Light and the Atom

Created 25Mar2005by Dan Smith

Why do we want to look inside an atom?

• We want to know how an atom is structured inside. This will help us understand how different atoms interact to make compounds, and why some atoms are very reactive (like F and K), but other atoms are not reactive at all (like He and Ne).

How can we look inside an atom to see how it’s put together?

• In order to look inside something very small, one must use a probe that is even smaller.

• Few things are smaller than an atom; only subatomic particles (protons, neutrons, electrons)…and light waves.

• Subatomic particles are too destructive to use, but light can look inside an atom without damaging it.

Let’s understand the tool we’re using.

• We call light “Electromagnetic Radiation”.

• It’s composed of both an electrical wave and a magnetic wave.

• Visit this website to see the interlocking waves: (click on the red & blue wave)

http://www.monos.leidenuniv.nl/smo/index.html?basics/light.htm

Wave or particle?

• Just like Dr. Jekyll and Mr. Hyde, light can be thought of both as a wave and as a particle (called a photon or a quantum).

• In its particle nature, light behaves like a bouncing ball – it can reflect off a surface.

Wave or particle?

• In its wave nature, light can bend, just like when sound waves bend and spread out when passing through an open door. We call this bending refraction and diffraction.

• We’re going to concentrate on light’s wave nature.

First, the terminology

• Wavelength – the distance from one wave top or crest to the next crest

• The waves of light are so small that they’re measured in nanometers (1 x 10-9 meters)

• The symbol for wavelength is , the Greek letter “lambda”.

wave crestwave troughnode

Describing Light Waves

• 3 things are needed to adequately describe a light wave:

– its wavelength () in nanometers

– its frequency () in “waves per second”, also known as Hertz (Hz)

– its speed (c) in meters per second.

Wave frequency

• Frequency is the number of waves that pass a given point in 1 second. The unit is also known as a Hertz (Hz), or “cycles per second.”

• Your car’s radio dial is marked in kHz (kiloHertz) for ‘am’ stations and MHz (MegaHertz) for ‘fm’ stations.

More about Frequency

• The symbol for frequency is the Greek

letter “nu” ().

• Frequency and wavelength are inversely related to each other. As the wavelength gets shorter, more waves pass in 1 second (the frequency increases.)

How are they related?

• Frequency and wavelength are inversely related through the equation:

xc“lambda times nu = c”

“wavelength times frequency = c”

where “c” is the speed of light.

C - the speed of light

• c is the speed of light, a constant in a vacuum or in air.

• c = 3.00 x 108 meters / second (300,000,000 m/s or 300,000 km/s.)

• This ‘c’ is the same thing in Einstein’s famous E = mc2 equation.

Let’s try some calculations

• Example 1: What is the frequency of light, if the wavelength is 500 nanometers (nm)?

• Step 1: convert nanometers to meters.

500 nm = 500 x 10-9 m = 5.00 x 10-7 m

• Step 2: re-arrange the equation

If xc, then c /

• Step 3: insert the numbers

frequency = 3.00 x 108 m/s

5.00 x 10-7 m

= 6.00 x 1014 Hertz

You try this one…

• What is the frequency of light that has a wavelength of 400. nm?

• Don’t go on until you’ve tried this!

If you answered 7.50 x 1014 Hz, you’re right!

3.00 x 108 m/s / 4.00 x 10-7 m = 7.50 x 1014 Hz

Finding wavelength from frequency

• Finding the wavelength is similar to finding the frequency:

If xc, then c /

• What is the wavelength of light that has a frequency of 2.5 x 1014 Hz? Go to the next page after you’ve solved the problem.

If you said 1.2 x 10-6 m, you were right!

Now convert this answer into nanometers.

If you said 1.2 x 10-6 m = 1200. x 10-9 m, and this is 1200. nm, you were right again.

The energy value of light

• Light is energy, right? So how much energy is there in 1 wave or photon of light?

• The amount of energy depends on the light’s wavelength or frequency.

Max Planck

• In the early 1900’s a German physicist, Max Planck, determined that each photon or wave of light carries energy.

• Since shorter wavelengths of light have a higher frequency (more waves per second), shorter wavelength light should pack more energy.

That’s Max…groovy guy, huh?

So…shorter wavelength

= higher frequency…

= higher energy!

How much energy?

