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[accordion title=”1. Particle Nature of Light”]

- A
**photon** is a quantum of electromagnetic radiation.
- The energy of a photon is given by
*E*=*hf*, where *h* is Planck’s constant (6.63 $$\times$$ 10^{-34} J s) and *f* is its frequency.

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[accordion title=”1.1 Photoelectric Effect”]

- The
**photoelectric effect** is the emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequency is shone on it.
- The energy of an incident photon is the sum of the maximum kinetic energy $$K.E._{max}$$ of the emitted electrons from the metal surface and the work function $$\Phi$$ of the metal. Einstein’s photoelectric equation states that

$$hf=\Phi +K.E._{max}=hf_o +K.E._{max}$$

- where $$f_o$$ is the
**threshold frequency** or minimum frequency of the electromagnetic radiation below which no electrons are emitted from the metal surface regardless of the intensity of the radiation.
- The
**work function** $$\Phi$$ of a metal is the minimum energy needed to remove an electron from the metal surface.
- $$K.E._{max}$$ can be measured by applying a voltage to prevent the emitted electrons from reaching the electrode that collects them. This voltage is known as the stopping voltage $$V_s$$ and since the charge of an electron is
*e*, the equation can be rewritten as

$$hf=\Phi + eV_s$$.

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[accordion title=”1.2 Line Spectra”]

- An atom is in the ground state when its electron occupies the lowest energy level. When the atom gains energy, its ground state electron makes a transition to a higher energy level. The atom is said to be in an excited state.
- At this excited state, the electron is unstable. It will jump to a lower energy level by emitting a photon whose energy is equal to the energy difference between the two levels. The photon energy is given
*hf = E*_{higher} – E_{lower.}
- The
**emission line spectra** are the spectra of light radiated by individual atoms in a hot gas when the electrons in the atoms jump from higher energy levels to lower energy levels. Each spectrum consists of coloured lines on a dark background.
- The
** absorption line spectra **consists of dark lines on a coloured background. When a beam of white light is passed through a cool gas, photons whose energies are equal to the excitation energies of the gas atoms, are absorbed. These photons are re-emitted in all directions, so the intensity of these wavelengths in the transmitted white light beam is reduced.

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[accordion title=”2. Wave Nature of Particles”]

- Louis de Broglie postulated that, because photons have wave and particle characteristics, perhaps all forms of matter have both properties. Electron diffraction provides evidence for the wave nature of particles.
- The de Broglie wavelength of a particle is given by $$\lambda = \dfrac{h}{p}$$
* *where *p* is the momentum (*mv*) of the particle and *h* is Planck’s constant.

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[accordion title=”3. X-ray Spectrum”]

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[accordion title=”4. Heisenberg Uncertainty Principle”]

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[accordion title=”5. Wave Function and Probability”]

- An electron can be described by a wave function $$\Psi$$ where the square of the amplitude of the wave function $$|{\Psi}|^2$$ gives the probability of finding the electron at a point.

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[accordion title=”6. Quantum Tunneling”]

- Classically, an electron of energy
*E* approaching a potential barrier, whose height *U* is greater than *E*, cannot penetrate the barrier but would simply be reflected and return in the opposite direction.
- However, quantum mechanics predicts that since $$|{\Psi}^2|$$ is non-zero beyond the barrier, there is a finite chance of this electron tunnelling through the barrier and reaching the other side of the barrier.
- The transmission coefficient
*T* represents the probability with which an approaching electron will penetrate to the other side of the barrier. The transmission coefficient* T* is given by $$T=e^{-2kd}$$ where $$k=\sqrt{\dfrac{8\pi^2m(U-E)}{h^2}}$$

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