# How to tell which property is changing?

Discussion in 'MCAT Study Question Q&A' started by Sammy1024, Sep 5, 2014.

1. ### Sammy1024

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I always get confused about what changes in an equation. For example, it might say something like light is going from air to water, which of the following changes?

frequency
intensity
wavelength
speed

but for other situations I still don't know what is changing. Since f and wavelength, and speed are connected via an equation then how can one change but not the other.

2. ### justadream 7+ Year Member

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@Sammy1024

It is super confusing for me too.

In general, however:

Switching from one medium to another: Frequency stays the same. Speed and Wavelength directly proportional.

Staying in the same medium: Speed stays the same. Frequency and wavelength inversely proportional.

Intensity has to do with amplitude. I'm pretty sure that's independent of the rest of the stuff except when you consider light as photons (in which case E=hf).

3. ### type12 5+ Year Member

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Frequency can only change when there is a relative change in velocity (doppler effect) for a wave that has already been emitted.

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