MODERN PHYSICS

Photoelectric Effect is the phenomenon that when light shines on a metal surface, electrons are emitted from the surface.

Photoelectric Effect – A photon may knock an electron out of an atom and in the process itself disappear.

Electrons should be emitted when light shines on a metal is consistent with the electromagnetic wave theory of light – that is the electric field of an EM wave could exert a force on electrons in the metal and eject some of them

The process of measuring maximum kinetic energy can be done by using a variable voltage source and reversing the terminals so that the electrode C is negative and P is positive. The electrons emitted from P will be repelled by the negative electrode. But if this reverse voltage is small enough, the fastest electrons will still reach C and there will be a current in the circuit.

If the reverse voltage is increase, a point is reached where the current reaches zero, so no electrons have sufficient energy to reach C. This called stopping potential or stopping voltage

Simulation 1: Click to run

Wave theory

Assume a monochromatic light. Two important properties of light are intensity and frequency. When two properties varied, the wave theory make prediction as below:

· If the light intensity increase, numbers of electrons ejected and their maximum kinetic energy should be increased because – the higher the intensity, greater electric field amplitude and greater electric field should eject electrons with higher speed.

· The frequency of the light should not affect the kinetic energy of the ejected electrons.

Photon theory

In monochromatic beam, all photons have the same energy, E=hf. Increasing the intensity of light mean increasing the number of photons.

· If the frequency remains the same, it does not affect the energy of each photon
· An electron is ejected from the metal by a collision with a single photon. Consequently, all the photon energy is transferred to the electron and the forces some minimum energy W₀ (Work Function) is required to get an electron out through the surface.
· hf<W₀- The photon will not have enough energy to eject any electron
· hf>W₀– The electrons will be ejected and energy will be conserved.

Consideration of photon theory:

An increase in intensity of the light beams means more photons are incident, so more electrons will be ejected.
Since the energy of each photon is not changed, the maximum kinetic energy of electrons is not changed by increase in intensity
If the frequency is increased, the maximum kinetic energy of the electrons increases linearly.

Compton Effect

Compton Effect – a photon can be scattered from an electron and in the process, lose some energy. But the photon is not slowdown; it still travels with speed, c but its frequency will be lower.

Compton scattered short wavelength light, which is X-rays, from various materials. He found that the scattered light had a slightly longer wavelength than the incident light, therefore, there is a slight lower frequency indication loss of energy.

Since the photon is relativistic particle that travel with the speed of light, v= c, the momentum of photon is

Figure 1: Compton Scattering

Atomic Structure

In 1911, Ernest Rutherford (1871-1937) theorized that the atom must consist of a tiny but massive positively charged nucleus, surrounded by electrons some distance away. The electrons would be moving in orbits about the nucleus.

Figure 1: Atomic Structure

Line Spectrum of Hydrogen Atom

Figure 2: Hydrogen Atom

Hydrogen is simplest atom that has only one electron orbiting its nucleus. It atomic number is 1.

In 1885, J. J. Balmer showed that the four visible lines in the hydrogen spectrum (with wavelength 656 nm, 486 nm, 434 nm and 410 nm) fit the following formula

Later was found that this Balmer series of lines extended to UV region, ending at

= 365 nm

Figure 3: Electron transitions for the Hydrogen atom

Wave-Particle Duality

Some indicate that light behaves like waves and the other indicates light behaves like stream of particles. These behaviours of light come in to conclusion as wave-particle duality.

In 1923, Louis de Broglie suggests that the wavelength of a particle would be related to its momentum as in the same way with photon.

p = linear momentum

= wavelength

Sometimes it is called the de Broglie wavelength of a particle

Video 2: Waves-Particle Duality

Diffraction of X-Ray

X-ray is produced when electrons accelerated by a high voltage strike the metal target inside the X-ray tube. W. C. Roentgen in 1895 discovers the X-ray using voltages of 30kV – 150 kV. H-rays scattered from a crystal did indeed show the peaks and valleys of a diffraction pattern. It was shown that X-rays have a wave nature and the atoms are arranged in a regular way in crystals (serve as diffraction) Today, X-rays are recognized as electromagnetic radiation.

The diffraction of X-rays with wavelength, that a reflection from a crystal as described by Bragg equation. Strong reflections are observed at grazing angles