# Rotation of PPL by opticaly active compounds (mathematical feeling)

1. May 6, 2014

### namanjain

i read formulae in textbook :

[α]λt°C = Θ/L*C ........(i)

where Θ is rotation in light
and L is length of tube
and C is concentration in gm/litre

if we consider tube of lenth L and cross section area 'A'
then C= M/A*L
M is mass of sample

so (i) becomes

[α]λt°C = Θ/[M/A]

so are scientist so blind to observe or i am doing something wrong.

2. May 6, 2014

### UltrafastPED

This appears to be a variation on Verdet's equation for measuring the concentration of an optically active substance, allowing for temperature variations.

You have neatly removed the variable for concentration.

3. May 6, 2014

### DrDu

To blind to observe what?
Your formula is correct, however, I doubt that it is more practical. The point is that you would have to fill the sample of the given mass in a tube which has precisely the cross section you sprecified. Have you tried to fill a tube with precisely the amount of substance you want and to put glass windows on it without neither liquid spilling out or gas bubbles remaining inside?

4. May 7, 2014

### namanjain

DO IMPROVIZE MY LAST STATEMENT I WANT TO ASK:
the older equation has a feeling that if length of tube is increased one of factor will have to change and my equation marks that rotation(θ) and [α] are both independent of length of tube. I am just a bit confused for example help out with this

Q) if length of tube is doubled what will be the effect on Θ?

5. May 7, 2014

### DrDu

Depends on what you hold constant. If M is constant, nothing, if c is constant, theta will double.