Waves and Particle Nature of Light

• Absorption Spectrum

   The frequencies at which a certain element absorbs photons at a higher rate as these frequencies correspond to the spacings between energy levels in the element’s atoms.
• Amplitude

  The maximum displacement of a vibrating particle or wave from its equilibrium position.
• Angle of Incidence

  The angle at which a light ray hits a medium. Measured from a line perpendicular to the surface of the medium.
• Angle of Reflection

  The angle at which a light ray reflects off a surface. Measured from a line perpendicular to the surface of the medium.
•  Angle of Refraction

   The angle at which light rays travel after transferring into a different refractive index material. Measured from a line perpendicular to the surface of the medium.
• Antinode

  The point on a stationary wave where the incoming and reflected wave are in phase forming a maximum point.
• Coherence

   Waves with the same frequency and constant phase difference.
• Constructive Interference

  Interference when the two waves are in phase. If the two waves are both at their peak the addition of the two peaks becomes large.
• Converging Lens

  A lens that takes a parallel set of light rays and causes them to meet at a point.
• Critical Angle

  The angle of incidence when the angle of refraction is exactly 90 degrees. It is when the refracted ray travels along the boundary line.
• De Broglie Hypothesis

  All particles have a wave-like nature and a particle-like nature. The wavelength of a particle is inversely proportional to the particle’s momentum.
• De-excitation

  The movement of an electron from a high energy level to a lower energy level. This occurs in excited atoms over time and causes photons to be released.
• Destructive Interference

   Interference when the two waves are in antiphase. When one wave is at a peak and one is at a trough their addition results in a minimum point.
• Diffraction

  The spread of a wave as it passes through a gap or over an edge.
• Displacement

  The distance and direction that a vibrating particle or wave has travelled from its equilibrium position.

   
•  Diverging Lens

  A lens that takes a parallel set of light rays and causes them to travel away from each other and not meet.
• Electronvolt

  The kinetic energy gained by one electron that is accelerated through a potential difference of 1 volt. Equal to 1.6x10-16 J.
• Emission Spectrum

  When a certain element’s atoms de-excite they move from one energy level to another with a specific energy gap between these levels. This creates a photon of that specific energy and it is the frequencies of these photons that make up the emission spectrum.
• Excitation

   The movement of an electron from a low energy level to a higher energy level. This occurs when another electron or photon transfers energy to an orbital electron causing it to move to a higher energy level.
• Focal Length

  The distance from the centre of the lens to the focal point.
• Focal Point

  The point at which rays of light passing through a lens converge (or appear to converge).
• Frequency

  The number of complete oscillations of a wave (wave cycles) per second.
• Fundamental Frequency

  The oscillation of an entire object forming the lowest possible frequency for that object. For a string fixed at both ends this is where there is only a single antinode in the middle of the string.
• Huygen’s Principle

   Every point on a wavefront can be treated as a point source of a secondary wavelet.

   
• Ionisation

   When an electron is removed from an atom giving the atom a positive charge.
• Antiphase

  When the phase difference between two waves is 180 degrees or π radians.
• In-Phase

  When the phase difference between two waves is 0 degrees or 0 radians.
• Intensity

  The power transferred by a wave per unit area.
• Lens Power

  The inverse of the focal length.
• Longitudinal Wave

  A wave whose oscillations are parallel to the direction of propagation of the wave (e.g. sound waves).
• Magnification

  The height of an object divided by the height of its image.
• Node

  The point on a stationary wave where the incoming and reflected wave are in antiphase forming a minimum point.
• One-to-one Interaction

  When one photon transfers all of its energy to a single electron in a photoelectric process.
• Path Difference

  For two waves starting from different points and arriving at the same point the path difference is the difference in the length travelled by each wave, measured in metres.
• Period

  The time taken for a wave to complete one full cycle.
• Phase

  If the full cycle of a wave (peak to peak) is represented on a circle with the first peak at 0 degrees (0 radians), the trough at 180 degrees (π radians) and the next peak at 360 degrees (2π radians), the phase is where on that circle the wave is at a given point in space and time. Measured in radians or degrees.
• Phase Difference

  The difference in phase if the phase of two waves is determined at a single point in time, measured in radians or degrees. It can be seen as the amount by which one wave lags behind another.
• Photoelectric Effect

  When light of a high enough energy shone on a metal surface causes electrons to be emitted. The electrons are given enough kinetic energy by the photons to overcome the attractive force of the ions in the metal.
• Polarised Wave

  A wave whose oscillations have been restricted to the same plane.
• Principal Axis

  A passing through the centre of the lens. Passing through the axis of the lens where it is thinnest.