Lecture9 Blackbody

# Lecture9 Blackbody - Learning From Light Continue...

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Continue developing your conceptual model for how light & matter interact: + what is an object’s luminosity? + how does a star’s temperature, luminosity & size relate? + how do we measure temperature of an object from the light it emits? Learning From Light

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We learned last week that: Light can act like a particle or a wave (wave/particle duality). Wavelength is the distance between two adjacent wave peaks. Frequency is the number of times per second that wave goes through an area. Particles of light are called photons . Each photon has a specific wavelength and frequency, c = ! ƒ Frequency " wavelength Energy of a photon depends on its frequency, E ~ ƒ Frequency " Energy
We learned last week other kinds of light: Wavelength of the visible light is very small (400 -- 700 nm), where the width of human hair is 10,000 nm! Electromagnetic spectrum is a full range of wavelengths. Only a fraction of this radiation reaches us!

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Carefully studying the features in a spectrum of the Sun or any astronomical object tells us a great deal! In astronomy, light is all we have! composition temperature motions (velocity) How: compare to lab spectrum Assume: laws of physics are universal
Nearly all dense objects emit heat radiation, or thermal radiation , including: planets, stars & you! We disperse the light (spread it out) into a spectrum to gain the most information. Sp ec tr a l curve : intensity of light at a given wavelength over a range of wavelengths. How Do We Learn From Light? continuous spectrum

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Which statement about the spectrum is true ? A. equal light is emitted at every color B. the spectrum peaks (is brightest) in yellow light C. the spectrum peaks at green-blue wavelengths D. the spectrum peaks at a wavelength of 400nm Question Energy Output (per second)
Which statement about the spectrum is true ? A. equal light is emitted at every color B. the spectrum peaks (is brightest) in yellow light C. the spectrum peaks at green-blue wavelengths D. the spectrum peaks at a wavelength of 400nm Question Energy Output (per second)

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Blackbody absorbs all electromagnetic radiation that strikes it. It heats up and emits energy (at each wavelength different intensities than received ). Emitted energy depends only on T and size (surface area) of BB. Blackbody Iron bar, light bulb & stars are examples of blackbody. Blackbody’s spectrum has a continuous energy distribution over a particular wavelength range.

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Temperature of a Blackbody • Iron rod heated in a darken room: 1. gets brighter : emits more radiation at all ! 2. changes color: from red (coolest of all glowing bodies) to blue/white (hottest of all glowing bodies) • What happens if you put a light emitted by the rod through a prism or diffraction grating? All ! are always present in its spectrum, but the ! of maximum emission is changing.
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