Which of the following scientists has not contributed significantly to photoelectric effect?

The correct option is C.J. Davission.

To understand why this is the correct answer, let's look at the contributions of each of the listed scientists:

1. Wilhelm Hallwacks:
Wilhelm Hallwacks, along with Heinrich Hertz, was one of the early pioneers who investigated the photoelectric effect. In 1888, Hallwacks observed that an uncharged zinc plate connected to an electroscope loses its charge when irradiated with ultraviolet light, and a negatively charged zinc plate loses its charge rapidly under ultraviolet light. This established the foundational experimental observations of the photoelectric effect.

2. Philipp Lenard:
Philipp Lenard conducted systematic experimental studies of the photoelectric effect between 1899 and 1902. He observed that the kinetic energy of the emitted electrons (photoelectrons) depends on the frequency of the incident light, while the number of emitted electrons (photoelectric current) depends on the intensity of the light. His detailed experiments provided the key data that classical wave theory could not explain.

3. Albert Einstein:
Albert Einstein successfully explained the photoelectric effect in 1905 by proposing that light behaves as packets of energy called photons or quanta. He formulated the famous photoelectric equation:
$K{\max }_{}=h\nu -\Phi$
where $K{\max }_{}$ is the maximum kinetic energy of the emitted electrons, $h\nu$ is the energy of the incident photon, and $\Phi$ is the work function of the metal. Einstein was awarded the Nobel Prize in Physics in 1921 primarily for this explanation.

4. C.J. Davisson:
Clinton Joseph Davisson (along with Lester Germer) is famous for the 1927 Davisson-Germer experiment, which demonstrated the wave-particle duality of matter by demonstrating that electrons can be diffracted by a nickel crystal. This confirmed the de Broglie hypothesis that particles like electrons possess wave-like properties. Therefore, Davisson's major contribution was to the wave nature of matter, not to the photoelectric effect.