• The energy equation for light is…

Energy = a constant x frequency

or

E = h x

h is a constant called Planck’s Constant.h = 6.6 x 10-34 Joule / Hertz

Calculating the energy

• Example 2: How much energy is in light that has a wavelength of 450 nm?

• Step 1: Convert 450 nm to meters:

450 nm = 450 x 10-9 m = 4.50 x 10-7 m

• Step 2: Divide the speed of light by

wavelength to get frequency: = c 3.00 x 108 m/s / 4.50 x 10-7 m = 6.67 x 1014 Hz.

• Step 3: Multiply the frequency (result of step 2) by Planck’s Constant to get the energy: E = h x

(6.67 x 1014 Hz) x (6.6 x 10-34 J/Hz) =

4.40 x 10-19 Joules (in 1 photon

of this color of light).

Your turn to practice

• You try the following problem:

What is the frequency and energy of yellow light that has a frequency of 510 nm?

Did you get these answers?

= 5.10 x 10-7 meters = 5.88 x 1014 Hz 3.00x108 m/s / 5.10x10-7 m

E = 3.88 x 10-19 J 5.88x1014Hz x 6.6x10-34 J/Hz

Time for the quiz!

• Rank these 3 different colors of light from the least to the most energy:

• Light A has a wavelength () of 700 nm.

• Light B has a frequency () of 5.00 x 1014 Hz.

• Light C has an energy of 2.5 x 10-19 Joules/photon.

What’d you get?

• If you ranked them C, A, B (from least to most energy), you’re right on target.

Shift Gears – The Electromagnetic Spectrum

• Visible light is only a very tiny portion of the entire Electromagnetic Spectrum.

• Visible light has wavelengths from 400 nm (violet light) to 700 nm (red light).

• Most light we can’t even see, but other types of light are useful in chemistry.

Look at the previous slide again

• Notice how as the wavelength gets longer, the energy decreases.

• The different parts of atoms and molecules can be probed by different types of light.

Types of light & chemistry

• Low energy radio waves can be used to vibrate the nuclei of atoms.

• If you’ve ever had an MRI (magnetic resonance imaging), radio waves were shot into your body. The atoms of your body responded by giving back a “radar image” of your organs, bones, etc.

An MRI scanRadio waves in action!

Microwaves and Infrared

• Microwaves and Infrared (IR) light can be used to examine the chemical bonds between atoms in a molecule.

• Lots of chemical reactions also give off IR light – it’s the same as heat!

Chemical bonds

A reaction givingoff lots of infraredand visible light.

Ultraviolet light

• Ultraviolet (UV) light can cause lots of chemical reactions.

• 2 chemical reactions that are harmful are the tanning of your skin and the formation of cataracts in the lenses of your eyes.

X-rays and Gamma rays

• The last 2 types of light can (again) be used to look into the nuclei of atoms, and to strip electrons from atoms (ionization).

So just how does light affect the electrons within atoms?

• You’ve seen pictures of atoms that look like this:

• The circles represent “energy levels” within the atom.

Energy levels

• The energy levels are like floors within a building. Electrons can only exist on the energy levels, but never between them.

When a wave of light hits an atom,an electron can ride the wave up toa higher energy level. In the process, it absorbs the light’s energy. The electron gains energy.

Losing the energy that was gained

• After a short time, the energy that the electron gained is lost, and the electron falls back down to the lowest energy level that has room for it.

• Exactly how far the electron falls determines the energy (the color) of light that is given off.

A short fallproduceslow energylight.

A long fallproduceshigherenergylight.

It’s all in the spacing

• While every element is built basically the same way, the spacing between the energy levels is different for every element.

• Since the spacing is different, each different element will give off its own distinct set of colors when its electrons fall back to lower energy levels.

Identifying elements by their light

• The science of identifying the chemical elements by the colors of light that they give off is called spectroscopy.

• Spectroscopy can tell us just how the electrons within an atom are arranged.

Some characteristic colors

• On the following slides are some characteristic emission colors for different elements:

Lithium and Strontiumgive off red flames.

Copper gives offblue and green.

Potassium gives off alilac flame.

Sodium gives offa yellow flame...

…as does Carbon.

Try some spectroscopy for yourself

• Go to the following websites to explore the emission spectra of some common elements.

http://jersey.uoregon.edu/vlab/elements/Elements.html

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/linesp16.swf

Remind me

• Remind me when we’re back in the regular classroom…I’ll show you some first-hand emission colors from different elements.

• In the meantime, have fun with your homework!

• Yeah, that’s all folks.