Light continues to propagate after the source is removed due to phenomena such as afterglow, phosphorescence, and the finite travel time of photons. This is a matter of everyday observation and fundamental physics, requiring no citation, though various materials science papers document delayed optical emission.
The claim that light continues to propagate after the source is removed is trivially true by everyday observation (turning off a flashlight in a dark room still leaves a beam traveling through space, and afterglow/phosphorescence emits stored light over time). Thus, STEP 0 classifies this as COMMON_KNOWLEDGE. Retrieved papers 4, 5, and 8 discuss persistent luminescence and afterglow, which are specific material examples of light emission continuing after excitation is removed, and are included as supporting evidence.