A rapid series of still pictures can be made to appear as a smoothly moving picture in cinema and animation because the human eye and brain have which property?

Introduction / Context:
Movies, television and many forms of animation rely on the fact that a rapid sequence of still images can create the illusion of continuous motion. This basic optical illusion depends on the way the human visual system processes information over time. The underlying phenomenon is often described as persistence of vision. This question asks which description best captures that effect.

Given Data / Assumptions:

• We are considering a series of still pictures shown in quick succession.
• The time interval between successive frames is small, typically less than a tenth of a second.
• The human eye and brain work together to form a visual impression.
• We neglect details of modern display technologies and focus on human perception.

Concept / Approach:
Persistence of vision refers to the tendency of the eye and brain to retain a visual image for a short interval after the object is removed. If a second image appears within this retention period, the two images overlap in perception. When many such images are shown quickly one after another, the brain blends them into a smooth, continuous sequence. The critical interval is around one tenth to one fifteenth of a second for most people, which is why traditional film and video formats use frame rates of about 24 frames per second or more.

Step-by-Step Solution:
Step 1: Recognise that each frame of a movie is actually a separate still photograph. Step 2: These frames are projected in rapid succession, for example 24 frames per second. Step 3: The eye and brain do not instantly erase one image when the next appears; they retain each image briefly. Step 4: This retention time, on the order of one tenth of a second, means that successive images overlap in perception. Step 5: As a result, the sequence of slightly different still images is interpreted as continuous motion rather than separate pictures. Step 6: Therefore, the property responsible is the short persistence of visual impressions in the eye and brain.

Verification / Alternative check:
Simple experiments support this idea. If you spin a sparkler in a circle, you see a continuous ring of light rather than a single moving point, because the light impression persists briefly as the sparkler moves. Toys such as thaumatrope discs, where two images on opposite sides of a disc appear merged when the disc is spun quickly, also rely on persistence of vision. In all of these cases, the visual system blends closely spaced images into one continuous perception.

Why Other Options Are Wrong:
They can refocus extremely quickly: While the eye can refocus on objects at different distances, this is not the main reason still images appear as motion.

The eye is physically faster than the brain: The brain and eye work together, but the crucial point is not speed difference; it is the persistence of the signal in the visual system.

They can shut out some of the light: The eye does not deliberately shut out images to create motion; rather, it retains them briefly, causing overlap in perception.

Common Pitfalls:
Students sometimes think movies work by literally moving pictures, not by flashing separate frames, so they overlook the role of human perception. Others may recall the term persistence of vision but confuse it with the speed of eye movement or focusing. Remember that what matters is the short time during which an image remains in the visual system after it disappears, allowing rapidly changing images to blend into a smooth sequence.

Final Answer:
Fast series of still pictures appear as motion because the eye and brain retain an image for about one tenth of a second, so two images close in time merge into one.