When light from a flashlight is moved quickly back and forth on a wall in a darkened room, it can appear to observers that there is a trail of light moving across the wall, even though physically the light is only in one place at any given time.

this experience is an effect of memory that occurs because of:

a. a visual delay effect.
b. echoic memory.
c. persistence of vision.
d. top-down processing.

Explanation:

c. persistence of vision.

It is a type of optical illusion that happened when visual perception of any object is not deferred when light pass from it

It is happened because human eye is capable of retaining only 10 to 12 images every second for 15 sec. In simple it can defined as  holding of visual for a moment of time after removal of source that produce that visual.

that tool is called an altimeter which measures your altitude which can be used to tell which atmosphere you are currently in, including the stratosphere.

hope this ! : )

+15-35=-20

going in opposite directions