Companion to the reading passage The Light in the Dark: Einstein, the Photoelectric Effect, and the Battle for Reality. Answer each question, then click Check. Multiple-choice and number questions grade themselves; for written answers, compare yours with the model answer and checklist, then score yourself honestly.
In 1905, Albert Einstein proposed that light transfers energy in separate packets, now called photons, an idea that later earned him the Nobel Prize. A group of students tested this idea. They placed a clean zinc plate on top of an electroscope and gave the plate a negative charge, which made the electroscope’s leaves spread apart. If light frees electrons from the zinc, the plate loses charge and the leaves fall. To free one electron from zinc, a photon must have at least 6.9 × 10−19 joule of energy. The students shone three different light sources on the plate for 60 seconds each. Their results are shown in Table 1.
Table 1: Light Sources Shone on a Negatively Charged Zinc Plate
| Light Source | Frequency (Hz) | Brightness | Did the Leaves Fall? |
|---|---|---|---|
| Red laser | 4.6 × 1014 | very bright | no |
| Green laser | 5.6 × 1014 | very bright | no |
| Ultraviolet lamp | 1.2 × 1015 | dim | yes |
Data are representative values for practice.
1. A student makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response using a mathematical relationship.
2. The students then made the ultraviolet lamp brighter without changing its frequency. Which row correctly describes the effect of this change?
A silicon solar cell uses the same idea. When a photon is absorbed by the silicon, it can free an electron so that the electron is able to move through a circuit as part of a current. In silicon, a photon must have at least 1.8 × 10−19 joule of energy to do this. Engineers tested whether a silicon cell responds to the invisible infrared light from a TV remote control, which has a wavelength of 9.40 × 10−7 meter. They also tested whether it responds to the infrared radiation given off by a warm rooftop, which has a frequency of about 3.0 × 1013 hertz.
3. Which mathematical representation can be used to determine the frequency of the light from the TV remote control?
4. Substitute numerical values into a mathematical representation that can be used to determine the energy of one photon of the infrared radiation given off by the warm rooftop. Then calculate the energy.
On a clear day, sunlight delivers 1600 joules of energy each second to a rooftop solar panel. In that same second, the panel delivers 400 joules of electrical energy to a home. The rest of the energy is reflected from the panel’s surface or warms the panel, and is grouped as Eother. Engineers note that a hotter panel converts sunlight less efficiently, so they mount panels with an air gap underneath to let them cool.
5. Complete the energy bar graph for the sunlight–solar panel system during one second. The fixed ET column represents the light energy reaching the panel (1600 J), already completed for you. Set the Electrical column (electrical energy delivered) and the Eother column (thermal and reflected). This system has no kinetic-energy column in the original worksheet, so leave the KE bar at 0.
Click a box to set each bar’s height; click it again to clear the bar to 0. The ET bar is drawn for you.