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#### 究竟什么是对数

Logarithms, Explained - Steve Kelly

How does the difference between
0.0000000398和
point 0000000398 and
3.98e-09
point 00000000398

cause one to have red eyes after swimming?

To answer this, we first need a way of

dealing with rather small numbers,

or in some cases extremely large numbers.

This leads us to the concept of logarithms.

Well, what are logarithms?

Let’s take the base number, b,

and raise it to a power, p,

like 2 to the 3rd power,

and have it equal a number n.

We get an exponential equation: b raised to the p power, equals n.

In our example, that’d be 2 raised

to the 3rd power, equals 8.

The exponent p is said to be

the logarithm of the number n.

Most of the time this would be written:
log，底数b，数字，等于p，指数。
"log, base b, of a number equals p, the power."

This is starting to sound a bit confusing with all the variables,

so let’s show this with an example.

What is the value of

log base 10 of 10,000?

The same question could be asked using exponents:
10的多少次方是10000?
"10 raised to what power is 10,000?"

Well, 10 to the 4th is 10,000.

So, log base 10 of 10,000

must equal 4.

This example can also be completed very simply on a scientific calculator.

Log base 10 is used so frequently

in the sciences

that it has the honor of having its own button on most calculators.

If the calculator will figure out logs for me,

why study them?

Just a quick reminder:
log键只能计算以10为底的对数。
the log button only computes logarithms of base 10.

What if you want to go into computer science

and need to understand base 2?

So what is log base 2 of 64?

In other words, 2 raised to what power is 64?

Well, use your fingers. 2, 4, 8, 16, 32, 64.

So log base 2 of 64 must equal 6.

So what does this have to do with my eyes turning red

in some swimming pools

and not others?

Well, it leads us into an interesting

use of logarithms in chemistry:

finding the pH of water samples.
pH值是告诉我们水的酸碱性的样本，
pH tells us how acidic or basic a sample is,

and can be calculated with the formula:
pH值等于以10为底氢离子或H＋浓度的对数的负数。
pH equals negative log base 10 of the hydrogen ion concentration, or H plus.

We can find the pH of water samples

with hydrogen ion concentration of point 0000000398

and point 00000000398

quickly on a calculator.

Punch: negative log of each of those numbers,

and you’ll see the pH’s are 7.4 and 8.4.

Since the tears in our eyes have a pH of about 7.4,
H＋的浓度是0.70398时
the H plus concentration of .70398

will feel nice on your eyes,

but the pH of 8.4 will make you feel itchy and red.

It’s easy to remember logarithms "log base b of some number n equals p"

by repeating: "The base raised to what power equals the number?"

"The BASE raised to what POWER equals the NUMBER?"

So now we know logarithms are very powerful

when dealing with extremely small or large numbers.

Logarithms can even be used instead of eyedrops after swimming.