Wave Properties

Measuring Waves:

The frequency of a wave is the number of complete waves passing a point per second.
The amplitude of a wave is the maximum displacement of a vibrating particle.
The wavelength of a wave is the least distance between two adjacent vibrating particles with the same displacement and velocity at the same time.

The higher the frequency of a wave, the shorter its wavelength. The equation for
wave speed = frequency x wavelength

Wave Properties

Reflection - Straight waves directed at a certain angle to a hard flat surface reflect off at the same angle. The angle between the reflected wavefront and the surface is the same as the angle between the incident wavefront and the surface. The angle between the incident ray and the mirror is equal to the angle between the reflected ray and the mirror.

Refraction - When waves pass across a boundary at which the wave speed changes, the wavelength also changes. The direction of the refracted waves is closer to the normal than the directed of the incident waves. An example is, a light ray directing into a glass block. The light ray changes direction when it crosses the glass boundary. This happens because light waves travel more slowly in glass than in air.

Diffraction - This occurs when waves spread out after passing through a gap or around an obstacle. The effect can be seen in a ripple tank when straight waves are directed at a gap. The narrower the gap, the more the waves spread out. The longer the wavelength, the more the waves spread out.

Matters and Radiation

Inside the atom

Every atom contains a positively charged nucleus composed of protons and neutrons and electrons surrounding the nucleus. The word nucleon is used for a proton or a neutron in the nucleus. The table below shows the charges and mass of protons, neutrons and electrons.

 

Isotopes are atoms of the same elements with the same number of protons but different number of neutrons.

 

The specific charge of a charged particle is defined as its charge divided by its mass.

 

For example, a charged particle has a charge of 1.60 x 10^-19 C and a mass of 1.67 x 10^-27  kg.

Therefore, the specific charge is : 1.60 x 10^-19 C /1.67 x 10^-27  kg = 1.76 x 10^¹¹   Ckg^-1

 

Stable and unstable nuclei

 

A force that holds a stable nuclei is called the strong nuclear force, as it overcomes the electrostatic force of repulsion between the protons in the nucleus and keeps the protons & neutrons together.

 

Alpha radiation consists of alpha particles which comprise of 2 protons and 2 neutrons. Therefore its mass number is 4. As the nucleus emits an alpha particle, its nucleon number decreases by 4 and its atomic number decreases by 2.

 

 

For example:

 







Gamma radiation is electromagnetic radiation emitted by an unstable nucleus. It can pass through thick metal plates. It has no mass and no charge.

 

 

Photons

 

All electromagnetic waves travel at the speed of light (c), which is 3.0 x 10^⁸ .

Electromagnetic waves are emitted by a charged particle when it loses energy. They are emitted as short 'bursts' of waves, each burst leaving the source in a different direction. Each burst is a packet of electromagnetic waves and is referred to a photon. This is the emission of electrons from a metal surface when light is directed at the surface.

 

photon energy E = hf

 

h=Planck's constant (6.63 x 10^^-34 Js)

 

Example: Calculate the frequency and the energy of a photon of wavelength 650nm

 

h = 6.63 x 10^^-34 Js

c = 3.00 x 10^^-8  ms^^-1

 

 

 

 f = (3.00 x 10^^-8  ms^^-1 ) / (650 x 10^^-9 ) = 4.62 x 10^⁵ 

E = hf= 6.63 x 10^^-34  x 4.62 x 10^⁵

 

= 3.06 x 10^^-33 J