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Absolute refractive index
Property of the optical material equal the ratio : speed of light in a vacuum/speed of light in the material
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Acceleration
The rate of change of velocity : change in velocity/time taken ; unit m s-2
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Acceleration due to gravity
The rate at which all objects Accelerate under gravity if air resistance is neglected ; also known as the acceleration of free fall ; on earth its usually taken as 9.81 m s-2 , but it varies slightly from place to place
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Amplitude
The maximum height of a wave , or the largest displacement from equilibrium
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Anti node
A point on a standing wave where the amplitude is at a maximum
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Anti phase
Two points on a wave , or points on two waves , are in anti phase if their vibrations are 180° out of phase with each other
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Breaking stress or ultimate tensile stress
The maximum stress (force per unit area) that a material can withstand before it breaks
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Brittle
A brittle material fractures before it undergoes plastic deformation
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Centre of gravity
The point at which the weight of an object can be taken to act ; an object will balance if its supported at its centre of gravity
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Centre of mass
The point at which the mass of an object can be taken to be concentrated ; in a uniform gravitational field this is the same as the centre of gravity
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Cladding
A layer of glass (or plastic) that surrounds the central core of an optical fibre
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Coherent
Two or more waves that have a fixed phase difference are said to be coherent
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Component
A vector can be split up into perpendicular components ; the vertical component is the part of the vector that acts in a vertical direction
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Compression
An object in compression is under the influence of forces that tend to squash it
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Coplanar forces
A two dimensional system of forces that all act in the same plane ; they can be drawn on a piece of paper
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Couple
Two equal forces that act in opposite directions on an object so as to cause rotation
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Critical angle
The minimum angle of incidence at an optical boundary at which total internal reflection occurs
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Density
The amount of mass per unit volume
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Diffraction
The spreading of waves through an aperture or round an obstacle
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Diffraction grating
A series of closely spaced parallel slits through which light can diffract ; used to create spectra
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displacement
a vector describing the difference in position of two points
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Drag
Resistive force , such as air resistance , which acts to oppose motion in a fluid
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Ductility
The ability of materials to show extended plastic deformation and become elongated under tension ; a ductile metal can be drawn out into wires
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Efficiency
The ratio : useful energy transferred (or work done) / total energy input ; this is always less tHan 10
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elastic behavIour (elasticity)
When a material returns to its original dimensions after deforming force is removed
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Elastic strain energy
The potential energY stored in an elastic material that has been extended
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Endoscope
A medical device that used optical fibres to see inside the body
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energy
The ability to do work I.e. Move a force through a distance ; a scalar quantity , measured in joules
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Equilibrium
An object is said to be in equilibrium if its not accelearting
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First order maximum
A point at which the waves passing through a diffraction grating interfere constructively ; waves from adjacent slits have a path difference one wavelength , and so all to wave arrive in phase
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Free body diagram
A simplIfied picture of a physical situation which shows all of the relevant forces acting on a body
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Frequency
The number of waves passing a point in one second , measured in hertz , Hz
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Friction
A force that acts between surfaces , acting so as to oppose their relative motion
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Fundamental frequency
The lowest resonant frequency of a vibrating system or a standing wave
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Gravitational potential energy
The energy stored by a mass due to its position in a gravitational field ; in a uniform field ,the gravitational potential energy of a mass , m , that is raised by a distance , delta h , is given by Ep = mg delta h
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hookes law
Law stating that , for an object under tension , such as a wire or a spring , the extension is proportionAl to the applied force
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Inertia
An objects resistance to acceleration ; for linear motion , this is the mass
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instantaneous velocity
The rate of change of displacement , as measured over a very small time interval
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Interference partern
A Series of maxima (points of constructive interference) and minima (Points of destructive interference) in a region where two or more waves overlap
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Kinetic energy
The energy of a mass , m , moving at a velocity Ek = 1/2mv2
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Laser
A device that produces a highly monochromatic , coherent , non diverging light beam
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Longitudinal wave
A wave that has oscillations parallel to the direction of travel of the wave
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Newton
the S.I. unit of force ; 1 newton is the force that will Accelerate the mass of 1 kg at 1ms-2
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Node
A poInt on a standing wave at which the amplitide is zero
