**Real Image** â is an image formed by the actual intersection of light rays.

**Virtual Image** â is formed by the apparent intersection of light rays.

**Refraction** â is the bending of light as it passes from one medium to another.

**The Refractive Index of a Medium** â is the ratio of the sine of the angle of incidence to the sine of the angle of refraction when light travels from a vacuum into that medium.

**Snellâs law of refraction** â states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant .

**Transverse wave** â is a wave where the direction of vibration is perpendicular to the direction in which the wave travels.

**Longitudinal Wave** â is a wave where the direction of vibration is parallel to the direction in which the wave travels.

**Refraction** â is the changing of direction of a wave as it travels from one medium to another.

**Diffraction** â is the spreading of waves around an obstacle.

**Interference** â occurs when two waves combine to produce a wave of a different amplitude.

**Coherent Sources** â are sources which have the same frequency and are in phase with each other.

**Polarised wave** â is a wave which vibrates in one plane only.

**Stationary waves** â are formed when two periodic travelling waves of the same frequency and amplitude travelling in opposite directions meet.

**Doppler Effect** â is the apparent change in the frequency in of a wave due to the motion of the source of the wave.

**Natural frequency** â of an object is the frequency at which the object will vibrate if free to do so.

**Resonance** â is the transfer of energy between two objects which have the same natural frequency.

**Sound Intensity** â at a point is the rate at which sound energy is passing through unit area.

**The threshold of hearing** â is the smallest sound intensity detectable by the average human ear at a frequency of 1 KHz.

**Frequency limits of audibility** â are the highest and lowest frequencies that can be heard by a normal human ear.The range is 20 Hz â 20,000 Hz.

**Overtones** â are frequencies which are multiples of the fundamental frequency.

**Diffraction grating** â consists of a piece of transparent material on which a very large number of parallel lines are engraved.

**Dispersion** â is the separating of white light into its different colours.