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# Help with graphs of diffraction gratings watch

1. I've been drawing a graph and my 2 variables are Fringe Spacing W (cm) on the vertical axis and Distance to screen D (m) on the horizontal. Should those 2 variables both be in the same unit (both metres or both centimetres) because when I find the gradient, it gives me 32.5 which is quite a big number, having both variables in metres would give me 0.325 which might be more sensible but I don't know.
2. We're always taught (at least I've always been taught) to give the units in metres, and merely to scale the actual scale.

Example could be such that if you have 30 millimetres worth on your scale, 30mm -=- 3 cm -=- 3E-2 M. Make the scale in metres, but make the intervals very small and make it very large. That way it's still in metres, but the scale is just appropriated.
3. (Original post by AxSirlotl)
I've been drawing a graph and my 2 variables are Fringe Spacing W (cm) on the vertical axis and Distance to screen D (m) on the horizontal. Should those 2 variables both be in the same unit (both metres or both centimetres) because when I find the gradient, it gives me 32.5 which is quite a big number, having both variables in metres would give me 0.325 which might be more sensible but I don't know.
Not necessarily. You can have your scales on either axis in whatever (appropriate) units you like.

To restate:

- you do not need to have consistent units for your axis on the graph
- you do need to have consistent units for your gradient calculation, and for that matter, any calculation

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