how fructose is laevorotatry?

Some chiral compounds (which don't have plane of symmetry) have some property to rotate plane polarised light (either clockwise or counter clockwise direction). 
those compounds which rotates plane polarised light toward clockwise direction are called dextrorotatory ( denoted by d or +) and those which rotates plane polarised light toward counter clockwise direction are called laevorotatory ( denoted​ by l or -).
If an experiment is carried out in which plane polarised light of wavelength $\lambda$ is passed through a sample of concentration C, at temperature T, and the angle by which light rotates is called angle of rotation $\alpha$.
and specific rotation is equals to
  ​_{$[\alpha {]}_{\lambda }^T$  =}  $\frac{\alpha }{l x C}$
l is the path length in dm

now it has three chiral centers, that is why it is optically active.
specific rotation of fructose is -92.4
negative sign show that light rotates toward counter clockwise direction. so it is laevorotatory.
this specific rotation is experimental phenomenon, we cant predict specific rotation after seeing the structure..