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# Light

## Light is a small range of the electromagnetic spectrum that we perceive with our eyes.

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Night Vision Goggles

### Night Vision Goggles

Credit: PEO Soldier
Source: http://en.wikipedia.org/wiki/File:PEO_ANAVS-6_NVG.jpg

A U.S. Army aviator is using a pair of helmet mounted night vision goggles to help him see during the night.

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Night vision goggles (NVG) operate by collecting small amounts of light and amplifying the signal to the point that we are able to see an image. This is achieved through the following process:

1. Light is initially captured by an objective lens.
2. 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.
3. 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 of electrons to be emitted.)
4. The electrons speeding down the tube towards a phosphors screen. When the screen is hit the electrons get stuck. This causes photons to be emitted from the screen and sent towards another lens.
5. The resulting image is then seen through a final lens by your eye.

#### Explore More

Using the information provided above, answer the following questions.

1. Why does the phototube need to be in a vacuum?
2. What is the objective of the photocathode?
3. Do the electrons that are emitted from the photocathode have the same energy as the incoming photons? If they don't have the same energy, where is the missing energy?

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