Definition Questions Flashcards
(26 cards)
What is a spectral line?
When light is split up l, each photon energy frequency is seen as a separate discrete line of colour
Why do atoms of different elements produce different colours?
Different atoms have different discrete energy levels.
The photon energy depends on the difference between these energy levels.
Different colours correspond to photons of different energies
Explanation of how electrons can escape from the surface of a cathode
Electrons near the surface of the cathode absorb photon and gains energy.
The work function is the energy needed for the electron to escape the surface.
Electrons released this way are called photoelectrons.
Explanation of why photocathodes have a threshold wavelength
Electrons require a certain amount of energy to escape from the surface.
This energy comes from a photon and so the photon needs a minimum frequency to provide enough energy.
Minimum frequency corresponds to the threshold wavelength
Explanation of why some photoelectrons will be emitted with less than the maximum kinetic energy
Photon transferred to an electron below the surface
So some energy transferred to atoms on way to the surface so will leave with less than max
OR: lower energy photon could provide electron with just under max
The absorption and emission of a narrow band of frequencies by electrons in some liquids
When blue light (for example) is absorbed and the electron is excited to a higher energy level
As the electron loses its energy it emits green light (for example) and falls to the intermediate energy level
Due to liquids having close together particles, there are bands rather than discrete levels
Hence a range of frequencies
How plane polarised light is different from ordinary light
Plane polarised light oscillates in 1 plane.
Ordinary light oscillates in all planes
What effect does a polarising filter have on polarised light
Only allows light in 1 plane.
When filter is at 90° to light, blocks it out
A method of measuring the angle of rotation of a plane of polarisation
Polaroid filters placed at both ends of a tube.
Polaroids crossed to block out light, with no sugar solution in between.
Solution introduced
One Polaroid rotated, angle measured
What is a transverse wave?
Oscillations occur at right angles to the direction of energy transfer
What is a longitudinal wave?
Oscillations occur parallel to the direction of energy transfer
What is frequency?
The number of oscillations per unit of time
What is fundamental frequency?
The longest wavelength standing wave that fits in the wire (in the column of air)
What is wavelength?
The distance between successive points in phase
What is superposition?
When two or more waves meet, the resultant wave has a displacement equal to the sum of the two
How is a standing wave formed?
Waves are reflected from a barrier.
The incident and reflected waves superpose.
Where they meet in phase, constructive interference occurs and forms antinodes
Where they meet in antiphase they form a displacement of zero, nodes
What is coherence?
Same frequency and a constant phase relationship
Why does the resistance of a metal increase with temperature?
Increased temp leads to increased lattice vibrations
Causes electrons to collide and not get through as easily (increased resistance)
Why does the resistance of a semiconductor decrease with temperature?
Increased temp leads to more electrons released
This increases current and lowers resistance
Dominates effect of lattice vibrations
What effect does internal resistance have on power?
Power has a maximum value when external resistance is = to internal resistance
Why does the p.d across a load resistor increase as current falls?
Load resistor is in series with internal resistance
Current falls, load resistor has a greater share of current than the internal resistance
Explanation of how ultrasound can be used to build up a picture
Reflects from boundaries of different densities.
Time taken to reflect gives depth
Size of reflection gives density
Definition of Doppler shift
A change in frequency caused by the relative moment between source and receiver
Principal of operation of a Doppler probe
Arrange probe so fluid is approaching
Fluid reflects
Change in frequency depends on speed
