# Need some help with half life

1. Dec 15, 2015

### thetexan

So I'm getting ready to buy some Tritium gun sights that will glow in the dark. The advertisement says Tritium has a half life of 12.3 so the sights will be about half as bright (50%) after 12.3 years.

I'm wondering how bright they will be after 6 years and 2 months...will they be one quarter as bright?

tex

2. Dec 15, 2015

### PietKuip

Simple estimate would be 75 %, but 70.7 % (square root of one half) is accurate.

3. Dec 15, 2015

### Staff: Mentor

That's 1/4 LESS bright, not "as" bright.

4. Dec 18, 2015

### sophiecentaur

Yes - "Threequarters as bright".
This way of stating things is getting very common - particularly when describing the cost of things but it is potentially very confusing. It is a shame that decreases are not expressed in fractions of the original (or possibly percentage).
"Reduced to 75%" or "reduced to 3/4" of the original can't be misconstrued.
Even worse is "A hundred times less", instead of "one hundredth". This is used throughout the media and must be a nightmare for children, learning to do arithmetic. It's also a great way to hoodwink innocent consumers and voters.

5. Dec 18, 2015

### gmax137

Don't get me started. My local weatherman told us this morning "It's 46 degrees now, that's twice the normal 23 degree morning temperature for December 18..."

6. Dec 18, 2015

### SteamKing

Staff Emeritus
Could be worse. It could be half the normal temperature for December 18 ...

7. Dec 18, 2015

### sophiecentaur

And answer me this one: then the bank manager tells you that the interest he's charging you has only gone up by 1% (i.e. from 1% to 2%) how has he the nerve not to admit that the cost has actually increased by 100%?