compared to blue)
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
… how an eye works
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
• 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.
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 waterPath 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
Toiletpaperroll
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
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
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 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