Lecture 11 ASTR 111 – Section 002

Outline Short review on interpreting equations Light –Suggested reading: Chapter and of textbook

Volume 2 meters 1 meter Side view of sphere Increase r by a factor of 2. What happens to volume?

Volume 2 meters 1 meter Increase r by a factor of 2 and Volume increases by 2x2x2 = 8 The radius changed from 0.5 to 1.0. Compute Volume using radius of 0.5 m and 1.0 m to see if you still get 8x the volume! You should know volume is proportional to the radius cubed Side view of sphere

Related questions What happens to answer if measurements were 1 yard and 2 yards instead of 1 meter and 2 meters? What happens if diameter increases by a factor of two? How much more paint is required to cover the larger sphere?

Area 2 meters 1 meter Disk Increase r by a factor of 2 and Surface Area increases by 2x2 = 4 The radius changed from 0.5 to 1.0. Compute Area using radius of 0.5 m and 1.0 m to see if you still get 4x the Area!

Outline Short review on interpreting equations Light –Suggested reading: Chapter and of textbook

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Lecture Experiment Need student with stopwatch Need student that can run Suppose running student is photon who informs me when an eclipse occurs. Stopwatch student is on Jupiter and he says eclipse every 15 seconds and then the photon runs to meet me to tell me an eclipse occurred.. Earlier we saw that periods depended on the observer. For example, we talked about synodic and sidereal time and days (a day is a period of time). Now the job of the audience is to tell the time interval that I perceive between eclipses. I want to know the time between eclipses.

Particle versus Wave Light is composed of photons. To fully understand light, we need to think of it in terms of both a particle and a wave.

He spoke with the wisdom that can only come from experience, like a guy who went blind because he looked at a solar eclipse without one of those boxes with a pinhole in it and now goes around the country speaking at high schools about the dangers of looking at a solar eclipse without one of those boxes with a pinhole in it.

Particle When thinking about the speed of light, we will think of a photon as a wave particle. But you can think of it as a wave too: – 8Yhttp:// 8Y –Or – 3i0&feature=related

Lecture Experiment X Y X Launches a photon every 5 seconds. How often does Y get hit by a photon? photon

Light travels through empty space at a speed of 300,000 km/s In 1676, Danish astronomer Olaus Rømer discovered that the exact time of eclipses of Jupiter’s moons depended on the distance of Jupiter to Earth

Light travels at 300,000 km/sec About how fast does your car travel in km/hour when you are on the freeway? About how fast does your car travel in km/second when you are on the freeway? If it takes light 8 minutes to travel from the sun to Earth, how long would it take you to drive?

About how fast does your car travel in km/hour when you are on the freeway? When you are going 60 miles/hour you are going 100 km/hour.

About how fast does your car travel in km/second when you are on the freeway?

If it takes light 8 minutes to travel from the sun to Earth, how long would it take you to drive?

Approximately what was the difference in timing of the eclipses between these two points ?

Approximately what was the difference in timing of the eclipses between these two points ? About 16 minutes. But difference is not caused by difference in position! It is caused by relative motion!

Determining the Speed of Light Galileo tried unsuccessfully to determine the speed of light using an assistant with a lantern on a distant hilltop

Amplification of Angular Changes If I hold a laser pointer and look at my hand, I say that I can’t see it moving. But if I point it at a wall, you can see the beam jump around. So my hand must be moving a very small amount. I have amplified the amount by projecting onto something far away. How can I use this principle to detect changes in speed of a pitcher. Give diagram. Emphasize angle in = angle out (which also works for light). Many solutions, but the one that works best (and is related to angular change) is to rotate wall a bit. This example can work, but it needs more set-up and diagrams.

In 1850 Fizeau and Foucalt also experimented with light by bouncing it off a rotating mirror and measuring time The light returned to its source at a slightly different position because the mirror has moved during the time light was traveling d=rate x time again gave c

What would happen if the stationary mirror was actually moving towards the rotating mirror?

