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Basic Idea:

• Measure the stopping potential and binding energy for ejected electrons.

• 3 (each setup can accomodate two groups)

	What:	How Many:	Where:
	Pasco Mercury Light Source See photo below	1	non-Majors storeroom, shelf DD3
	Light Aperture See photo below	2	non-Majors storeroom, shelf DD4
	Lens/grating assembly (blazed) See photo below	2	non-Majors storeroom, shelf DD4
	Photoelectric head See photo below	2	non-Majors storeroom, shelf DD4
	Support base See photo below	2	non-Majors storeroom, shelf DD4
	Coupling bar See photo below	2	non-Majors storeroom, shelf DD4
	Filters (green & yellow) See photo below	2 each	non-Majors storeroom, shelf DD4
	Digital Multimeter	2	Majors storeroom, shelf A3
	Wires	6 – 36" or 48" 4 – 24"	104, 152 lab
The following components are shared:
	40v regulated power supply	2	Majors storeroom, shelf C2
	Digital Multimeter	1	Majors storeroom, shelf A3
	Wires	3 – 12" 4 – 24"	104, 152 lab

Computers Needed?

• Yes (see KaleidaGraph setup note below!).

• Lab setup (See this page for photo details; place apparatus at an angle for easier access):

  

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• Mercury light source:

  
  Image courtesy PASCO Scientific.

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• Photoelectric head, yellow & green filters:

  
  Image scanned from PASCO Scientific catalog.

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• Light aperature, support base, lens/grating assembly (blazed), coupling bar:

  
  Image scanned from PASCO Scientific Instruction Manual.

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• The whole setup (connect two per mercury source):
  • Note: Attach the lens/grating assembly so that its label is on the same side as the electrical connections in the photoelectric head! This will put the brighter side of the spectrum on the "near" side of the student.

  
  Image scanned from PASCO Scientific Instruction Manual.

• Power supply connection:
  • Note: Main DC power panel must be disconnected before the 40v power supplies is used! Jeff will take care of this.
• Two 40v DC power supplies are connected in series; both are set to 9.0 volts.

Photoelectric head power connection:

Note the additional ground wire connected to the green connector on the power panel. Only the newer photo heads require this connection, yet all will work properly when connected thusly.

• Aligning the photo head:

  Turn on the Hg light source. Slide the lens/grating assembly on the Hg source back & forth on its supports until the light is focused on the reflective white mask of the photo head. Roll the light shield of the photo head out of the way, noting the white photodiode mask inside. Rotate the photo head until the image of the aperature is centered on the window in the photodiode mask. Tighten thumbscrew on the base support rod. Again slide the lens/grating assembly back and forth to focus a sharp image of the aperature on the window of the photodiode mask. Tighten thumbscrews to lock in place.

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• KaleidaGraph setup note:
  If students don't enter their frequencies in PHz (as stated in the directions), the "Linear w/uncertainties" fit must be adjusted to the following to prevent an overflow error:

  a + b*x; a=100; b=1

  Normally, a = 1 would suffice. This adjustment will allow the students to enter their frequencies with the 10¹⁴ power.

• Instructor's Note: Why can you see the UV line at 365 nm?
  • From Pasco's Instruction Manual: "The white reflective mask on the h/e apparatus is made of a special fluorescent material. This allows you to see the ultraviolet line as a blue line, and it also makes the violet line appear more blue. You can see the actual colors of the light if you hold a piece of white non-fluorescent material in front of the mask. (The palm of your hand works in a pinch, although it fluoresces enough that the UV line will still be visible.)"
  • For demonstration, use a piece of cardboard (e.g. from the pad of graph paper) or a manilla folder!

©	St. Lawrence University	Department of Physics

Revised: 23 Apr 09

Canton, NY 13617