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optical fibre
A think strand of glass or plastic which carries light signals
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Overtone
A vibration with a frequency that is a multiple of the fundamental frequency
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Parallelogram law
A method for finding the resultant force of two vectors
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path difference
The difference in the distance travelled by two waves ; commonly expressed as the number of wavelengths
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phase difference
The difference in phase (the position in the cycle) of two waves , expressed in degrees or radians
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In phase
Two waves are in phase if they are at the sAme point in their cycle at the same time
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plastic behaviour
When a material if permanently deformed , even after the applied force is removed
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Polarised
A transverse wave that is constrained to vibrate in one direction only is said to be polarised
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Power
The rate at which energy is transferred or the rate at which work is done , measured in joules per second , or watts , W ,
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principle of conservation of energy
law stating that the total energy of a closed system is constant
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principle of moments
law stating that if an object is in equilibrium , the sum of the clockwise moments about any point must equal the sum of the anticlockwise moments about that point
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principle of superposition
law stating that when two similar waves overlap , the total disturbance caused is the vector sum of the individual disturbances
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progressive wave
a wave that transfer energy in the direction of the wave travel
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rarefaction
a region of lower pressure or density in a longitudinal wave
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refraction
the change in direction of a wave as it crosses a boundary between two mediums in which its speed differs
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refractive index (absolute refractive index) (n)
property of an optical material equal to the ratio : speed opt light in a vacuum / speed of light in the material
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relative refractive index n2
the relative refractive index of material 2 relative to material 1 (n2) is the ratio of : speed of light in medium 1 / speed of light in medium 2
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resolution or resolving
the splitting up of a vector into components , usually perpendicular
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resultant
the sum of two or more vectors such as forces
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scalar
a physical quantity that is fully specified by its magnitude (size) ; it has no direction associated with it
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snells law
law of refraction connecting the angle of incidence and angle of refraction with the absolute refractive indices of the materials either side of the boundary : n1sinθ1 : n2sinθ2
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spectrometer
a device that uses a diffraction grating to produce spectra or to measure the wavelength of monochromatic light
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spectrum
the distribution of wavelengths in a light source
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spring constant k
the force needed to stretch a spring by unit extension k = force/extension unit Nm-1 its usually a measure of the stiffness of a spring
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stationary or standing wave
a wave that doesn't transfer energy in the direction of wave travel ; it has stationary points called nodes
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stiffens
the resistance to extension of a material under tension
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strain or tensile strain ε
- the fractional increase in length of a wire , l , under tension
- ε = Δl/l
- it has no unit
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strength
a measure of the force (stress) needed to cause fracture of a material
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stress or tensile stress σ
- the force per unit cross sectional area
- σ = F/A
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tensile force
a force acting to cause extension
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tension
an object in tension is under the influence of of forces which tend to extend it
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terminal velocity
the steady velocity reached by a falling object when the drag is equal to the weight
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torque
- the rotational equivalent of a force
- torque produces rotational acceleration
- unit Nm
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[total internal reflection
the complete reflection of a light ray at the boundary of two media , when the ray is in the medium with a lower speed of light
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transverse wave
a wave that has oscillations perpendicular to the direction of wave travel
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ultimate tensile stress or breaking stress
the maximum stress (force per unit area) that a material can withstand before it breaks
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upthrust
the upward force on an object that is submerged in a fluid ; it is equal to the weight of the fluid displaced
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vector
a physical quantity that is specified by its magnitude and direction
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velocity
the rate of change of displacement ; velocity = change in displacement/time ; unit ms-1
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watt
unit of power equal to the rat elf energy transfer of 1 joule per second
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wavelength
the distance between consecutive points on a wave that have identical motion
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work
work done = force x distance moved in the direction of the force
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yield point
the minimum stress at which plastic deformation occurs
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young modulus
the stiffness constant of a material , defined by the ratio : tensile stress/tensile strain
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zero orde maximum
the central point at which the wave passing through a diffraction grating interfere constructively ; waves from adjacent slits have zero path difference and so all waves arrive in phase
