NOTES FOR THE DAY: Waves: Characteristics: wavelength, λ --distance of repetition (crest to crest).

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# NOTES FOR THE DAY: Waves: Characteristics: wavelength, λ --distance of repetition (crest to crest). - PowerPoint PPT Presentation

NOTES FOR THE DAY: Waves: Characteristics: wavelength, λ --distance of repetition (crest to crest). frequency, f --no. of complete cycles passing a point in a given time (cycles/sec = Hertz or Hz). wave velocity, v --the speed of a crest (m/s).

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NOTES FOR THE DAY: Waves:

Characteristics:

wavelength, λ--distance of repetition (crest to crest).

frequency, f--no. of complete cycles passing a point in a given time

(cycles/sec = Hertz or Hz).

wave velocity, v--the speed of a crest (m/s).

Wave formula:v = fλ. Note: if you know f you can get λor vice versa

(for light v = c is known).

Ex. Calculate the frequency of light with wavelength = 3 m.

f = c/λ = 3 x 108 m/s/3m= 108 cycles/s (Hz).

The Electromagnetic Spectrum: light is an electromagnetic wave.

All wavelengths are possible. (Overlay.)

Spectral Types:

1. Continuous (Black Body)--a hot, dense gas with a common

temperature. (See Planck curve overlay.) Wien's Law

relates surface temperature, T, to peak wavelength.

T = constant/λpeak .

2. Bright line (emission)--a hot sparser gas with temperature

variations. Collisions excite electrons, and they give off

well-defined wavelengths.

3. Dark line (absorption)--a cool gas in front of a hot, dense gas

with a continuous spectrum. It absorbs certain frequencies.

Light As Messenger:

Waves:

Characteristics:

wavelength, λ--distance for repetition, crest to crest.

wave velocity, v--the speed of a crest (in m/s).

frequency, f--no. of complete cycles passing a point in a given time (cycles/sec = Hertz or Hz).

Wave formula:v = fλ.

Note: if you know f you can get λor vice versa

(for light v = c is known).

Ex. Calculate the frequency of light with

wavelength = 3 m.

f = c/λ = 3 x 108 m/s/3m= 108 cycles/s (Hz).

Spectral Types:

• Continuous (Black Body) spectrum
• --a hot, dense gas with a common temperature. Planck curve.

Wien's Law for continuous spectrum

relates surface temperature, T, to peak wavelength.

T = constant/λpeak .

2. Bright line (emission) spectrum--a hot sparser gas with temperature variations. Collisions excite electrons, and they give off well-defined wavelengths.

3. Dark line (absorption) spectrum--a cool gas in front of a hot, dense gas with a continuous spectrum. It absorbs certain frequencies.

Kirchoff’s Laws of Spectral Formation:

NOTES FOR THE DAY: Why are there spectral lines?

Note the spectrum of the sun in the overlay.

How did the dark lines get there?

Atoms are made up of protons and neutrons in the nucleus

and electrons 'in orbit' about them.

Example--The Hydrogen Atom:

Quantum Theory says electrons have orbits of certain energies—

this is like different sized stair steps. They give off light when

they fall to a lower lever (emission) and absorb it

when they encounter just the right energy light (absorption).

See diagram of H atom energy levels.

Elements are identified in the periodic table and have their individual

'thumbprints' of spectral lines.

The number of the element = the atomic number = #protons

= #electrons in a neutral atom.

Spectroscopy--study of spectra.

Spectroscope (spectrometer)--device to study light spectra.

Planck's Law--the energy of a photon, or light quantum

(particle or bundle), is proportional to its frequency: E = hf.

Why are there spectral lines?

Note the spectrum of the sun in the picture.

this is like different sized stair steps.

Example: the Hydrogen atom

n is the principal

quantum number

Balmer lines are the visible lines of Hydrogen

and have n = 2 as their lower level.

Lyman lines are UV and have n = 1

as the lower level in the transition.

Elements are identified in the periodic table

and have their individual 'thumbprints' of spectral lines.

The number of the element = the atomic number

= # protons = # electrons in a neutral atom.

Spectroscopy--study of spectra.

Spectroscope(spectrometer)--device to study light spectra.

Planck's Law (1900)--the energy of a photon, or light quantum (particle or bundle), is proportional to its frequency:

E = hf.

The Doppler Effect:

Light lowers in frequency when source has relative velocity

away from observer (red shift),

raises in frequency when approaching (blue shift).

Relative velocity can be obtained from the frequency shift.