Q.A microscope using suitable photons is employed to locate an electron in an atom within a distance of 0.1 Å. What is the uncertainty involved in the measurement of its velocity?

De Broglie Wavelength: When Particles Start Acting Like Waves

Imagine you're holding a cricket ball. You know exactly where it is, and if you throw it, you can predict its path. That's a particle — localised, definite, following Newton's laws. Now think of light. You can't "hold" a beam of light; it spreads out, bends around corners, creates interference patterns. That's a wave — spread out, not localised.

For centuries, physics kept these two worlds separate. Particles were particles. Waves were waves. Never the twain shall meet.

Then came a young French physicist, Louis de Broglie, in 1924. He asked a question that seemed almost absurd: If light — which we thought was a wave — can behave like a particle (the photoelectric effect), then why can't a particle — say, an electron — behave like a wave?

That question turned physics upside down.

The Core Idea

De Broglie proposed that every moving particle has a wave associated with it. The wavelength of that wave depends on the particle's momentum. The faster or heavier the particle, the shorter the wavelength.

This is not a mathematical trick. It's a physical reality. An electron moving through a crystal actually behaves like a wave of this wavelength — it can diffract, interfere, and form patterns just like light does.

Why You Don't See It in Daily Life

Here's the crucial point: the de Broglie wavelength is incredibly tiny for everyday objects.

Take a cricket ball of mass 0.16 kg moving at 30 m/s. Its de Broglie wavelength is:

That's about a hundred trillion trillion times smaller than the nucleus of an atom. No experiment can detect such a wave — it's effectively zero for all practical purposes.

Now take an electron (mass $9.1 \times 10^{-31}$ kg) accelerated through 100 volts. Its speed is about $5.9 \times 10^6$ m/s. Its de Broglie wavelength:

That's about 0.12 nanometres — comparable to the spacing between atoms in a crystal. This is measurable. And indeed, in 1927, Davisson and Germer fired electrons at a nickel crystal and observed diffraction — the unmistakable signature of a wave.

What This Means Physically

The wave is not a physical wave in space like a water wave. It's a probability wave — its amplitude at any point tells you the probability of finding the particle there. Where the wave amplitude is large, you're likely to find the particle; where it's zero, you won't.

This wave-particle duality is not a compromise. It's the actual nature of reality. An electron is neither a pure particle nor a pure wave — it's something that shows particle-like behaviour in some experiments (like hitting a screen at a point) and wave-like behaviour in others (like passing through two slits and interfering with itself).

A Quick Way to Remember

For an exam, you'll often need to compute the de Broglie wavelength of an electron accelerated through a potential difference $V$ volts. The kinetic energy gained is $eV$, so:

Substituting into $\lambda = h/(mv)$ gives:

Plug in the numbers ($h$, $m_e$, $e$) and you get a handy formula:

The Bottom Line

De Broglie wavelength is the bridge between the particle and wave pictures of matter. It tells you that momentum and wavelength are two sides of the same coin. For large objects, the wavelength is negligible — Newtonian physics works fine. For tiny particles, the wavelength dominates — and you must use quantum mechanics.

When you see $\lambda = h/p$, remember: that's nature saying that everything — from electrons to planets — has a wave nature. It's just that for most things, the wave is too small to notice.

Searches like "de Broglie wavelength formula and examples" and "dual nature of matter class 12 physics" are very common, since this concept is central to the Dual Nature of Radiation and Matter chapter of the NCERT/CBSE Class 12 Physics curriculum. The handy $\lambda = 12.27/\sqrt{V}$ shortcut for accelerated electrons is a frequent JEE Main and NEET numerical question.