From lowest energy to highest energy, which
of the following correctly orders the different
categories of electromagnetic radiation?
A) infrared, visible light, ultraviolet, X rays, gamma rays, radio.
B) visible light, infrared, X rays, ultraviolet, gamma rays, radio.
C) radio, X rays, visible light, ultraviolet, infrared, gamma rays.
D) gamma rays, X rays, visible light, ultraviolet, infrared, radio.
E) radio, infrared, visible light, ultraviolet, X rays, gamma rays.
From lowest energy to highest energy, which
of the following correctly orders the different
categories of electromagnetic radiation?
A) infrared, visible light, ultraviolet, X rays, gamma rays, radio.
B) visible light, infrared, X rays, ultraviolet, gamma rays, radio.
C) radio, X rays, visible light, ultraviolet, infrared, gamma rays.
D) gamma rays, X rays, visible light, ultraviolet, infrared, radio.
E) radio, infrared, visible light, ultraviolet, X rays, gamma rays.
The frequency of a wave is
The frequency of a wave is
A) the number of peaks passing by any point each second.
B) measured in hertz (Hz).
C) measured in cycles per second.
D) equal to the speed of the wave divided by the wavelength
of the wave.
E) all of the above.
A) the number of peaks passing by any point each second.
B) measured in hertz (Hz).
C) measured in cycles per second.
D) equal to the speed of the wave divided by the wavelength
of the wave.
E) all of the above.
We can see each other in the classroom
now because we
A) emit infrared light.
B) emit thermal radiation.
C) reflect visible light.
D) reflect infrared light.
E) emit visible light.
We can see each other in the classroom
now because we
A) emit infrared light.
B) emit thermal radiation.
C) reflect visible light.
D) reflect infrared light.
E) emit visible light.
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How are wavelength, frequency, and
energy related for photons of light?
How are wavelength, frequency, and
energy related for photons of light?
A) Longer wavelength means higher frequency and lower energy.
B) Longer wavelength means lower frequency and higher energy.
C) There is no simple relationship because different photons travel
at different speeds.
D) Longer wavelength means higher frequency and higher energy.
E) Longer wavelength means lower frequency and lower energy.
Learning from
from Light
Learning
Light
A) Longer wavelength means higher frequency and lower energy.
B) Longer wavelength means lower frequency and higher energy.
C) There is no simple relationship because different photons travel
at different speeds.
D) Longer wavelength means higher frequency and higher energy.
E) Longer wavelength means lower frequency and lower
energy.
Learning goals
•
•
•
•
Radio
Optical
What types of light spectra can
we observe?
What types of light spectra can we observe?
How does light tell us about composition?
How does light tell us about temperatures?
How does light tell us the speed of an object?
X-ray
Example: Solar Spectrum
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How does light tell us what
things are made of?
Hydrogen atom
Distinct energy levels lead to
distinct emission or absorption
lines
• Electrons in atoms
have distinct energy
levels.
• Each chemical
element, ion,
molecule, has a
unique set of energy
levels.
Light contains chemical
fingerprints
Thought Question
Which letter(s) labels absorption lines?
• Every atom, ion, and molecule has a unique
spectral “fingerprint.”
• We can identify the chemicals in gas by their
fingerprints in the spectrum.
• With additional physics, we can figure out the
chemicals’ abundances, densities,
temperatures, and more.
A
B
C
D E
C
D E
Thought Question
Which letter(s) labels the peak (greatest
intensity) of infrared light?
Thought Question
Which letter(s) labels absorption lines?
A
B
A
B
C
D E
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Thought Question
Which letter(s) labels the peak (greatest
intensity) of infrared light?
A
B
C
D E
Thought Question
Which letter(s) labels emission lines?
Thought Question
Which letter(s) labels emission lines?
A
B
C
D E
How does light tell us the
temperatures of planets and
stars?
Thermal Radiation
A
B
C
D E
Two Properties of Thermal Radiation
1. Hotter objects emit more light at all frequencies per unit area.
2. Hotter objects emit photons with a higher average energy.
• Nearly all large or dense objects emit thermal
radiation, including stars, planets, you…
• An object’s thermal radiation spectrum
depends on only one property: its
temperature.
Thought Question
Which is hotter?
a) A blue star.
b) A red star.
c) A planet that emits only infrared light.
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Thought Question
Which is hotter?
a) A blue star.
b) A red star.
c) A planet that emits only infrared light.
Thought Question
Why don’t we glow in the
dark?
a) People do not emit any kind of light.
b) People only emit light that is invisible to
our eyes.
c) People are too small to emit enough light for
us to see.
d) People do not contain enough radioactive
material.
Doppler shift tells us ONLY about the part of
an object’s motion toward or away from us!
Thought Question
Why don’t we glow in the
dark?
a) People do not emit any kind of light.
b) People only emit light that is invisible to our
eyes.
c) People are too small to emit enough light for
us to see.
d) People do not contain enough radioactive
material.
How does light tell us the
speed of a distant object?
The Doppler Effect
The amount of blue or red shift tells us an
object’s speed toward or away from us.
Thought Question
I measure a line in the lab at 500.7 nm.
The same line in a star has wavelength
502.8 nm. What can I say about this star?
a) It is moving away from me.
b) It is moving towards me.
c) It has unusually long spectral lines.
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Doppler Effect Summary
Thought Question
I measure a line in the lab at 500.7 nm.
The same line in a star has wavelength
502.8 nm. What can I say about this star?
a) It is moving away from me.
b) It is moving towards me.
c) It has unusually long spectral lines.
Motion toward or away from an observer causes a shift in
the observed wavelength of light:
• blueshift (shorter wavelength) ⇒ motion toward you
• redshift (longer wavelength) ⇒ motion away from you
• greater shift ⇒ greater speed
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