# The differential of

1. Jan 27, 2005

### Hyperreality

From what I've learnt in the first year of calculus, a derivative is always written in the form of dy/dx and it must be written in this form, for dy and dx to have any mathematicl meaning.

But in many physics text book I've been self studying from, dy, dx, ds, dt... of such have been used frequently. My guess is that it has something to do with the "geometrical aspect" of the variable, where can I find a definite explanation for these notations?

For instance the, the intensity in the range of $$\lambda + d\lambda$$,

$$dI = R(\lambda)d\lambda$$

Why can't they just write as

$$dI/d\lambda = R(\lambda)$$?

2. Jan 27, 2005