What is the main idea of the passage?

Quantum tunneling lets particles appear across sufficiently narrow energy barriers with predictable probabilities and important atomic-scale applications. (The passage explains the wave-based mechanism, controlling factors, scale, examples, and limits.)

Why can a scanning tunneling microscope map tiny surface changes?

Its tunneling current responds very strongly to the distance between the tip and surface. (The application relies on current varying sensitively as the narrow gap changes.)

What does “nonzero” mean in the second paragraph?

greater than zero, even if very small (Some wave remains beyond the barrier, creating a possible rather than guaranteed measurement there.)

Which nuclear process does tunneling help explain?

radioactive alpha decay (The third paragraph directly connects tunneling with radioactive alpha decay.)

The Tunnel Through a Barrier: Level 5 Reading Comprehension Questions

A low-energy ball cannot cross a hill; it rolls back. At quantum scales, particles do not behave like tiny balls with exact paths. Their state is described by a wave-like probability that can extend into and sometimes beyond an energy barrier. This effect is called quantum tunneling.

The word “tunnel” is a metaphor. A particle does not dig a passage or borrow energy while hidden inside. When its quantum state meets a barrier, the mathematical wave decreases through that region rather than ending sharply at the boundary. If the barrier is thin enough, a nonzero part can remain on the far side. A measurement may then find the particle there. Tunneling probability falls rapidly as a barrier becomes wider or harder to cross. The particle's mass and energy also matter. For everyday objects, the probability is so fantastically small that classical expectations work. At atomic scales, the effect can be important.

Tunneling helps explain radioactive alpha decay, in which a particle escapes a nucleus through a barrier. It also enables scanning tunneling microscopes. When a conducting tip comes extremely close to a surface, electrons can tunnel across the narrow gap. The resulting current changes sensitively with distance, allowing instruments to map surfaces at atomic scale.

The effect does not make every forbidden event possible in practice. It replaces a classical certainty with a probability governed by precise conditions. Quantum tunneling is strange because the far-side result conflicts with the ball-and-hill picture, yet its measured patterns are dependable enough to reveal atoms and support electronic devices.

Comprehension questions