Thermal Radiation and The Doppler Effect (5.4 & 5.5) Our goals for learning How does light tell us the temperatures of planets and stars? How does light tell us the speed or rotation rate of a distant object?

How does light tell us the temperatures of planets and stars?

Properties of Thermal Radiation Nearly all objects emit thermal radiation (also called blackbody radiation), including stars, planets, you… An object’s thermal radiation spectrum (blackbody spectrum) depends on its temperature

Video -Blackbody Radiation https://www.youtube.com/watch?v=sUp_WZKZID4

This graph shows how many photons are given off at each wavelength for objects at four different temperatures. The wavelengths corresponding to visible light are shown by the colored bands.

Note that at hotter temperatures, more energy (in the form of photons) is emitted at all wavelengths.

 is a constant = 5.97 x 10-8 Watt/m2 K4 Stefan-Boltzmann Law P = power T = temperature  is a constant = 5.97 x 10-8 Watt/m2 K4 Remind students that the intensity is per area; larger objects can emit more total light even if they are cooler.

The higher the temperature, the shorter the wavelength at which the peak amount of energy is radiated (Wien’s law).

(the temperature is in Kelvin) Wien’s Law (the temperature is in Kelvin) Remind students that the intensity is per area; larger objects can emit more total light even if they are cooler.

How does light tell us the speed of a distant object? This figure from the book can give an introduction to the Doppler effect. The Doppler Effect: the change in wavelength due to motion between the wave source & the observer.

picture waves in water

With light, the change in wavelength is seen as a color change.

Measuring the Shift Stationary Moving Away Away Faster Moving Toward Toward Faster We measure the Doppler Effect from shifts in the wavelengths of spectral lines

How does light tell us the rotation rate of an object? Different Doppler shifts from different sides of a rotating object spread out its spectral lines This figure from the book can give an introduction to the Doppler effect.

Spectrum of a Rotating Object This figure from the book can give an introduction to the Doppler effect. Spectral lines are wider when an object rotates faster

Redshift The Redshift (z) is an approximation to the Doppler shift.  

Video- What is redshift? https://www.youtube.com/watch?v=FhfnqboacV0

What have we learned? How does light tell us the temperatures of planets and stars? Nearly all large or dense objects emit a continuous spectrum that depends on temperature. The spectrum of that thermal radiation tells us the object’s temperature. How do we interpret an actual spectrum? By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.

What have we learned? How does light tell us the speed of a distant object? The Doppler effect tells us how fast an object is moving toward or away from us. Blueshift:objects moving toward us Redshift: objects moving away from us How does light tell us the rotation rate of an object? The width of an object’s spectral lines can tell us how fast it is rotating