The diameter of a human pupil in millimetres is given by
D= (2 + 4x^-0.3) / (1 + x^-0.3)
where x is the luminance measured in candelas per square metre. Using this function,what is the diameter of the pupil when there is no light? What happens to the diameter of the pupil as it gets brighter and brighter?
try multiplying top and bottom by x^0.3
(2x^0.3+4)/(x^0.3+1)
Now you have a nice simple limit of 4/1
As x gets big, x^-0.3 vanishes, and the limit is 2/1