Q.What will be the wavelength of a ball of mass 0.1 kg moving with a velocity of 10ms−1?
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Concept understanding — De Broglie Wavelength
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.
λ=ph=mvh
Where:
λ = de Broglie wavelength (in metres)
h = Planck's constant (6.626×10−34J⋅s)
p = momentum of the particle (mv for non-relativistic speeds)
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:
λ=0.16×306.626×10−34≈1.38×10−34m
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 9.1×10−31 kg) accelerated through 100 volts. Its speed is about 5.9×106 m/s. Its de Broglie wavelength:
λ=9.1×10−31×5.9×1066.626×10−34≈1.23×10−10m
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.
Note
The de Broglie wavelength is only observable when it is comparable to the size of objects the particle interacts with. For macroscopic objects, it's far too small to matter. For subatomic particles, it's the key to understanding their behaviour.
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).
Important
De Broglie's hypothesis is not just a clever idea — it's the foundation of quantum mechanics. Every particle has a wavelength, and that wavelength determines how it moves, where it can be found, and even why electrons in atoms occupy only certain discrete energy levels (standing waves around the nucleus).
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:
21mv2=eV⇒v=m2eV
Substituting into λ=h/(mv) gives:
λ=2meVh
Plug in the numbers (h, me, e) and you get a handy formula:
Tip
For an electron accelerated through V volts:
λ(in A˚)=V12.27
So a 100 V electron has λ≈1.23A˚ — right in the X-ray range.
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 λ=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 λ=12.27/V shortcut for accelerated electrons is a frequent JEE Main and NEET numerical question.
Why this formula?
De Broglie Wavelength: Why the Formula Holds
Let's build this from the ground up — understanding why matter has a wavelength, not just memorizing λ=ph.
The Core Insight: Nature's Symmetry
Before de Broglie, physics had two separate worlds:
Light — showed wave behaviour (diffraction, interference) but also particle behaviour (photoelectric effect)
Matter — showed particle behaviour (momentum, collisions) but no wave behaviour yet
De Broglie asked a daring question in his 1924 PhD thesis:
If light (a wave) can behave like a particle, why can't a particle (like an electron) behave like a wave?
Nature should be symmetric — what applies to one should apply to the other.
Step 1: Start with Light (What We Already Knew)
For a photon, Einstein had given us two key relations:
Energy:E=hf (Planck's relation)
Momentum:p=λh (from E=pc for light, combined with c=fλ)
So for light:
λ=ph
This was experimentally verified for photons.
Step 2: De Broglie's Bold Hypothesis
De Broglie said: This relation is not special to light. It is universal.
For any particle with momentum p:
λ=ph
Where:
λ = de Broglie wavelength
h = Planck's constant (6.626×10−34 J⋅s)
p = momentum of the particle
Step 3: Why Momentum and Not Velocity?
This is crucial. The formula uses momentum (p=mv), not just velocity.
For a non-relativistic particle (slow compared to light):
λ=mvh
For a relativistic particle (like an electron at high speed):
p=γmvwhereγ=1−v2/c21
λ=γmvh
Why momentum? Because momentum is the more fundamental quantity — it's conserved, it's frame-independent in a deeper sense, and it connects directly to the wave's phase.
Step 4: The Deeper Reasoning — Wave-Particle Duality
De Broglie didn't just guess. He reasoned:
Every moving particle has an associated wave — called the "matter wave" or "pilot wave"
The frequency of this wave comes from energy: f=hE
The wavelength comes from momentum: λ=ph
These two relations are linked by the phase velocity of the wave:
vphase=fλ=hE⋅ph=pE
For a free particle with kinetic energy E=2mp2:
vphase=2mp=2v
This is half the particle's speed — a strange but mathematically consistent result.
Step 5: Experimental Confirmation (Why We Believe It)
De Broglie's idea was confirmed when electrons showed wave behaviour:
Davisson-Germer experiment (1927): Electrons scattered off a nickel crystal produced diffraction patterns — exactly like X-rays (waves!)
The measured wavelength matched λ=h/p perfectly
This was Nobel Prize material — de Broglie won in 1929.
Key Takeaways for Exams
Concept
Formula
When to Use
De Broglie wavelength
λ=ph
Always — fundamental definition
Non-relativistic
λ=mvh
For v≪c (most exam problems)
Relativistic
λ=γmvh
For v≈c (rare in school exams)
For an electron accelerated through V volts
λ=2meVh
Derive from p=2mEk
The Deeper "Why" — One Sentence
De Broglie wavelength exists because nature is symmetric: just as light has both wave and particle aspects, so must matter — and the bridge between them is Planck's constant h.
The formula λ=h/p is not derived from deeper principles — it is the fundamental postulate that connects the particle's momentum to its wave's wavelength. Its validity comes from experiment, not from pure mathematics.
The key idea is the De Broglie wavelength — every moving particle has a wavelength associated with it, given by λ=ph, where h is Planck's constant and p is the linear momentum.
Step 1: Write the de Broglie relation:
λ=mvh
Step 2: Substitute the given values: m=0.1kg, v=10m/s, and h=6.63×10−34J s.
λ=0.1×106.63×10−34
Step 3: Simplify:
λ=16.63×10−34=6.63×10−34m
✓Final answer
The wavelength is 6.626×10−34m.
The de Broglie wavelength of a macroscopic object is extremely small. For a 0.1kg ball moving at 10m/s, the wavelength is 6.63×10−34m — far below any detectable scale.
The idea of matter waves (de Broglie wavelength) applies to everything that has momentum — not just electrons or photons, but cricket balls, planets, and people. The reason we don't see diffraction of a moving ball is that its wavelength is unimaginably tiny. The formula is the same for all objects:
λ=ph=mvh
where h=6.63×10−34J⋅s (Planck's constant), m is mass in kg, and v is speed in m/s.
Let's apply it step by step.
Identify the given quantities
Mass m=0.1kg
Velocity v=10m/s
Planck's constant h=6.63×10−34J⋅s
Compute the momentum
Momentum p=mv=0.1×10=1kg⋅m/s
This is a very ordinary, human-scale momentum — about the same as a briskly thrown apple.
Apply de Broglie's relation
λ=ph=16.63×10−34=6.63×10−34m
Interpret the result
This wavelength is 6.63×10−34 metres — that's 1019 times smaller than the diameter of a hydrogen atom. No experiment can detect wave-like behaviour for such an object; the ball behaves purely classically.
Watch out
A common mistake is to forget that h is in joule-seconds and to use grams or cm/s without converting. Always work in SI units: kg, m/s, J·s. Here, 0.1kg and 10m/s are already correct, so no conversion is needed.
Tip
Notice that the momentum came out to exactly 1kg⋅m/s. That makes the wavelength numerically equal to h itself — a neat coincidence that helps you check your arithmetic: if mv=1, then λ=h.
✓Final answer
The de Broglie wavelength of the ball is 6.63×10−34m.