FSc ICS Notes Physics XI Short Questions & Definitions Chapter 10 Optical Instruments 1st Year Physics Notes Online Taleem Ilm Hub
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Optical instrument: The device based on the principles of reflection and refraction.
Lens: A portion of a transparent medium such as glass bounded by two faces, one of which is at least curved.
Convex lens (or Converging lens): A lens which converges parallel light rays. It is thicker in the middle than it is at the edge.
Concave lens (or Diverging lens): A lens with concave surfaces which diverges parallel light rays.
Reflection: The turning back of a wave from the boundary of a medium.
Refraction: The bending of a wave disturbance at it passes obliquely from one medium into another of different density.
Principal axis: Line through center of curvature and center of a lens or mirror.
Aperture: The angular portion of a sphere included by the mirror or lens.
Focus: A point at which light rays meet or from which rays of light diverge.
Principal focus: A point to which rays parallel to the principal axis converge, or from which they diverge, after reflection or refraction.
Focal length: Distance from an optical device to the point where it focuses parallel rays.
Refractive index: A measure of the extent to which a ray of light is bent as it passes from one transparent medium to another.
Optical center: A fixed point of a lens lying inside or outside on its axis such that a ray of light passing through it suffers no deviation.
Image: The reproduction of an object formed by lenses or mirrors.
Real image: An image which is formed by actual rays of light.
Imaginary (or Virtual) image: Image formed by rays not actually passing through the position the image appears to occupy.
Least distance of distinct vision: The distance equal to 25 centimeter for a normal person to see clearly an object.
Fuzzy: Not clear; indistinct; moving the camera causes.
Iris: The circular coloured membrane of the eye.
Retina: The inner, sensitive coating of the eye containing the ends of the nerves of sight; that part of the eye which receives images of object.
Pupil: The opening in the iris of the eye, through which rays of light pass to the retina.
Magnification: The ratio of the angle subtended by the image as seen through the optical device to that subtended by the object at the unaided eye.
Linear (or Transverse) magnification: The ratio of the linear dimensions of the image to those of the object.
Magnifying power (or Angular magnification): The ratio of the angle subtended by the image of an object seen through a telescope and the angle subtended by the same object seen without the telescope.
Resolving power: The ability of an instrument to reveal the minor details of the object under examination.
Spectrometer: Optical instrument used for the study of spectra. It consists of collimator, turntable and telescope.
Grating spectrometer: A spectrometer attached with diffraction grating.
Magnifying glass (or Simple microscope): An ordinary convex lens held close to the eye.
Chromatic aberration: The non-focussing of light of different colours.
Spherical aberration: The failure of parallel rays to meet at a single point after reflection or refraction.
Compound microscope: It is used to produce a very large magnification of very small objects. It consists of an objective and an eyepiece.
Telescope: A device for collecting and producing an image of distant objects.
Astronomical telescope: A telescope used to see heavenly bodies; it consists of two converging lenses.
Objective: That part of a telescope or binoculars that faces the objects; it has large focal length and large aperture.
Eye piece: A convex lens of short focal length and small aperture.
Unaided eye: Looking with naked eye without using optical device; such as, telescope or microscope.
Planet: Any one of the larger celestial bodies of the solar system, revolving round the sun in a nearly circular orbit and shining by reflected light; distinguished from the stars.
Star: Any celestial body visible as a point of light.
Reflecting telescope: A device used to see distant objects in which concave mirror is used as an objective.
Light year: A unit of distance. It is the distance that light travels through space in one year; equal to 9.46 x 10(15) meters.
Spectrum: A band of seven colours formed by the dispersion of the components of white light, when it is passed through a prism.
Spectrometer: Optical instrument used for the study of spectra. It consists of collimator, turntable and telescope.
Collimator: The part of the spectrometer consisting of slit and a convex lens, which is called collimating lens.
Turn-table: The part of the spectrometer between collimator and telescope. This turn-table is provided with three leveling screws. It is used for supporting the prism or the diffraction grating.
Grating (or diffraction grating): An optical surface, either transmitting or reflecting with several thousand equally spaced and parallel lines ruled on it.
Speed of light in vacuum: Speed of electromagnetic radiation; a fundamental constant is defined as,
c = 2.99792458 x 108 m s(-1).it is recommended since 1975 for universal use. The speed decreases when the radiation enters a material medium.
Photo phone: An instrument for talking along a beam of light instead of telegraph wire; telephoning without wires by varying the intensity of a beam of light by the action of voice, and allowing the light to fall upon a piece of crystalline selenium.
Modulation: i) Variation of radio frequency waves by means of audio frequency waves. ii) Variation of the amplitude, frequency or phase of electromagnetic waves, especially for carrier waves.
Radio: The process of communication across space by the transmission and reception of an electromagnetic wave of radio frequency without the use of connecting wires or other material link.
Radio frequencies: Frequencies of radio waves above audio frequency; between 3 KHz and 300 GHz (wavelength 1 cm to 100 km).
Audio frequencies: Frequencies to which the ear is sensitive; about 20 Hz to 20 kHz.
Technology: The practice of any or all of the applied sciences that have practical value and/or industrial use.
