Lecture 7 ASTR 111 – Section 002. Outline Discuss Quiz 5 Light –Suggested reading: Chapter 5.1-5.2 and 5.6- 5.8 of textbook.

Lecture 7

ASTR 111 – Section 002

Outline

• Discuss Quiz 5

• Light– Suggested reading: Chapter 5.1-5.2 and 5.6-

5.8 of textbook

Light travels through empty space at a speed of 300,000 km/s

• In 1676, Danish astronomer Olaus Rømer discovered that the exact time of eclipses of Jupiter’s moons depended on the distance of Jupiter to Earth

• Light travels at 300,000 km/sec

• About how fast does your car travel in km/hour when you are on the freeway?

• About how fast does your car travel in km/sec when you are on the freeway?

• If it takes light 8 minutes to travel from the sun to Earth, how long would it take you to drive?

• Approximately what was the difference in time of the eclipses that Olaus Rømer observed?

Determining the Speed of Light

• Galileo tried unsuccessfully to determine the speed of light using an assistant with a lantern on a distant hilltop

• In 1850 Fizeau and Foucalt also experimented with light by bouncing it off a rotating mirror and measuring time

• The light returned to its source at a slightly different position because the mirror has moved during the time light was traveling

• d=rt again gave c

Light is electromagnetic radiationand is characterized by its wavelength ()

Frequency and Wavelength

f or unit time

1unit length

The Greek letter “nu” and not the letter “v”

• Cars are traveling at 100 km/hr to the right• What would you need to know in order be

able to tell how often a car will pass the finish line?

Finish line

velocity = 100 km/hr

Finish line

v = 100 km/hr

Replace cars with lines

• Cars are traveling at 100 km/hr to the right• What would you need to know in order be

able to tell how often a peak will pass the finish line?

Finish line

v = 100 km/hr

Finish line

cDistance between peaks

How often peakpasses finish line

How fast wave moves to right

Interference - destructive

A

B

Interference - destructive

C

D

Interference

A

B?

Interference - simple

C

D?

?

The electromagnetic spectrum

• Because of its electric and magnetic properties, light is also called electromagnetic radiation

• Visible light falls in the 400 to 700 nm range

• Stars, galaxies and other objects emit light in all wavelengths

• Which of the following has the highest frequency?– Visible light– Radio waves– Microwaves– X-Rays– Infrared light– Ultraviolet light– Gamma rays

• Which of the following has the highest wavelength?– Visible light– Radio waves– Microwaves– X-Rays– Infrared light– Ultraviolet light– Gamma rays

• Which of the following has the highest speed?– Visible light– Radio waves– Microwaves– X-Rays– Infrared light– Ultraviolet light– Gamma rays

• Which of the following has the highest energy E (h is a constant)?– Visible light– Radio waves– Microwaves– X-Rays– Infrared light– Ultraviolet light– Gamma rays

hc

E

The dual nature of light

• Particle

• Wave

Particle

What would you expect if instead of a laser beam you used yellow spray paint beam?

Draw it!

Wave

The atom and light

• An atom has a small dense nucleus composed of protons (and neutrons)

• Rutherford’s experiments with alpha particles shot at gold foil helped determine the structure

Probing the atom

Spectral lines are produced when an electron jumps from one energy level to another within an atom

• The nucleus of an atom is surrounded by electrons that occupy only certain orbits or energy levels

• When an electron jumps from one energy level to another, it emits or absorbs a photon of appropriate energy (and hence of a specific wavelength).

• The spectral lines of a particular element correspond to the various electron transitions between energy levels in atoms of that element.

• Bohr’s model of the atom correctly predicts the wavelengths of hydrogen’s spectral lines.

Measurements in Astronomy

• In astronomy, we need to make remote and indirect measurements– Think of an example of a remote and indirect

measurement from everyday life

Using Light

• Light has many properties that we can use to learn about what happens far away

• Light interacts with matter in a special way

• Only photons with special wavelengths will interact with atom

• How will this affect what a person will see at point X?

• When is the atom “hotter”?

X

• Why is UV light usually blamed for skin cancer? What is special about it compared to other light sources?

What will the spectrum look like here?

Emission line spectrum

Continuous Spectrum

• A blackbody emits photons with many energies (wavelengths) – a continuous spectrum

• A prism bends photons more or less depending on their wavelength

What will the spectrum look like here?

Absorption Spectrum

Absorption vs. Emission

• What type of spectrum is produced when the light emitted from a hot, dense object passes through a prism?

• What type of spectrum is produced when the light emitted directly from a cloud of gas passes through a prism?

• Describe the source of light and the path the light must take to produce an absorption spectrum

• There are dark lines in the absorption spectrum that represent missing light. What happened to this light that is missing in the absorption line spectrum?

From Lecture Tutorials for Introductory Astronomy, page 61.

Each chemical element produces its own unique set of spectral lines

• Stars like our Sun have low-density, gaseous atmospheres surrounding their hot, dense cores. If you were looking at the spectra of light coming from the Sun (or any star), which of the three types of spectra would be observed?

• If a star existed that was only a hot dense core and did not have a low-density atmosphere surrounding it, what type of spectrum would you expect this particular star to give off?

• Two students are looking at a brightly lit full Moon, illuminated by reflected light from the Sun. Consider the following discussion between two students about what the spectrum of moonlight would look like:– I think moonlight is just reflected sunlight, so we will see the Sun’s

absorption line spectrum.– I disagree, an absorption spectrum has to come from a hot, dense

object. Since thie Moon is not a hot, dense object, it can’t give off an absorption line spectrum.

Do you agree or disagree with either or both of these students? Explain your reasoning.

• Imagine that your are looking at two different spectra of the Sun. Spectrum #1 is obtained using a telescope that is in a high orbit far above Earth’s atmosphere. Spectrum #2 is obtained using a telescope located on the surface of Earth. Label each spectrum below as either Spectrum #1 or Spectrum #2.

• Would this make sense?

This dark line was removed

Energy and electromagnetic radiation Planck’s law relates the

energy of a photon to its frequency or wavelength

E = energy of a photonh = Planck’s constantc = speed of light = wavelength of light

The value of the constant h in this equation, called Planck’s constant, has been shown in laboratory experiments to be

h = 6.625 x 10–34 J s

hc

E

Three Temperature Scales

Color and Temperature

An opaque object emits electromagnetic radiationaccording to its temperature

http://www.straightdope.com/mailbag/mhotflame.html

Blue: Hot or Not?