Aim: To investigate the light emitted from the sunApparatus: Computer Lab with the following software programmes and pictures

Tool: Download at http://www.cabrillo.edu/~dbrown/tracker/Other Software: Update to Java 1.5 (for windows) and Java 1.6 (Mac)Pictures: “solar spectrum.png”, “H.png”, “He.png”, “Na.png”, “Mg.png”

Contact: Lee Tat Leong (lee_tat_leong@moe.edu.sg), Wee Loo Kang (wee_loo_kang@moe.gov.sg)

Background:

A star often is modeled as a blackbody, and electromagnetic radiations emitted from these bodies are known as blackbody radiation. The figure shows a highly schematic cross-section to illustrate the idea. The photosphere of the star where the emitted light is generated is idealized as a layer within which the photons of light interact with the material in the photosphere.

The sun generates its energy primarily through nuclear fusion of hydrogen (isotopes) to helium.

For details, refer to the lecture on Nuclear Physics.

Activity 1 The solar emission spectrum1. Launch the software tracker.

Fig. 1: Tracker’s analysis tools

2. Click (or File|Open menu item) and select the picture “solar spectrum.png”.

3. To calibrate the points on the picture, click the calibration tool and select Calibration Points.

Fig. 2: calibrating the picture

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4. Click Create and choose a track type from the menu of choices. Most moving objects are tracked using a Line Profile.

By holding down the shift key, left click & drag with the mouse across the spectrum to create a line profile.

Change the spread to about 5 to take an average reading of the intensity of the spectrum. The plot on the right hand side shows the brightness/intensity (luma) against the

wavelenght in nanometres.

Fig. 3: Creating a line profile

5. Right-click on the plot and choose analyse… in the popup menu. Select the Coordinate checkbox, move the crosshair to read the wavelength (x-axis) and

the corresponding intensity (y-axis).

What are the wavelengths that the spectrum intensity dips by a large amount? Suggest with your reasoning why such dips occur.

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Fig. 4: detail data analysis

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Select the Data Builder … to build the Planck’s law for blackbody radiation.

where I, h, c, , k and T are the intensity, Planck’s constant, speed of electromagnetic radiation, wavelength (in m), Boltzmann constant and temperature of the body respectively. The constant is about for this picture.

Change the temperature T from the Data Builder… ranging from 4000 K to 7000 K, observe how the smooth curve changes.

Deduce the surface temperature of the sun.

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Activity 2 Exploring the absorption lines in the solar spectrum for specific wavelengths(Note: You do not need to use the Data Builder function for this activity)

Without closing the tracker software programme, analyse the emission spectrum of the Hydrogen, Helium, Sodium and Magnesium (“solar spectrum.png”, “H.png”, “He.png”, “Na.png”, “Mg.png”) in the same manner as the previous activity. Be systematic in naming the profile of the different pictures.

Suggest how scientists might have deduced the elements that are present in the sun.Hint: Summarise your finding for the two activities and relate the emission spectrum of the various

elements and the emission spectrum (and absorption lines) of the sun.

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