Understanding Light: Wave-Particle Duality, Spectrum, and Interaction with Matter - Prof. , Study notes of Astronomy

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ASTR1030 - FALL, 2008. LECTURE 12; PAGE 1

ASTR

Announcements

http://lasp.colorado.edu/~ergun/

ASTR1030/

Next Observing: Thursday, Oct. 2, 7:30 pm. SBO.

• This weeks reading: Chapter 6.

Today

• LIGHT!

ASTR1030 - FALL, 2008. LECTURE 12; PAGE 2

Light: The “Wave-Particle Duality”

Waves

:.

Frequency

Wavelength

×

Speed

f

λ

×

c

ASTR1030 - FALL, 2008. LECTURE 12; PAGE 4

Light

Spectrum

Radio Waves

are the same as light - the lowest frequency (longest wave-

length) part of the spectrum. We (humans) cannot sense radio waves. Infrared Radiation

has frequencies less than that of visible light (longer

wavelengths). We cannot see infrared radiation, but we can feel intense infra-red radiation as heat. Visible Light

is a small portion of the spectrum from 700 nm (red) to 400 nm

(violet). Remember, 1 nm = 1x

m. We (humans) see light.

ASTR1030 - FALL, 2008. LECTURE 12; PAGE 5

Light

Spectrum

Ultraviolet Radiation

has frequencies greater than that of visible light (shorter

wavelengths). We cannot sense ultraviolet radiation, but after exposure, we canfeel it’s effects (sunburn). X rays

have frequencies greater than that of ultraviolet (shorter wavelengths).

We cannot sense X rays. Many materials are transparent to X rays (i. e., theylook like glass or water to someone with X-ray vision), so X rays are very use-ful to “look” inside an object. Gamma Rays

have frequencies greater than that of X rays. Individual gamma

ray photons have a lot of energy and can damage a cell if they strike it - makingthem dangerous to humans.

ASTR1030 - FALL, 2008. LECTURE 12; PAGE 7

Light: Emission

ALL OBJECTS EMIT LIGHT!

The emitted light generally covers a large range in the spectrum.The peak in the spectrum is proportional to the temperature.

λ

peak

×^ T

Knm o (

ASTR1030 - FALL, 2008. LECTURE 12; PAGE 8

Light: Emission

ASTR1030 - FALL, 2008. LECTURE 12; PAGE 10

Light: Emission

What is the spectral peak of light emitted from our body?What is the spectral peak of light emitted from the sun? Important Point: We can tell how hot stars are from their color.

λ

peak

×^ T

Knm o (

λ

peak

×

K

o

Knm o (

λ

peak

nm

(Infared)

λ

peak

×

K

o

Knm o (

λ

peak

500

nm

(Yellow)

=

ASTR1030 - FALL, 2008. LECTURE 12; PAGE 11

Light: Emission

Every substance has a

distinct emission pattern

or finger print.

Important Point: We cantell what a planet or star

is made of by analyzingthe emission spectrum.

Magnified View

ASTR1030 - FALL, 2008. LECTURE 12; PAGE 13

Light: Doppler Shift

• As a car or train approaches, one hears a higher pitch sound. As it moves

away, one hears a lower pitched sound.

• Light is the same. As an object approaches, one sees higher-frequency light

(blue shift). As it moves away, one sees lower-frequency light (red shift)

ASTR1030 - FALL, 2008. LECTURE 12; PAGE 14

Light: Doppler Shift

Important Point: We can if a star is moving toward or away by analyzing

the light spectrum.

ASTR1030 - FALL, 2008. LECTURE 12; PAGE 16

Light

Doppler Shift

Example: The 557.7 nm O

green line is from a comet is seen at

557.8 nm. Is Jupiter moving toward or away from us? How fast inkm/s?A: Away. The wavelength is longer, so the light is red-shifted.

v - - c

λ

shift

λ

laboratory

λ

laboratory

v

c

∆λλ

0

---

v

c

∆λλ

0

---

(^8)

m

s ⁄

×

nm

nm

x

(^4)

m ---- s

km

s

• ASTR1030 - FALL, 2008. LECTURE 12; PAGE