Physics · Dual Nature Of Radiation And Matter · NEET
It proved that electrons show wave nature. When a beam of electrons hit a nickel crystal, they did not scatter in all directions equally. Instead, a strong beam appeared at one special angle, just like light waves do when they pass through a grating (diffraction). Since only waves can diffract, this showed electrons behave as waves. It was the first direct experimental proof of de Broglie's idea that matter has a wave nature.
A crystal has atoms arranged in a regular repeating pattern. The spacing between rows of atoms in nickel is about 0.09 nm, which is close to the de Broglie wavelength of the electrons (about 0.165 nm). For diffraction to happen, the wavelength must be near the spacing of the obstacles. So the nickel crystal acts as a natural grating for electron waves. An ordinary rough metal plate has no regular spacing, so no clear diffraction pattern would form.
When the electrons were accelerated through 54 volts, and the detector was placed at a scattering angle of 50 degrees, the scattered electron current reached a sharp peak (maximum). This peak is the signature of constructive interference of electron waves. At other voltages or angles the peak was weaker. So 54 V and 50 degrees are the exact conditions where the wave effect showed up most clearly.
Theory (de Broglie): wavelength = 1.227 / sqrt(V) nm. For V = 54 V, wavelength = 1.227 / sqrt(54) = 1.227 / 7.35 = 0.167 nm. Experiment (from the crystal diffraction using Bragg's law with the 50 degree peak): wavelength = 0.165 nm. The two values are almost equal. This close match confirmed that the accelerated electron really has the wavelength de Broglie predicted.
An electron accelerated from rest through a potential difference V gains kinetic energy eV. Its de Broglie wavelength is lambda = h / sqrt(2 m e V). Putting in the constants gives a shortcut: lambda = 1.227 / sqrt(V) nm, where V is in volts. This one formula is all you usually need in NEET numericals about accelerated electrons.
No. The photoelectric effect shows the particle nature of light (photons knocking out electrons). The Davisson-Germer experiment shows the opposite: the wave nature of particles (electrons behaving as waves). Students mix these up because they are in the same chapter. Remember: photoelectric = light acts as particle; Davisson-Germer = electron acts as wave.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
C. J. Davisson and L. H. Germer performed it in 1927 in the United States. G. P. Thomson did a similar electron diffraction experiment the same year. Their work confirmed de Broglie's 1924 idea of matter waves.
Using lambda = 1.227 / sqrt(V) nm, we get lambda = 1.227 / sqrt(54) = 1.227 / 7.35 = 0.167 nm, which is about 1.67 angstrom.
Bragg's law, n times lambda = 2 d sin(theta). Using the nickel atomic spacing and the diffraction angle from the 50 degree peak, the experimental wavelength comes out to about 0.165 nm.
A peak means constructive interference of electron waves scattered from the regular rows of atoms. If electrons were only particles, the scattered current would change smoothly with angle, with no sharp peak. The peak is direct evidence of wave behaviour.
Yes. Later experiments showed neutrons, protons and even whole atoms diffract in the same way. Any moving particle has a de Broglie wavelength, though for heavy or fast objects the wavelength is far too small to detect.
