Optical activity. Image 1 of 2. Optical activity of fructose demonstrated by using polarized light. Optically active compounds rotate light waves that pass through them. This can only be seen under polarized light, light that vibrates in one direction only and is produced by using a polarizing filter. Polarized light passes through the fructose, and then through another polarizing filter. The second filter shows fructose's optical activity strengthening one of the colours of the light. The colour depends on the relative angles of the filters (here 75 degrees). Different angles produce different colours, as seen in A350/126-7.