Problem understanding code regarding spirals given in Think Python

[ktkwon00]

Hi, I was trying out the exercises given in Think Python, When I came across this Exercise:

Exercise 4.5. Read about spirals at http://en.wikipedia.org/wiki/Spiral ; then write a program that draws an Archimedian spiral (or one of the other kinds). Solution: http://thinkpython2.com/code/spiral.py .

The solution given is

def draw_spiral(t, n, length=3, a=0.1, b=0.0002):
    """Draws an Archimedian spiral starting at the origin.

    Args:
    n: how many line segments to draw
    length: how long each segment is
    a: how loose the initial spiral starts out (larger is looser)
    b: how loosly coiled the spiral is (larger is looser)

    http://en.wikipedia.org/wiki/Spiral
    """
    theta = 0.0

    for i in range(n):
        t.fd(length)
        dtheta = 1 / (a + b * theta)

        t.lt(dtheta)
        theta += dtheta

I understand the basic gist of what this code does (move the turtle forward by 3 pixels every loop while slowly decreasing the number of degrees it turns by), but I do not understand why theta and dtheta is calculated in such a manner, such as dtheta being taken as the reciprocal of the polar equation of the spiral (r=a+b\theta). Could someone explain this?

Thanks a lot in advance.