Bandwidth: i) The difference between the upper and lower frequency limits of a band, normally measured in hertz. ii) The range of frequencies over which a particular characteristic of an electronic device or system lies within specified limits.
Illumination: A measure of the visible-radiation energy reaching a surface in unit time.
Fibre optics: The use of fine transparent fibres to transmit light. The light passes along the fibres by a series of internal reflections.
Fibre: Raw material, which can be separated into threads for making up textile or some, woven like system.
Internal reflection: Reflection within a medium.
Total internal reflection: i) The reflection of light at the boundary of two transparent media when the angle of incidence exceeds the critical angle. ii) Reflection which occurs when the angle of incidence is greater than the critical angle.
Refractive index (Absolute index of refraction): The ratio of velocity of light in vacuum to its velocity in a given substance. n = c1 / c2
Relative refractive index (relative index of refraction): The ratio of velocity of light in one medium to its velocity in another medium.
Critical angle: The angle of incidence for which the angle of refraction is 90 degree .
Axial: Of, relating to, or having the characteristics of an axis; situated around, in the direction of, on, or along an axis; extending in a direction essentially perpendicular to the plane of a cyclic structure.
Laws of reflection: The angle of incidence equals the angle of reflection, and the incident ray, the reflected ray and the normal to the reflecting surface at the point of incidence lie in the same plane.
Cross section: A cutting or piece of something cut off at right angles to an axis.
Propagation: Spreading or transmitting through a medium.
Hypothetical: Based on supposition, or on something assumed.
Optical fibre: An optical fibre consists of a single flexible rod of high refractive index, less than 1mm in diameter, having polished surfaces coated with transparent material of lower refractive index.
Multi layer fibre: The fibre having many layers.
Core: The central part of a wire.
Cladding: i) A layer of lower refractive index (less intensity) over the central core of high refractive index (high density). ii) The process of bonding one metal to another to prevent corrosion of one of the metals.
Stepped-index fibre: Here the indexes of both cladding and core are constant throughout.
Graded index fibre: In it the refractive index of the core decreases radially outwards.
Light rays then spiral smoothly around the central axis rather than zig zagging.
Single mode fibre: Here the core is very narrow relative to the cladding and rays travel parallel to the central axis; it may be stepped or graded index.
Single (or mono) mode step index fibre: It has a very thin core of about 5 μm diameter and has a relatively larger cladding of glass or plastic.
Multimode step index fibre: Here a core of relatively larger diameter such as 50 μm is used. The fibre core has a constant refractive index such as 1.52.
Multimode graded index fibre: i) An optical fibre in which the central core has high refractive index which gradually decreases towards its periphery. ii) Its core ranges in diameter from 50 to 1000 μm, which has relatively high refractive index and the refractive index decreases gradually from the middle to the outer surface of the fibre. There is no noticeable boundary between core and cladding.
LED: (Light emitting diode) A semi-conductor diode, made from certain materials (e.g. gallium arsenide), in which light is emitted in response to the forward-bias current.
Ultraviolet light: Light of shorter wavelength than visible light but longer than X-rays; frequencies from 8x10(14) to 2x10(16) Hz.
Infra-red light: Light waves longer than waves of ordinary light but shorter than radio-waves; frequencies from 3x10(11) to 4x10(14) Hz.
Audio signal: i) Sound or information transmitted electrically at the normally audible frequencies of 20 to 20,000 cps or Hz. ii) An alternating voltage proportional to the sound pressure produces in an electric circuit by a microphone.
Ear piece: In a signal transmitter, the device nearest the ear which converts electrical signals into sound.
Microphone: A device for converting sound energy into electrical energy.
Modulation: i) Variation of radio frequency waves by means of audio frequency waves. ii) Variation of the amplitude, frequency or phase of electromagnetic waves, especially for carrier waves.
Digital modulation:
i) A modulation by digital method. The laser is flashed on and off at extremely fast rate. The communication is represented by code of 1s and 0s. the receiver is programmed to decode 1s and 0s.
ii) Variation of the amplitude, frequency or phase of electromagnetic waves and sending signals in digital form of zero or one shape, especially for carrier waves.
Laser: (Light Amplification by Stimulated Emission of Radiation) a device which is able to produce a beam of radiation with unusual properties, generally the beam is coherent, monochromatic, parallel with high intensity.
Stimulate: To rouse and produce greater activity in.
Coherent: Having same phase, i.e. peaks and troughs always come together.
Phase coherence: Producing of two waves at the same instant, having same wavelength and time period.
Monochromatic: Light consisting of only one colour.
Intensity: The energy transmitted per second through a unit area by the light waves.
Repeater: A device used to amplify or regenerate signals in order to extend the transmission between two stations.
Photodiode: A semiconductor diode that produces a significant photo-current when illuminated.
Photo-current: Current produced due to photo electric effect.
Photo electric effect: The emission of electrons by a substance when illuminated by electromagnetic radiation.
Dispersion: i) The separation of polychromatic light into its component wavelengths. ii) Separation of white light into colours.
Biconvex lens: A type of convex lens whose both sides curved outwards.
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