The Tunnel Through a Barrier
Written by Studio AM.
Your result
- Words per minute
- Score
- Pace
Words per minute mean something only with the score beside them. They describe this passage on this read. With fewer than 3 right, try the next slower pace.
Choose a pace and select Start. The passage is paced and timed, then the questions check what you understood. How pace mode works
Reading time 0:00
A low-energy ball cannot cross a hill; it rolls back. At quantum scales, particles do not behave like tiny balls with exact paths. A mathematical wave function describes where a particle may be found. That function can extend into and 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. At a simple barrier, the wave function generally decreases through that region rather than ending sharply at the boundary. A nonzero value on the far side means there is some probability of finding the particle there. Thinner barriers generally make that probability larger. 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 probability depends on precise conditions, which researchers can control in instruments. 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.
The passage is hidden while you answer. It comes back with your result.
Questions
Choose an answer. The explanation appears after you answer.
-
Question 1 of 4
What is the main idea of the passage?
The answer is A: 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.
-
Question 2 of 4
Why can a scanning tunneling microscope map tiny surface changes?
The answer is B: 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.
-
Question 3 of 4
What does “nonzero” mean in the second paragraph?
The answer is C: greater than zero, even if very small
Some wave remains beyond the barrier, creating a possible rather than guaranteed measurement there.
-
Question 4 of 4
Which nuclear process does tunneling help explain?
The answer is D: radioactive alpha decay
The third paragraph directly connects tunneling with radioactive alpha decay.
Score: none answered yet.