Semi-log Plots
| English | Chinese | Pinyin |
|---|---|---|
| exponential | 指数 | zhǐ shù |
| semi-log plot | 半对数图 | bàn duì shù tú |
| logarithmic scale | 对数刻度 | duì shù kè dù |
| slope | 斜率 | xié lǜ |
Straightening a curve
- A straight line is the easiest graph to read, fit, and extend.
- So scientists play a clever trick: re-scale an axis to turn a curve into a line.
- For exponential data, log-scaling the vertical axis does exactly that.
- The result is a semi-log plot, and it reveals hidden exponential behavior at a glance.
What a semi-log plot is
- A semi-log plot 半对数图 uses a logarithmic scale 对数刻度 on one axis, usually the vertical.
- On a log scale, equal spaces mean equal multiplications: $1, 10, 100, 1000$ are evenly spaced.
- The horizontal axis stays ordinary (linear).
- "Semi" means only half the plot — one axis — is logarithmic.
The exponential that a semi-log plot straightens
y = a·bˣ
This exponential curves upward on ordinary axes. Re-scale the vertical axis logarithmically and it becomes a perfect straight line.
A semi-log plot uses a logarithmic scale on…
"Semi" means half: a semi-log plot log-scales just one axis, most often the vertical one.
Why exponentials become straight
- Take an exponential 指数 $y = a\,b^x$ and apply a log to both sides.
- You get $\log y = \log a + x\log b$ — a linear equation in $x$.
- So plotting $\log y$ against $x$ gives a perfectly straight line.
- The log scale does the "$\log y$" for you automatically.
An exponential relationship appears as a straight line on a semi-log plot.
Taking $\log$ of $y = a\,b^x$ gives $\log y = \log a + x\log b$ — linear in $x$, so the plot is straight.
Select all true statements about semi-log plots.
Only exponential data straightens on a semi-log plot — not every data set. The other three are correct.
Reading slope and intercept
- The straight line's slope 斜率 equals $\log b$ — bigger slope, faster growth.
- Its intercept equals $\log a$ — that recovers the initial value $a$.
- So you can read both numbers of the exponential straight off the line.
- Fitting a line is far easier and more accurate than fitting a curve.
On a semi-log plot of an exponential, the ____ of the straight line equals $\log b$ (the growth rate).
From $\log y = \log a + x\log b$, the slope is $\log b$ and the intercept is $\log a$.
If data plots as a straight line on a semi-log graph, the best model is…
Straight on a semi-log plot is the signature of an exponential relationship in the original data.
Building a model from the plot
- Plot your data on semi-log axes and see if it falls on a line.
- If it does, the underlying relationship is exponential.
- Measure the slope and intercept, then convert back to $a$ and $b$.
- This is a standard scientific tool for spotting exponential growth or decay.
Only exponential data straightens on a semi-log plot — it is not a magic wand for every curve. If the points still bend on a semi-log plot, the relationship is not exponential, and you should try a different model.
Bacteria counts double each hour: $1, 2, 4, 8, 16, \dots$
- On ordinary axes these curve upward steeply.
- On a semi-log plot ($\log$ of the count vs time) they fall on a straight line.
- The constant slope confirms a constant growth factor — genuine exponential growth.
A semi-log plot puts a logarithmic scale on one axis. An exponential relationship $y = a\,b^x$ becomes a straight line, because $\log y = \log a + x\log b$. The line's slope gives the growth rate and its intercept gives the initial value — a quick test for exponential data.