Lights, flashing: A sensation that is created when the clear, jelly-like substance that fills the middle of the eye (vitreous humor) shrinks and tugs on the retina. These flashes of light can appear off and on for several weeks or months. As a person ages, flashes are likely to occur increasingly often. Flashes usually do not reflect a serious problem. However, if flashing lights suddenly appear or increase, an ophthalmologist should be consulted immediately, to see whether the retina has been torn. Flashes of light that appear as jagged lines or 'heat waves' in both eyes, often lasting 10 to 20 minutes, are frequently caused by migraine, a spasm of blood vessels in the brain.