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physical quantities can be classified into two groups
scalars or vectors
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give an example of a scalar quantity
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scalar quantities must have
a magnitude but no direction associated with them
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give an example of a vector quantity
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a vector quantity is only fully specified when
the magnitude and direction is given
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draw a table to show examples of scalar and vector quantities met in this unit
x
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a vector quantity has
magnitude and direction whereas a scalar quantity only has magnitude
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vector quantities are often identified by the use of
bold type
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when two vectors are added
we need to take account their direction as well as their magnitude
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two vectors can be added by
drawing a scale diagram showing the effect of one vector followed by the other
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the sum of a number of vectors is known as the
resultant
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the resultant is
the single vector that has the same effect as the combination of the other vectors
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it is vital to take into account the relative direction of vectors when
adding them together , for example the resultant of two 5N forces could be anything from 0 to 10N depending on their directions
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the resultant of two vectors can be also be found by the
parallelogram law . a parallelogram is constructed using the two vectors as adjacent sides . the resultant is the diagonal of the parallelogram
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if the vector diagram is drawn to scale , the resultant vector can be found by
direct measurement from the diagram
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for two vectors at right angles , the magnitude of the resultant can be found from
calculation by using pythag
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subtracting a vector quantity can be though of as
adding a negative vector . the vector which is to be subtracted is reversed in direction . this reversed , or negative , vector is then to be added in the usual way
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a single vector can be replaced by a combination of two of more vectors that would have the same effect this is known as
resolving a vector into its components and can be found as the reverse of finding the resultant . the components of a vector could be at any angle but it is often useful to use two components that are at right angles to each other . this might be to find the horizontal and vertical components of a force or a velocity
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remember that the vector you are resolving is always the
hypotenuse of a triangle . the components will always be smaller than the original vector
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it is often important to be able to identify and add together all the forces that are acting on an object . the size and direction of the resultant will
determine what happens to the object
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everyday objects are subjected to a variety of forces such as
- weight
- contact forces
- friction
- tension
- air resistance
- buoyancy
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all of those forces are
electromagnetic in origin except for weight . they arise because of the attraction or repulsion of the charges in atoms
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the weight is the
force that acts on mass due to the gravitational attraction the earth
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the gravitational field strength on the earth is
- 9.81 Nkg-1
- this means that every kilogram of mass is attracted towards the earth with an attraction of 9.81 N
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the weight of an object in newtons is given by
- weight (N) = mass (kg) 8 gravitational field strength (Nkg-1)
- w=mg
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the goal weight of a real object is
the sum of the gravitational attractions acting on every particle in the object . the resultant of these forces is the weight of the object which can be treated as a single force acting at one point in the object , this is known as the centre of gravity
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whenever two solid surfaces touch they
exert a contact force on each other . this force is known as the reaction . it is the contact force between the floor and your feet that stops gravity pulling your feet through the ground . the resultant contact force between two surfaces could be at any angle
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we usually split the contact force into two components
- the normal contact force acting perpendicularly to the two surfaces
- the frictional force acting parallel to the surfaces
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a frictional force acts
between two surfaces whenever there is a relative motion between them , or when an external force is trying to slide past each other
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an object is said to be in tension when
a force is acting to stretch the object . elastic materials , like ropes or metal cables resist this stretching and exert a force on the bodies trying to stretch them
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ay object that is moving through a fluid is subject to
a resistive force or drag . any object moving through the atmosphere has to push the air out of the way , this gives rise to the drag force that acts to oppose relative motion between the object and the fluid
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the size of the air resistance acting on an object depends on
the area of the object and the density of the air . the air resistance also increases as the relative speed between the object and the air increases . so as you go faster the force trying to stop you increases
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buoyancy - any objects that are partly or fully submerged in a fluid like a boat floating on water or a hot air balloon floating in the atmosphere are subject to
an upthrust from the surrounding fluid
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free body diagrams - the forces acting on a real object may be ver complex . a free body diagram is an attempt to
model the situation so that we can analyse the effect of the forces . the free body diagram is used to show all external forces that are acting on an object . since forces are vector quantities they are represented by arrows , drawn to scale and acting in the correct direction
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