Night Vision Goggles
In the photo above, a U.S. Army aviator is using a pair of helmet-mounted night vision goggles to help him see in the dark.
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Night vision goggles (NVG) operate by collecting small amounts of light and amplifying the signal to the point that the human eye is able to see an image. This is achieved through the following process:
- Light is initially captured by an objective lens. The objective lens is the outermost lens of the goggles closest to the object itself.
- The collected light is sent into a photocathode. A photocathode consists of a photomultiplier tube that has a negatively charged plate inside of it. When the light hits the negatively charged electrode inside, electrons are emitted via the photoelectric effect.
- As the ejected electrons pass through the tube, the number of electrons is multiplied through the use of a micro channel plate. The micro channel plate works by causing the electrons to excite several atoms causing a chain reaction in which more electrons are emitted.
- The electrons speed down the tube towards a phosphor screen. When the electrons hit the screen, they stick, causing the light particles (known as photons) to be emitted and sent to another lens.
- The resulting image is then seen through a final lens by the eye.
- Learn More About the inner workings of a photomultiplier tube in the video below:
Using the information provided above, answer the following questions.
- Why does the phototube need to be in a vacuum?
- What is the objective of the photocathode?
- Do the electrons that are emitted from the photocathode have the same amount of energy as the incoming photons? If they don't have the same amount energy, where is the missing energy?