Final answer:
The quickly fading afterimage of the sparkler that Harry sees is a result of iconic memory, a type of brief visual retention within sensory memory, which typically lasts a fraction of a second. Additionally, the difference in time observed between seeing and hearing two fireworks explode is due to the fact that light travels faster than sound.
Step-by-step explanation:
The type of sensory memory that can account for the quickly fading afterimage of the sparkler is known as iconic memory. Iconic memory is a component of the visual memory system which includes a brief retention of a visual stimulus. This phenomenon occurs because sensory memory acts as a buffer for stimuli received through the senses.
When Rani waves the sparkler, Harry perceives a trail of light due to the persistence of vision provided by iconic memory. However, this memory is typically transient, lasting just a fraction of a second before fading away. This short-lived retention allows for a continuous perception of the visual world despite rapid eye movements and blinking.
Regarding the question about fireworks and the timing difference between sight and sound, this occurs because light travels faster than sound. When you see a firework explode, the light reaches you almost instantly, but the sound, traveling at a slower speed, arrives milliseconds later. This is clearly explained in Figure 17.4, where it states that we perceive the light energy before the sound energy due to the difference in their respective propagation speeds.