# Optics Waves & Light Sound – Leaving Cert Physics Terms

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

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

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

The Refractive Index of a Medium – 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.

Snell’s law of refraction – 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 – is a wave where the direction of vibration is perpendicular to the direction in which the wave travels.

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

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

Diffraction – is the spreading of waves around an obstacle.

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

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

Polarised wave – is a wave which vibrates in one plane only.

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

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

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

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

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

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

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

Overtones – are frequencies which are multiples of the fundamental frequency.

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

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

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