CEM 141 - Exam 2

What is electromagnetic radiation?

a wave that is oscillating in two perpendicular fields (light)

What are some examples of electromagnetic radiation?

• radio waves

• microwaves

• infrared

• visible

• ultraviolet

• x-rays

• gamma rays

What are the 3 ways that we characterize electromagnetic radiation?

• wavelengths

• frequency

• energy

What is different about radio waves and x-rays?

their wavelengths, frequencies, and energies differ

What is a light?

A. a wave

B. a particle

C. both

D. neither

C - both

True or false: electrons are both particles and waves

true

What makes light a wave?

Light doesn’t contain mass, yet it can transfer energy

When characterizing waves, what is the amplitude?

amplitude is the height of the wave (peak) or depth of the trough.

What is amplitude related to in light?

the intensity of the light

True or false: the lower the amplitude the higher the intensity (brightness) of light

False - the higher the amplitude the higher the intensity of light

When characterizing waves, what is wavelength?

The distance between any two identical points on the wave.

What greek letter is the symbol for wavelength?

lambda (wishbone & measured in m)

True or false: the type of light depends on its wavelength

true

What is the greek letter for frequency?

nu (squiggly v)

When characterizing waves, what is frequency?

The number of wave fronts that pass through a point in a given amount of time (typically 1 second).

What is the unit of frequency?

1 Hz = 1 1/s OR 1 s^-1

True or false: there is not an inverse relationship between wavelength and frequency

false - when you double frequency, wavelength is halved

How are wavelength and frequency related?

speed of light (c)

Energy of light

increases as frequency increases (and wavelength decreases)

True or false: energy of light is not related to amplitude

true

Which has higher intensity?

B

Which has higher frequency?

B

Which has a longer wavelength?

A

Which one is more likely to be red light (as opposed to blue)?

A

True or false: the violet/blue end of the electromagnetic spectrum has long wavelengths

false - the violet end has short wavelengths

What level of energy (high or low) does the violet end of the electromagnetic spectrum have?

high

What level of frequency (high or low) does the violet end of the spectrum have?

high

True or false: the red end of the electromagnetic spectrum has long wavelengths

true

What is the level of frequency (high or low) for the red end of the electromagnetic spectrum?

low

What level of energy (high or low) does the red end of the electromagnetic spectrum have?

low

List all of the light from right to left on the electromagnetic spectrum.

• long radio waves

• radio, TV

• microwave

• infrared rays

• visible light

• UV rays

• x-rays

• gamma rays

How many nanometers is the violet light on the electromagnetic spectrum?

~400 nm

How many nanometers is the red light on the electromagnetic spectrum?

~700 nm

True or false: the higher the logarithmic scale, the shorter the wavelength.

true - gamma rays (10^-12 = 10^-16 & 10^20 - 10^24)

Which electromagnetic radiation has wavelengths about the size of an atom?

x-rays (measured in nanometers)

What are the properties that provide evidence that light is a wave?

diffraction & interference —> shown in double-slit experiment

Diffraction

When the wave hits the barrier with the slit, it begins to spread out as they go through the opening.

True or false: particles diffract

false - only waves have the ability to diffract

If waves are in-phase (their peaks and troughs line up), does constructive or destructive interference occur?

constructive interference

constructive interference

crests and troughs reinforce (brighter light); only occurs if waves are in-phase

True or false: if waves are out of phase, destructive interference occurs.

true

destructive interference

crests and troughs cancel (darker light); only occurs if waves are out of phase

Double Slit Experiment

The marbles hitting a slit will strike the back wall and create a band of hits. If we have a double slit and aim a line of marbles at them, then we would expect to see a duplicate band of the first initial line. In the single slit, the waves hit the slit and radiate out, striking the back wall with the most intensity directly in line with the slit. When we add a double slit, the top of one wave will meet the bottom of another wave which causes them to cancel out and create an interference pattern of many bands on the back wall.

Which has the longest wavelength?

A. x-rays

B. visible

C. infrared

C - infrared

Why can’t we just classify light as a wave?

According to the wave model, energy should increase with the intensity but that does not occur in light (higher frequency & shorter wavelengths = higher energy).

Which has the highest frequency?

A. x-rays

B. visible light

C. infrared

A - x-rays

Which has the highest energu?

A. x-rays

B. visible

C. infrared

A - x-rays

What are the properties that provide evidence that light is a particle?

photoelectric effect

Photoelectric Effect

many metals emit electrons when electromagnetic radiation shines on the surface (garage door openers)

How does the photoelectric effect work?

the light is transferring energy to the electrons at the metal surface where it is transformed into kinetic energy that gives the electrons enough energy to “leave” the atoms in the metal.

Why are electrons stuck on atoms?

the electrostatic attraction between the negatively-charged electrons and positively-charged nucleus (protons).

If you increase the intensity of UV light does….

A. the number of electrons emitted increase

B. the number of electrons emitted decreases

C. no change

D. zero electrons are emitted

A - the number of electrons emitted increases

If you keep the intensity the same and increase the wavelength to blue light does…

A. the number of electrons emitted increase

B. the number of electrons emitted decreases

C. no change

D. zero electrons are emitted

C - no change in the number of electrons but they move slower

If you keep the intensity the same and increase the wavelength to yellow light does…

A. the number of electrons emitted increase

B. the number of electrons emitted decreases

C. no change

D. zero electrons are emitted

D - zero electrons are emitted

If you keep the yellow light and increase the intensity does….

