Big Questions in Science

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Color and spectra Atoms Schrödinger’s cat Radioactivity

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Light is a wave made of different wavelengths Blue light has short wavelength. Scattered all around atmosphere.

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Ni (78%) O (21 %)

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Cathode ray tube 1. Three electron guns 2. Electron beams 3. Focusing coils 4. Deflection coils 5. Anode connection 6. Separating beams 7. Phosphor layer 8. Close-up of screen

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http://en.wikipedia.org/wiki/File:CRT_color_enhanced.png

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• (Semi-) vacuum tube • Metal electrodes, high voltage in between • Postitively charged atoms attracted to cathode (neg.) • Kicked off electrons attracted to anode (positive charge)

Faraday: electrical current bent by a magnet. Thomson: electrical current consists of

particles much smaller than atoms. Mass 1600 smaller than hydrogen. Röntgen: new type of ‘cathode’ radiation.

Becquerel: uranium emits Röntgen radiation. Marie Curie researched other substances.

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Ernest Rutherford: radiation consists of:

Beta-rays. Could be bent, electrons.

Alpha-rays. Could be bent, much

heavier, positive charge. Helium ions.

Gamma-rays. Röntgen radiation. UV

Suggested internal structure of atoms.

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Rutherford: researched thorium, found emanation and radioactivity. Bombarded platinum foil with alpha particles, reflected in all directions.

Conclusion: mass concentrated in the nucleus.

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Rutherford: second, “planetary” model (inspired by Lord Kelvin).

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Different elements have different ‘absorption spectra’, specific wavelengths that they absorb (or emit).

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Planck’s law

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Assumption Planck needed to make: light carries energy in discrete amounts.

Each ‘energy packet’ has energy h

Big Questions in Science, fall 2012. SdH, AUC

Fascinated by planetary model. Putting things together: quantum hypothesis

applied to atom. Atomic numbers explain properties of atoms. Sommerfeld refined version.

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Radiatioactive decay is random. Same true for photon emission by atom:

unpredictable.

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PET scan X-rays

As long as radiation is not measured, the atom is in two states (decay/non-decay). This is called a superposition.

Property is only determined once a measurement is made.

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The single-slit experiment

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http://www.universetoday.com/83380/double-slit-experiment/

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http://www.hitachi.com/rd/research/em/doubleslit.html

26 http://russherman.com/Talks/FirstThreeMinutes_CollegeDay.pdf

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Waves that go through the slits interfere with each other.

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http://micro.magnet.fsu.edu/primer/java/interference/doubleslit/

Electrons and photons behave as waves while in transit: interference.

Detected as particles on the screen. Interference pattern disappears if

observation. No way to predict where single photon ends

up: intrinsic unpredictability. Only probabilities.

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de Broglie: if photons are both waves and particles, particles behave as waves.

Planck relation:

. Matter:

.

Frequency proportional to speed (squared).

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Waves of high frequency (short wavelength) are sharply localized, waves of low frequency (long wavelength) are spread out.

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Small

Large

Large

Small

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Electron microscope

Resolution ~ wavelength 0.1 to 0.5 nm

http://www.science.uva.nl/onderwijs/thesis/apart/phys/thesis.php?start=181&level=bachelor

Quantum computing

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Compare Bohr’s model of the atom with the Solar System.

1) Similarities. 2) Where analogy breaks down.

What is the role of the hydrogen atom in the development of modern quantum mechanics?

Hint: consider Kuhn’s notion of ‘exemplar’.

Think of similar examples in Newtonian physics and in the theory of relativity.

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