Physics Important Definitions and Formulae(Light + Waves)

Physics important definitions and formulae Waves A wave is a phenomenon in which energy is transferred through vibrations. Transverse waves are waves which travel in a direction perpendicular to the direction of the vibrations. Longitudinal waves are waves which travel in a direction parallel to the direction of vibrations. The line that joins all the peaks of a wave or all identical points on a wave is called a wavefront. Wave terms The highest points are called crests or peaks while the lowest 1.  points are called throughs. The amplitude, a, is the maximum displacement from the rest 2.  position. The wavelength, ,is the distance between any two identical 3.  points on successive waves. The frequency, f, is the number of complete waves generated 4.  per second. The unit of frequency is hertz (Hz). The period, T, is the time taken to generate one complete 5. wave. 6. The speed, v, of the wave is the distance moved by a wave in one second. •

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Important formulae: V=f  T=1/f 

Light Reflection The angle of incidence, i, is the angle between the incident ray and the normal. The angle of reflection, r, is the angle between the reflected ray and the normal. Laws of Reflection 1. The incident ray, the reflected ray and the normal at the  point of incidence all lie on the same plane. 2. The angle of incidence, i, is equal to the angle of reflection, r. Characteristics of an image formed by a plane mirror: 1. The image is virtual. 2. The image is upright. 3. The The ima image ge form formed ed has has the the same same size size as the the obj objec ect. t. 4. The image is as far behind the mirror as the object is in front of the mirror. The object and image are perpendicular to the mirror. 5. The im image is is la laterally in inverted. A virtual image cannot be formed on a screen, nor do the light rays pass through it. Uses of reflection: periscope, mirror in meter  •

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Refraction The change in direction (or bending) of light when it passes from one medium to another is called refraction. The angle of incidence, i, is the angle between the incident ray and the normal. The angle of refraction, r, is the angle between the refracted ray • and the normal. Laws of Refraction 1. The incident ray, the refracted ray and the normal at the  point of incidence all lie on the same plane. 2. For two given media, the ratio sin i/sin r is a constant, where I is the angle of incidence and r is the angle of refraction. The refractive index, n, of a medium may also be defined as the ratio of the speed of light in a vacuum to the speed of light in that medium. Total internal refraction The critical angle is defined as the angle of incidence in the optically denser medium for which the angle of refraction in the optically less dense medium is 90 0 . Conditions for total internal refraction: 1. The light ray must travel from an optically denser medium towards an optically less dense medium. 2. The angle of incidence must be greater than the critical angle. •

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Applications for total internal refraction: periscope, binoculars, Single Lens Reflex Camera, Fibre Optics(“light pipes”) -Uses total internal reflection to transmit light from one place to another. - particularly useful when one wishes to view an image produced at inaccessible locations e.g. a doctor uses an endoscope endoscope which works on this this principle, to examine examine the internal organs of his patients. Fibre optics is seeing higher usage in telecommunications, as: -optical fibres can carry a much higher volume of telephone calls, computer computer data or television  pictures than electrical wires. -much thinner and lighter  -being made of glass, optical fibre cables are much cheaper than metals such as copper. -they allow high quality transmission of information over long distances, with negligible signal loss.

Thin Lenses

-Converging lenses are thicker in the middle than at the edge and diverging lenses are thinner in the middle. -A converging lens converges (brings together) rays of light passing through it, while a diverging lens diverges (spreads out) rays of light  passing through it. Converging Lens Terms used in lenses The principal axis of a lens is line passing through the optical centre, C, of the lens and perpendicular to the plane of the lens. The optical centre, C, of a lens is the point midway between the lens surfaces on its principal axis. Rays passing through the optical centre are not deviated. The principal focus, F, of a thin converging lens is the point on the principal axis, to which an incident beam parallel to the  principal axis is made to converge. The focal length, f, of a lens is the distance between its optical centre and principal focus, F. The focal plane of a lens is the vertical plane which passes through the principal focus and perpendicular to the principal axis. Since light can pass through a lens from either left or right side, a lens has two principal foci. •

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