Introduction to Electromagnetic Radiation

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Why is the water molecule bent, but carbon dioxide is linear?

Why is the ammonia molecule shaped like a pyramid?

Why is sulfuric acid a strong acid,

but sulfurous acid is a weak acid?

Why does arranging C5H12 in different ways change the boiling point?

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2-

2-

https://www.thoughtco.com/carbon-dioxide-molecular-formula-608475

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http://www.daviddarling.info/encyclopedia/X/xanthophyll.html

chlorophyllsWhy

do

leaves

change

color?

The behavior of electrons determines the structure of matter and the colors we see.

What we understand about the behavior of electrons comes from learning about their relationship with light.

To understand what electrons are doing, we must understand what light is.

What is light?• Light is energy. Visible light is energy we can see.• Electromagnetic radiation is the general term • Visible spectrum extends from 400 nm to 780 nm

Electromagnetic radiation is a wave of pure energy

wavelength:

l

frequency:

n

ln = c

E = hn

speed of light

energy

Parts of the Electromagnetic Spectrum

• Visible• Seen by human eyes

• Ultraviolet (UV)• High energy; can break chemical bonds

• X-rays• Roentgen, higher energy than UV

• Gamma rays• Most energetic, most don’t reach Earth

• Infrared (IR)• Heat; used in commercial night vision equipment

• Microwaves• Efficiently absorbed by water, cooking

• Radio waves• Hertz; wavelengths as long as football fields, used to transmit

communication signals

Lasers emit monochromatic light

• LASER is an acronym for “light amplification by stimulated emission of radiation.”

• Laser light contains light of only ONE wavelength.

• These waves are not randomly oriented, but are aligned, or in phase.

• Laser light is pure, intense, and resists spreading in space.

Behavior of electromagnetic waves

• electromagnetic radiation produces an interference pattern if it encounters a repeating object with a size similar to its wavelength

• electromagnetic radiation is a wave

Interference patterns

• electromagnetic waves can interfere with each other constructively or destructively

Light travels at different speeds through different materials

• the speed of light depends on the medium though which it travels• when light slows down in a medium, its direction changes • the refractive index of a material indicates how fast light moves through it

Electromagnetic spectrum

The speed of light: c = 299,792,458 m�s-1

Date Author Method Result (km s-1)1676 Olaus Roemer Jupiter's Satellites 214,000

1726 James Bradley Stellar Aberration 301,000

1849 Armand Fizeau Toothed Wheel 315,000

1862 Leon Foucault Rotating Mirror 298,000 ±500

1879 Albert Michelson Rotating Mirror 299,910 ±50

1907 Dorsay RosaElectromagnetic

constants299,788±30

1926 Albert Michelson Rotating Mirror 299,796±4

1947 Essen, Gorden-Smith Cavity Resonator 299,792±3

1958 K. D. Froome Radio Interferometer 299,792.5±0.1

1973 Evanson et al Lasers 299,792.4574±0.001

198317th General Conference on

Weights and MeasuresAdopted Value 299,792.458

Key historical measurements of the speed of light.

Source: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html (taken from Twentieth Century Physics, Vol 2, IOP/AIP press)

ln = c

Interaction of matter and electromagnetic radiation

• transmission• absorption• emission• reflection• refraction• diffraction• dispersion• polarization

Emission spectrum

• electrons in atoms can absorb energy supplied from outside the atom

• when they relax, they emit the energy as electromagnetic radiation

• each element emits a characteristic combination of wavelengths of emitted radiation

Absorption spectrum

• when light shines on atoms, the atoms selectively absorb exactly the same wavelengths of light that they emit after absorbing other forms of energy

Emission spectrum of hydrogen: Balmer’s explanation

21

22

22

nnna-

=l

• electrons in atoms can have only very specific energies and can absorb only specific energies

• the energy emitted is the difference in energy between two electron energy levels or states

colors of light emitted from a H atom:

Emission spectrum of hydrogen: Rydberg’s explanation

÷÷ø

öççè

æ-= 2

221

11~nn

RHn

• RH is the Rydberg constant for hydrogen: 109,677.581 cm-1

• n1 and n2 are principal quantum numbers for the initial and final electronic states

• Rydberg decided to see how general Balmer's equation was

• Yes – it works for UV, IR, and other types of emr

• Rydberg's equation

n = 1/l~