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Q & A: photographic plate light sensitivity

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Most recent answer: 10/08/2012
Q:
why photographic plate are not affected by red light but are immediately affected by white light?
- suman (age 26)
nepal
A:
Great question. The reason lies in the quantum mechanical nature of how light interacts with matter.

In a traditional photographic plate, the image is formed when crystals of some silver halide (e.g. silver bromide) partially convert to silver crystals. (A more complete and detailed description of the process can be found on Wikipedia: ) The process is triggered by light. The first step in the process involves an electron picking up enough energy from the light to come loose and move freely. There's a minimum amount of energy required for that to happen.

Now here's the key point: the energy from the light is not gradually absorbed by the electrons, as one might expect classically. Instead, it's transferred in packets ("quanta") of hf of energy, where h is Planck's constant and f is the light frequency. When the frequency is too low, for red light, each packet just doesn't have enough energy to knock an electron loose.

Similar physics is involved in many important processes. For example, the most important damage to your skin from ultraviolet light occurs because the quanta (photons) of this high-frequency light are energetic enough to break DNA molecules, unlike the photons of visible light.

Mike W.


(published on 10/08/2012)

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