A. the number of electrons emitted increase

B. the number of electrons emitted decreases

C. no change

D. zero electrons are emitted

C - no change (zero electrons are still emitted)

True or false: as you move towards long wavelength colored light (yellow, orange, red) no electrons are emitted

True - electrons are emitted as frequency increases and the length of the wavelength decreases

True or false: if the frequency of the light is above the threshold frequency, then electrons are not emitted from the metal

False - electrons are emitted and the amount of electrons depends on the intensity of the light (current)

True or false: if the frequency of the light is below the threshold frequency, no electrons are emitted

true - intensity of light has no impact (no current)

Photons

a little packet of energy; light

True or false: the energy of a photon is not quantized

False - photons can only have certain values

True or false: the photoelectric effect allows us to conclude that the energy of light depends on frequency and not the intensity

true - this is evidence for why light is also a particle

True or false: if a photon has enough energy, it will cause an electron to be ejected

true

True or false: you can add photons together to create enough energy to eject an electron

false - photons cannot be combined. a singular photon must have enough energy to eject an electron

What are the two ways energy can be transferred to molecules to break covalent bonds?

1. collisions with other molecules

2. collision with a photon

how many nm are gamma rays?

0.001 nm

how many nm are x-rays?

0.1 nm

how many nm is UV light?

10 nm

How many nm is visible light?

1000 nm

how many nm is infrared light?

1 × 10³ nm

how many nm are microwaves?

1 × 10^7 nm (1 centimeter)

how many nm are radio waves?

1 × 10^9 nm (1 meter)

how many nm are long radio waves?

1 × 10^11 nm

How is a sunburn formed?

if the photons emitted by the sun (UV rays) contain enough energy , then they can damage biological molecules by breaking their internal bonds.

True or false: electromagnetic radiation can be described as either a particle or a wave

true

True or false: matter and energy behaves the same at a small scale and in the macroscopic world

false - the wave-particle duality is important at small scales and the wavelength of the macroscopic objects is much smaller than the object and does not affect its properties.

True or false: white light contains every visible color in the spectrum

true

True or false: light from the sun can be separated into their varying wavelengths with a prism

true (can view full visible spectrum)

True or false: atoms can not emit light

false - atoms can emit light but they will not contain all the colors of the spectrum because specific elements only have a few wavelengths

True or false: atoms can absorb light

true

Why does the atomic absorption spectrum contain black lines?

The black lines represent light that did not pass through the atoms because those wavelengths were absorbed by the atom.

True or false: the wavelengths of an atom's emission lines are different then the wavelengths of its absorption lines

FALSE - the black lines on the absorption spectrum appear as colored lines on the emission spectrum

Can each element absorb or emit every wavelength of light in the visible spectrum?

A. Yes

B. No

B - no

True or false: the spectrum of an element is the same whether that element is on Earth, in the sun, or in a galaxy light years away

true

Why did we alter Rutherford’s model?

the model does not explain the atomic absorption & emission spectrum. not sustainable because the atom would have imploded.

Bohr’s model

• electrons move in orbits around the nucleus

• the orbits have definite energies and are at definite distances from the nucleus (quantized)

• explained the emission & absorption spectrum with discrete energy levels

• photons are emitted or absorbed by atoms as electrons move from one energy level to another

• energy of photons corresponds to the difference in energy between orbits

True or false: an electron moves to a higher energy orbit when a photon is absorbed

true (electron moves to lower energy orbit when photon is emitted)

What is the only element that the bohr model works for?

hydrogen

What were the problems with Bohr’s model?

• only works for hydrogen

• transition of electrons upon absorption and emission of photons is better demonstrated with energy diagrams

Energy diagrams

• each energy level has a quantum number

• higher number = higher energy

• energy levels are not orbits

• electrons transition between energy levels by absorbing or emitting photons with energies equal to the exact difference in energy between two levels

True or false: when an emission spectrum is produced, the energy diagram shows the atoms moving down energy levels

true

True or false: when determining which electron will release the largest amount of energy from a diagram, it is more important to look at the number of energy levels than the difference in energy.

False - only look at the difference in energy

Why can a specific element only absorb or emit certain wavelengths of light in the visible spectrum?

Electrons can only have the energies of the levels. Basically, only certain colors are emitted because they are equivalent to the amount of energy required by each level.

How are absorption and emission different from the photoelectric effect?

In the photoelectric effect, you are shining a beam of light on a piece of metal and the atom absorbs a photon resulting in the ejection of an electron. The electron completely leaves the atom (ionization). In absorption and emission, electrons remain on the atom but change energies.

de Broglie

all matter has wave properties and, therefore, a wavelength.

True or false: de Broglie is for light only, not matter

FALSE - de Broglie is for matter only; not light

True or false: wavelength is much smaller than macroscopic objects and is about the same size as atomic-scale objects

true - this means that the wavelength impacts atomic-scale objects but not macroscopic objects

What is wavelength measured in?

meters

What is the evidence that electrons are waves?

when electrons are used in the double slit experiment, they show an interference pattern.

Heisenberg Uncertainty Principle

we cannot measure accurately both the energy and position of an electron

What must our model of the atom include?

• electrons have wave-like properties

• electrons in an atom can only have certain energies

• since we know the energy of the electron, we can’t know its exact position