7/29/2019 2 Light

1/31

Light and Matter

Chapter 2

7/29/2019 2 Light

2/31

enLIGHTened Objectives

Models of light

light is a wave

light is a particle

Absorption and radiation of light by atoms

Measuring light to know composition of stars temperature of stars

motion of stars

7/29/2019 2 Light

3/31

Models of light

light is a wave light is a particle

So which one is right?

They are both right...and they are both wrong.

Thats called wave-particle duality

In some experiments, the wave model works best.

In other experiments, the particle model works best.

Thus, we use both.

7/29/2019 2 Light

4/31

Light is a wave propagating wave of oscillating electric and magnetic

fields

described by wavelength, , and frequency, f.

f = v where v is the speed of the wave.

In a vacuum, v = c = 3.00 x 108 m/s. large wavelength corresponds to small frequency and

small wavelength corresponds to large frequency.

synonyms for light are

electromagnetic wave electromagnetic radiation

radiation

visible light is light that our eyes are sensitive to; however,

that is not the only type of electromagnetic radiation

7/29/2019 2 Light

5/31

Wavelength and Frequency

7/29/2019 2 Light

6/31

Light comes in many

wavelengths When white light passes through a glass

prism (or a diffraction grating), itseparates into colors.

These colors have different wavelengths.

This group of wavelengths is the visiblepart of the electromagnetic spectrum.

When you see the entire spectrum with

no thin dark bands, it is a continuousspectrum.

7/29/2019 2 Light

7/31

Electromagnetic spectrum

7/29/2019 2 Light

8/31

Wavelength and frequency

7/29/2019 2 Light

9/31

Practice What is the wavelength of white light?

Which color of light has a longerwavelength purple or red?

Suppose that a certain medical treatment

requires exposing certain tissues to high

frequency radiation. Would that radiation

likely be gamma rays or radio waves?

7/29/2019 2 Light

10/31

Light is a particle Albert Einstein proposed that light consisted of

photons.

A photon is a particle or packet of energy.

A photon has an energy of E=hf where h is called

Plancks constant and f is frequency.

High frequency (low wavelength) photons havehigh energy; low frequency (high wavelength)

photons have low energy.

7/29/2019 2 Light

11/31

Absorption and emission

How is light absorbed and emitted by

atoms in interstellar gases or stars?

7/29/2019 2 Light

12/31

Bohr model(I hope its not bohring)

The Bohr model is aplanetary model, wherethe electron orbits thenucleus like a planet orbitsthe Sun.

An electron is onlyallowed in DISCRETE

orbits (n=1, n=2, n=3,etc.)

The higher the orbit, thehigher the energy of theelectron.

7/29/2019 2 Light

13/31

Modern view of hydrogen

Now, we know that the electron has

discrete energy levels, but it does notorbit the nucleus at fixed distances

from the nucleus. In fact, it may be

found anywhere in certain allowed

regions called orbitals. Each orbital

corresponds to a certain energy of the

electron.

7/29/2019 2 Light

14/31

Absorption, emission, and energy When an atom absorbs a

photon, it gains energy.

When an atom loses energy, itemits a photon.

An atom can only absorbphotons or emit photons of justthe right energy.

Those right energiescorrespond to theDIFFERENCES in energy

between the allowed energylevels.

Absorption

photon

Emission

photon

7/29/2019 2 Light

15/31

Hydrogen

-13.6 eV

-3.40 eV

-1.51 eV-0.850 eV

-0.544 eVn=5

n=4n=3

n=2

n=1

energy level energy

only certain energies areallowed

the change in the energy

between two levels

corresponds to a certaincolor photon absorbed or

emitted by the atom

the lowest energy level

is the ground state

higher energy levels are

called excited states

7/29/2019 2 Light

16/31

Absorption If light of a continuous spectrum is incident on a gas of

hydrogen atoms, then electrons will absorb some of thelight.

As a result, bands of the spectrum are missing; these arecalled absorption lines.

By the way, these same atoms emit the same colors in anemission spectrum!

7/29/2019 2 Light

17/31

Emission If excited hydrogen atoms fall to lower energy states,

photons will be emitted.

The emitted photons will be detected as light of certainbands of frequencies (i.e. colors).

The collection of bands (or lines) forms an emissionspectrum.

7/29/2019 2 Light

18/31

Whats so EXCITING? Sure, electrons get excited when they change

energy levels, by why do we get so excited?

Each element absorbs and emits a different set ofspectra.

By measuring the spectral lines, we can knowwhat element a gas is made of.

Now, we have way of determining what elementsstars like the Sun are composed of

Here are spectra for the most abundant elementsthat compose the Sun.

http://localhost/var/www/apps/conversion/tmp/scratch_5/spectra/index.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_5/spectra/index.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_5/spectra/index.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_5/spectra/index.html

7/29/2019 2 Light

19/31

Clouds of gas (nebulae) emit light, someby absorption and some by emission

7/29/2019 2 Light

20/31

emission

nebula

7/29/2019 2 Light

21/31

Practice

-13.6 eV

-3.40 eV

-1.51 eV-0.850 eV

-0.544 eVn=5

n=4n=3

n=2

n=1

energy level energy

See the Spectrumhandout

If an atom is in the ground

state (n=1) and is excited to

n=3, what energy photon wasabsorbed? What part of the

spectrum does this

correspond to?

If a hydrogen atom is in the

state n=4, to what level must

it fall in order to emit a

blue photon?

7/29/2019 2 Light

22/31

Practice If an atom absorbs a photon, does the atoms

energy increase, decrease, or remain constant?

Suppose that a gas of 4 hydrogen atoms has anatom in each of the 4 lowest energy levels. How

many distinct photons can be emitted by this gas?

Suppose that a particular gas will only emit a redphoton and a yellow photon. What colors will it

absorb if visible light is incident on the gas with

many of its atoms in the ground state?

7/29/2019 2 Light

23/31

Using light to know

the temperature of stars

7/29/2019 2 Light

24/31

Blackbody radiation A perfect absorber of

light is a blackbody.

A blackbody is also aperfect emitter.

The emissionspectrum of a

blackbody iscontinuous anddepends ontemperature.

T ~ 4000 K

7/29/2019 2 Light

25/31

Blackbody curves

7/29/2019 2 Light

26/31

Temperature and brightnessAs T increases,

the wavelength

for peak

brightnessdecreases (i.e.

shifts toward the

violet and

ultravioletwavelenghs).

As T increases,

the brightness

increases.

7/29/2019 2 Light

27/31

Using light to know

the motion of stars, planets, and other

objects in the sky

7/29/2019 2 Light

28/31

Doppler shift As a star approaches you, the frequencies of

the absorption lines increase (so the

wavelengths decrease). They are

blueshifted.

As a star recedes away from you, the

frequencies of the absorption lines decrease(so the wavelengths increase). They are

redshifted.

7/29/2019 2 Light

29/31

Blueshift

7/29/2019 2 Light

30/31

Redshift

7/29/2019 2 Light

31/31

Light tells us

what something is made of

by analyzing emission and absorption spectral

lines what temperature it is

blackbody curve

how fast it is moving toward us or awayfrom us

doppler shift of spectral lines