Waves A review of waves

Light is electromagnetic radiation and is characterized by its wavelength ( )

Frequency and Wavelength The Greek letter “nu” and not the letter “v”

Cars are traveling at 100 km/hr to the right What would you need to know in order to be able to tell how often (frequently) a car will pass the finish line? Finish line velocity = 100 km/hr

Cars are traveling at 100 km/hr to the right What would you need to know in order to be able to tell how often a car will pass the finish line? Finish line v = 100 km/hr (Both are the same)

Cars are traveling at 100 km/hr to the right What would you need to know in order to be able to tell how often a car will pass the finish line? Finish line v = 100 km/hr (Both are the same)

Finish line v = 100 km/hr Replace cars with lines

Cars are traveling at 100 km/hr to the right What would you need to know in order to be able to tell how often a peak will pass the finish line? Finish line v = 100 km/hr

Finish line Distance between peaks How often peak passes finish line How fast wave moves to right Frequency and wavelength are intimately related for a wave.

Demo with snapping rope

Interference - destructive t=1 t=2

Interference - destructive t=3 t=4

Interference ? (Just before collision) t=1 t=2

Interference ? (After Collision) ? During Collision t=3 t=4

Interference t=1 t=2

Constructive interference t=3 t=4

Video

Interference Waves in two dimensions

x x x x xx x x x x x x

The electromagnetic spectrum

Because of its electric and magnetic properties, light is also called electromagnetic radiation Visible light falls in the 400 to 700 nm range Stars, galaxies and other objects emit light in all wavelengths

How an x-ray works. Give example of shooting paint balls at someone who is standing in front of a white sheet. Does this mean that big waves cannot penetrate a building? In general, yes.

Which of the following has the highest frequency? –Visible light –Radio waves –Microwaves –X-Rays –Infrared light –Ultraviolet light –Gamma rays

Which of the following has the highest frequency? –Visible light –Radio waves –Microwaves –X-Rays –Infrared light –Ultraviolet light –Gamma rays

Which of the following has the highest wavelength? –Visible light –Radio waves –Microwaves –X-Rays –Infrared light –Ultraviolet light –Gamma rays

Which of the following has the highest wavelength? –Visible light –Radio waves –Microwaves –X-Rays –Infrared light –Ultraviolet light –Gamma rays

Which of the following has the highest speed? –Visible light –Radio waves –Microwaves –X-Rays –Infrared light –Ultraviolet light –Gamma rays

Which of the following has the highest speed? –Visible light –Radio waves –Microwaves –X-Rays –Infrared light –Ultraviolet light –Gamma rays All the same! c = 300,000 km/s

Which of the following has the highest energy E (h is a constant)? –Visible light –Radio waves –Microwaves –X-Rays –Infrared light –Ultraviolet light –Gamma rays

Which of the following has the highest energy E (h is a constant)? –Visible light –Radio waves –Microwaves –X-Rays –Infrared light –Ultraviolet light –Gamma rays Small wavelength means large E

How is the energy of a photon related to its frequency?

The dual nature of light Particle Wave

Particle

What would you expect if instead of a laser beam you used yellow spray paint beam? Draw it!

Wave

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x x x x xx x x x x x x

x x x x xx x x x x x x The amplitude of the wave in the middle is the highest “Projection screen”

The atom and light

An atom has a small dense nucleus composed of protons (and neutrons) Rutherford’s experiments with alpha particles shot at gold foil helped determine the structure Probing the atom

Spectral lines are produced when an electron jumps from one energy level to another within an atom The nucleus of an atom is surrounded by electrons that occupy only certain orbits or energy levels When an electron jumps from one energy level to another, it emits or absorbs a photon of appropriate energy (and hence of a specific wavelength). The spectral lines of a particular element correspond to the various electron transitions between energy levels in atoms of that element. Bohr’s model of the atom correctly predicts the wavelengths of hydrogen’s spectral lines.