Hallo,(adsbygoogle = window.adsbygoogle || []).push({});

First time I've experienced a real problem rearding equations in physics, so:

As conclusions to the lab on focal points and and center of curvatures (which will be explained at a later date), we were given the following relationships:

So = Do - f

Si = Di - f

And, we were ably to verify the Gaussian? form of the equation through a series of experiments:

1/Do + 1/Di = 1/f

And yadayada, now the final conclusion for the lab simply asks for a relationship between f, So, and Si that can be expressed analytically, graphically, and verbally.

After wrecking my brain on this question, I eventually checked the following site: http://www.sasked.gov.sk.ca/docs/physics/u3b32phy.html [Broken] to find this relationship:

SoSi = f^2

However, I obviously cannot go that far, lol. Here is, as best as I can transcribe, how far I got:

So = do -f ----> do = So + f

Si = di -f ----> di = Si + f

then, I substituted those into the equation, getting this stuff:

1/(So +f) - 1/(Si + f) = 1/f

I know for sure (I think, well w/e) that that is the first correct step. After that, I tried a couple of different things, the most prominent being the LCM/LCD approach:

(Si + f) - (So + f)

all over or divided by

(So + f)(Si + f)

all equals = 1/f

then....some simplification:

Si - So

all over or divided by

(So + f)(Si + f)

all equals = 1/f

from there, a smart guy at school (whose done some college level physics...I thinK) told me I could flip the numerator/denominator in both sides of the equation...giving....

(Si + f)(So + f)

all over or divided by

Si - So

all equals = f

(I just put (si + f) in the front b/c it looks a bit better I think)

However, after this we were both rather clueless as to how further simplify the expression...I tried expanding the numerator:

SiSo + Sif + Sof + f^2

all over or divided by

Si - So

all equals = f

but, like, then what? Si, So, f...nothing is common in the numerator anyhow. Eventually I got kinda berserk and tried taking out a Si and an f from the numerator...not pretty:

(Si)(So + f) + (f)(So + f)

all over or divided by

Si - SO

all equals = f

yawn..now what?....mm then this:

(So + f)(Si + f)

all over or divided by

Si - So

all equals = f

and that last part, I'm not even sure about my algebra lol. so yeah, I'm sorta hopeless, and I can't find much besides the aformentioned site for the derviation of this formula, but I think I'm on the right track so if anyone has any ideas...go ahead.

--Zeros

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Mirror Equations: Derivation of Newtonian Form

**Physics Forums | Science Articles, Homework Help, Discussion**