Science

Physics Lab 5

The photoelectric effect

This lab will investigate the photoelectric effect. The photoelectric effect is the emission of electrons from a surface when illuminated with light of a certain frequency. The first insight to understanding this phenomenon was presented in 1900 by Max Planck. His formula, E = hf, related the energy of a photon to its frequency. Albert Einstein extended this idea of quantized photonic energy to a stream of photons (electromagnetic radiation) and explained the photoelectric effect.

This virtual experiment is a Java Applet. Applets require the JVM (Java Virtual Machine). It is not uncommon for the JVM to be outdated or not installed on a computer system, causing the applet to fail to run properly if it runs at all. In that case you should download (for free) and install the latest version from

http://java.com/en/download/manual.jsp

Some network administrators block Applets. In that case, contact your network administrator or try a different Internet access point.

Instructions:

1. Go to http://phet.colorado.edu/sims/photoelectric/photoelectric_en.jnlp

2. Increase the intensity to 50%. You should notice the ejection of elections from the surface.

3. Slowly increase the wavelength (λ) of the light until electrons are no longer ejected and record the wavelength (λ) in the table below.

4. Calculate the frequency (f) for that wavelength and record it in the table below. Remember that c=λf, where c is the speed of light (3 * 10^8 m/s).

5. Calculate the Energy (E) in joules for that wavelength and record it in the table below. Remember that E = hf, where h the Planck constant (6.6 *10^-34 j*s)

6. Complete the Energy (E) in electro-volt (ev) and record it in the table below. 1 electron-volt (eV) = 1.6 X 10-19 J

7. Repeat the above step for each of the metals under the pull down menu.

Metal

Wavelength λ (nm)

Frequency f

(Hz)

Energy E

(J)

Energy

(eV)

Sodium

Zinc

Copper

Platinum

Calcium

8. The minimum frequency of a photon that can eject an electron from a surface is called the threshold frequency, ft. What is the threshold frequency, ft, for each of the metals? HINT: revisit instructions 3 & 4.

Metal

Threshold Frequency ft (Hz)

Sodium

Zinc

Copper

Platinum

Calcium

9. The minimum amount of energy required for an electron to escape from a metal is called the work function (W) and is given by the equation W = hft. What is the work function for each of the metals in joules and electron-volts? HINT: revisit instructions 3 to 6.

Metal

Work Function E

(J)

Work Function E (eV)

Sodium

Zinc

Copper

Platinum

Platinum

Calcium

10. Make the following additional adjustments to the simulation.

· Check the box “current vs. light intensity”.

· Check the box “electron energy vs. frequency”

· Select the Sodium.

· Use violet light (about 400 nm).

· Vary the intensity of the light and observe any changes in the number of ejected electrons.

· Vary the intensity of the light and observe any changes in the number of ejected electrons.

11. What’s the relationship between the intensity of the incident light and the number of the ejected electrons?

12. What’s the relationship between the intensity of the incident light and the energy of the ejected electrons?

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