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The Basic Idea of Photoelectric Effect

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Author: Jerry Silver

The Idea

In 1905, during his "miracle year," Albert Einstein published five papers. These included special relativity, which dealt with space and time, as well as general relativity, which related mass and energy through the equation E = mc2. However, Einstein won his only Nobel Prize for work he did that same year on the photoelectric effect.

At the time, it was known that light shining on certain materials could knock out electrons to produce a current. It stood to reason that the stronger the light, the greater the current. Researchers also found that how much of a kick the electrons got (or how much kinetic energy they had) depended on the color of the light. Many scientists expected a stronger light would also release an electron with greater energy. It took Einstein's brilliance to understand why the color (or frequency) of the light played such a key role in determining how much energy the electrons came away with. The consequences of this insight, along with the contributions of many other scientists, lead to the development of quantum mechanics, which is the basis for the modern electronic world.

Photoelectric Effect

This project introduces you to the idea of the photoelectric effect and guides you to recreate the type of data Einstein interpreted.

What You Need

The basics

  • piece of zinc metal
  • sandpaper or steel wool
  • short jumper wire
  • source of ultraviolet light (a carbon arc lamp or possibly a strong "black light")
  • source of visible light (incandescent lamp)
  • plate of glass
  • electroscope, either purchased or built as a project

Photoelectric effect apparatus

  • photoelectric effect apparatus, such as the Daedelon EP-05 (available from www.daedelon.com)
  • variable DC voltage source
  • voltmeter or multimeter configured as a voltmeter
  • various light sources of known frequency: this includes laser pointers of known wavelength, incandescent, carbon arc, or ultraviolet lights
  • color filters with known wavelength of transmitted light

Method

The basics

This part introduces the basic idea of the photoelectric effect and brings you to the dilemma Einstein addressed.

  1. Rub the piece of zinc with a piece of sandpaper or steel wool. This removes oxides to expose the metal.
  2. Discharge the electroscope by touching your finger to the electrode.
  3. Using a very short jumper, attach the zinc to the electroscope.
  4. Darken the room.
  5. Shine the light from an ultraviolet source onto the zinc.
  6. Observe the effect on the electroscope leaves.
  7. Discharge the electroscope and compare the effect of the ultraviolet source and the visible source. Also compare the effect of shining the ultraviolet source through a pane of glass that transmits mostly visible range light, but hardly any ultraviolet light.
  8. Charge the electroscope positively and observe the effect of shining ultraviolet light on the zinc.
  9. Charge the electroscope negatively and observe the effect of shining the ultraviolet light on the zinc.
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