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Light and Temperature. Why Infrared Cameras Work. Goals. The light that we see: where does it come from? Understanding the light of heat. On a sunny day: Why does it seem hotter wearing a black T-shirt versus a white one? Why are they different?. Emission lines. Absorption lines.

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Light and temperature

Light and Temperature

Why Infrared Cameras Work


Goals
Goals

  • The light that we see: where does it come from?

  • Understanding the light of heat.

  • On a sunny day:

    • Why does it seem hotter wearing a black T-shirt versus a white one?

    • Why are they different?


A spectrum

Emission lines

Absorption lines

Continuum

A Spectrum

  • A spectrum = the amount of light given off by an object at a range of wavelengths.


Three reasons
Three Reasons

All objects do one or more:

  • Reflect light because of color or smoothness (same as scatter)

  • Emit light because of their temperature

    (thermal radiation)

  • Emit or absorb light because of their composition

    (spectral lines)

    A person, house, or the Moon: reflects visible light, and because each is warm, emits infrared light.


Reflection absorption and scatter
Reflection, absorption, and scatter

  • Why do you see me?

  • Why do I see you?

  • Why is your shirt blue?

  • Why is this paper white?

  • Why is the table top black?


Temperature and light
Temperature and Light

  • Warm objects emit light.

    • Thermal radiation


Thermal radiation laws
Thermal Radiation Laws

  • Hotter is bluer.

    • (peak at shorter wavelength)

  • Hotter is brighter.

    • (More intense at all wavelengths)


What is light
What is Light?

  • Recall: Light is an energy wave.

    • The wavelength (l) and frequency (n) are related:

      c = ln

  • New: The energy of the wave is inversely proportional to the wavelength (where h is a constant):

    E = hn

    E = hc/l


Atoms in motion
Atoms in Motion

  • Everything is composed of atoms which are constantly in motion.


Temperature
Temperature

  • The hotter the object, the faster the average motion of the atoms.

COOLER

HOTTER


Atoms and light
Atoms and Light

  • As atoms move they collide (interact, accelerate).

  • Collisions give off energy.

  • But light IS energy.

E = hc/l


Light and temperature1
Light and Temperature

  • The hotter the object the faster the average atom and the more energetic the average collision.

  • The faster the atoms the more collisions there are.

HOT

COLD


Energy and intensity
Energy and Intensity

  • The more energetic the average collision the bluer the average light that is given off.

    • Since E = hc/l

  • The more collisions that occur the more light that is given off per surface area.


Thermal radiation laws1
Thermal Radiation Laws

  • Put another way:

    Wien’s Law for peak wavelength (lpeak):

    lpeakis proportional to 1/T

    Stefan-Boltzmann Law for total Flux (F):

    F is proportional to T4


Graphically

lpeaka 1/T F a T4

Graphically


Result

IR

Result

  • HOT toasters are BRIGHTER than cool toasters.

  • HOT toasters are BLUER than cool toasters.

  • What is the peak wavelength for something at room temperature (a cool toaster or a cool person)?

    lpeaka 1/T

    lpeak= k* 1/T

    lpeak= (3 x 10-3 m/K) * 1/ 300 K

    lpeak= 10-5 m


Thermal versus reflection
Thermal versus Reflection

  • Thermal radiation is light given off because of an object’s temperature.

  • Don’t confuse with reflected light:

    • Buses are yellow not because they are hot enough to emit visible radiation but rather they reflect the yellow light given off by the Sun.

  • What kinds of thermal radiation do we see in our everyday life?


The ir world
The IR World

  • Everyday objects (at everyday temperatures) emit thermal radiation in the IR, this is why we equate IR with HEAT.

http://www.x20.org/library/thermal/blackbody.htm


The ir universe

Orion - visible

Orion – by IRAS

The IR Universe

  • Everyday things that are hot radiate in the IR:

  • Dust – There are interstellar clouds of dust.


The ir universe1
The IR Universe

Io from IRTF.

  • Molten Rock – There are lava flows on a moon of Jupiter.

Orion – by IRAS


The ir universe2

The Moon in eclipse.

R. Gendler

The IR Universe

  • In eclipse, there is no reflected light.

  • Only thermal radiation.

  • Differences in composition lead to differences in temperature.

Orion – by IRAS


The greenhouse effect
The Greenhouse Effect

  • Why is my car hot on a summer day?

  • At T = 6000 K, the Sun radiates mostly visible light.

    Windshield is transparent to visible light.

  • Car seat absorbs this visible light and warms up to 400 K.

  • At T = 400 K, my seat radiates mostly at longer wavelengths in the IR.

    Windshield is opaque in the IR.

  • Result: Energy is TRAPPED inside the car!


Venus and earth
Venus and Earth

  • Certain gases act the same way as your windshield: Carbon Dioxide (CO2).

  • Venus – Runaway greenhouse effect.

  • Earth – Could that happen here?


Color why s
Color Why’s

  • Why is that shirt blue?

  • Why is the Sun yellow?

  • Why is this paper white?

  • Why is the light filament orange?

  • Why is Mars red?

  • On a sunny day:

    • Why does it seem hotter wearing a black T-shirt versus a white one?


Homework 4
Homework #4

  • For 9/19:

  • Reread B6.4 – 6.5

  • For 9/23:

  • Read B14

  • Do: Review Questions 7, 11, 13 and:

    • Why do we see absorption lines in the spectrum of the Sun? In your answer, address the following: where is the source of the light which is the continuous thermal spectrum? What is the material doing the absorbing and where is it? Which is hotter, and why does this give rise an an absorption spectrum as viewed from Earth?