DIRECTIONS: Click the lamp to turn on the light. When reading currents, click on the red button each time the brightness or color is changed.

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The animation above is a representation of the photoelectric effect. The large oval with an opening represents a tube with a window to let light in and two metal plates on either end. When light from the lamp enters the opening and hits the plate on the left, electrons are emitted. They cross the tube and hit the plate at the right end of the tube. A current is then measured by the ammeter. The wave theory of light says that since light is a wave, the energy of the light (wave) depends on the amplitude (or brightness) of the light (wave). The kinetic energy of the resulting electrons (called photoelectrons) is equal to the energy of the incoming light minus the amount of energy needed to free the electrons from the atoms of the plate (called the work function). The kinetic energy of the photoelectrons can be experimentally measured by using a battery to make the plate on the right negatively charged. The electrons can still hit the plate if they have enough energy. As the plate is made more and more negative, fewer and fewer electrons have enough energy to hit the plate. Finally, the plate can be made so negative that even the electrons with the most energy are unable to hit the plate and the current in the circuit (measured by the ammeter) goes to zero. The negative voltage that causes the current to go to zero is called the stopping potential.

Try each experiment below and answer the questions on your answer sheet.

Experiment #1: Change the color in order from blue down to red. What effect does changing the color have?

Experiment #2: Set the color to blue and change the brightness. What effect does changing the brightness have?

Experiment #3: Read the definition of stopping potential above. Set the color to blue and the brightness to High. Click the battery button then click each battery setting in order from low voltage to high voltage. Find the voltage that causes the current to go to zero and record it on your answer sheet. Repeat this for each color. Remember to click the red battery button every time you change the light color. What effect does changing the color have on the stopping potential?

Experiment #4: Do an experiment to answer the following question. For a given color, does changing the brightness change the stopping potential?