Light & the Electromagnetic Spectrum

What do you see?Old woman? Or young girl?

Is turning a light on and off a chemical or physical change?◦ Physical change

What creates light?

Question

Bohr Model – electrons are found in nrg levels UV

Light

Visible Light

Infrared Light

Niels Bohr

1913, looked at a hydrogen atom

Suggested an e- could move between nrg levels if discrete photons or nrg (amounts of nrg) were absorbed or emitted by the e-

e- falls from high low nrg = photon emission◦ Photon has a frequency, or color, that matches the

e- fall

Niels Bohr

When an electron moves from an excited state to its ground state, light nrg is given off.◦ Excited atom~ e- not in lowest possible nrg

state They have absorbed nrg and jumped to a further nrg

level The color of the light depends on the

amount of nrg. The more nrg levels jumped, the greater the

nrg of light.

Light

e- at higher nrg states are not stable

These e- want to return to their ground state to become stable◦ Lowest possible nrg level

Distance between nrg levels is NOT all the same.

Light

R O Y G B I V Blue light has a higher frequency than

red.

Electromagnetic Spectrum

Frequency – the # of wave cycles passing a point in a period of time.

As frequency increases, wavelength decreases.

wavelength

frequency

lightofspeedc __

sm81000.3

Planck assumed energy can be released and absorbed in discrete packets called quanta.

The energy of one quanta is:

Where,

known as Planck’s constant.

Max Planck

hE

sJh 341063.6

Light of a particular wavelength (λ) has a particular frequency (ν) and nrg (E).

If ν, λ or E are known, the other 2 can be calculated.

hE c

smc 81000.3

sJh 341063.6

speed of light

Planck’s constant

Yellow light given off by a sodium vapor lamp has a wavelength of 589 nm. What is the frequency of this radiation?

Practice

c

smc 81000.3

nmsm

589

1000.3 8

mnm1109

s11009.5 14

Unit for frequency (ν)◦ Hertz

Unit for wavelength (λ) ◦ Meter (m) or nanometer (nm)

Unit for energy (E)◦ Joule ( J )

Beware of UNITS!

Hzss

11

Calculate the energy of one photon of yellow light whose wavelength is 589nm.

Practice

hE

ssJ 11009.51063.6 1434

nmsm

589

1000.3 8

mnm1109

s11009.5 14

sJh 341063.6J191037.3

What do the following symbols stand for? What are their units?

ν

λ

h

c

E

Pop Quiz!

Frequency

Wavelength

Planck’s ConstantSpeed of Light

Energy

ssHz 11

nmm,

sJ

sm

J

Light’s wave nature is seen as its ability to diffract, reflect, refract.

Planck stated that nrg is radiated in discrete packets called quanta.

A photon is a quantum of light having the energy

Light’s particle nature is seen by the emission spectra of elements.

Wave Particle Duality of Light

h

Why each element produces a unique line spectra.

Eventually leads to a very famous equation by a very famous scientist.

2cmE