Mainipulating Radical expression



Given the following:
sqrt(l2^2 - 8 * ca * l2 * r2^2)/(4 * ca * l2 * r2)
How can I force the denominator into the radical sign,
to get this:
sqrt((1/(16 * ca^2 * r2^2)) - (1/(2 * ca * l2)))
Thanks,
Dan Stanger