Go to World and click the checkbox for Indirect Lighting. Set Factor to 25.00. Set Bounces to 3.Turn Factor down if you want less indirect light. 25 is actually a pretty high value, but it makes the results more obvious. Ditto for Bounces, which should normally be 1.
Just underneath Indirect Lighting, there’s a Gather group. Click on the Approximate button. (Raytrace doesn’t work with indirect lighting.) Check Falloff and sets its Strength to 0.200 or so. The lower Strength is, the farther the indirect light will go.