Booklet 2B - Waves (Factual) Flashcards
What is the frequency of a wave?
The number of waves per second.
What is the wavelength of a wave?
Wavelength is the distance from a point on one wave to the same point on the next wave.
What is the period of a wave?
The time it takes one wave to pass a point.
What is the amplitude of a wave?
The distace from the centre of a wave (the line of zero disturbance) to the crest or trough.
What is meant by diffraction?
Diffraction is the bending of waves round an object or through a gap.
What will increase the amount of diffraction?
- Longer wavelengths diffract more
- Narrower gaps cause more diffraction
What is a coherent source?
Waves that have a constant phase relationship.
What is constructive intereference?
This occurs when two waves meet at a point in phase.
This results in a wave of larger amplitude.
What is destructive interference?
Destructive interference occurs when two waves meet at a point exactly 180º out of phase.
This results in smaller amplitude (zero if two waves are identical).
What is the test for a wave?
If it can cause interference.
Describe the interference pattern produced by a monochromatic light source and a double slit.
A series of bright and dark fringes symmetrical about the central, brightest fringe.
What is a minimum?
A point where destructive interference occurs e.g. dark, quiet.
What is a maximum?
A point where constructive interference occurs e.g. bright, loud.
What are the conditions for constructive interference in terms of path difference?
The path difference is a whole number of wavelengths.
(0, 1λ, 2λ, 3λ…)
What are the conditions for destructive interference in terms of path difference?
The path difference is an odd number of half wavelengths.
(½λ, 1½λ, 2½λ…)
What are the advantages of using a grating to produce an interference pattern instead of a double slit?
- Fringes are brighter
- Fringes are sharper
Using a grating how can you increase the spacing between the maxima?
- Use a grating with a smaller slit separation.
- Use a source with a greater wavelength of light
- Move the screen further away
If white light is shone through a grating what does the interference pattern look like?
- The central order maximum will be white
- At every other maximum a spectrum will be produced
- The pattern is symmetrical
- Violet is closest to the centre and red the farthest away
What is refraction?
The change in speed of light (and wavelength) as it enters a different material, which can result in a change in direction.
What is meant by absolute refractive index?
It is the ratio of speed of light in a vacuum/air to the speed of light in the material.
It is also the ratio sinθ1/sinθ2 and λ1/λ2 where medium 1 is a vacuum/air.
Describe an experiment to measure refractive index.
- Setup the equipment as shown.
- Measure the angle in air (θa) and in the medium (θp) for a range of incident angles.
- Calculate the sin of all these angles.
- Plot a graph of sin(θa) versus sin (θp).
- The refractive index n is the gradient of this graph.
What happens to the frequency when a wave enters a different medium?
Frequency doesn’t change - it is constant.
A triangular prism can be used to split white light into a spectrum. Why does this occur?
Higher frequencies have a higher refractive index.
What are the 4 differences in spectra produced by refraction (in a prism) and by a grating?
Prism Grating
One spectrum Many spectra
Produced by Refraction Produced by interference
Red light deviated least Red light deviated most
Spectrum is dim Spectrum is bright
What is the critical angle?
The angle of incidence that gives an angle of refraction of ninety degrees.
When will total internal reflection occur?
- Incident light must arrive at the boundary at an angle greater than the critical angle
- Occurs when light tries to pass from an optically dense material to a less dense material
Define irradiance.
The power per unit area
What is a point source?
This is a source where the light spreads evenly in all directions.
This means that its irradiance is inversely proportional to the square of the distance (I = k/d2)
Describe how to investigate Irradiance and distance for a point source.
Set up the apparatus as shown below.
Measure the background irradiance when the lamp isn’t switched on. Subtract this reading from all subsequent measurements to correct for background.
Take corrected measurements of irradiance at regular distances away from the lamp.
Either
Plot a graph of I versus 1/d2, which should be a straight line.
or
Calcualte I x d2 for each point to show this value is roughly constant.
What are the main features of the Bohr model of the atom?
- Nucleus - central dense region containing most of the atoms mass. Contains positive protons and neutrons (no charge)
- Electrons (negatively charged) are only allowed in certain energy levels
- Electrons only emit/absorb energy when changing level.
What is the ground state of the Bohr model of the atom?
The ground state is the lowest energy level in an atom.
What is the ionisation level?
This is the energy needed by an electron to leave the atom altogether.
The convention is that the ionisation level is 0 as an electron reaching this level will just have escaped from the atom with no excess kinetic energy. All other energy levels are therefore negative.
Why do the energy levels in the Bohr model have negative values?
Zero potential energy is defined when an electron is able to just escape from an atom (i.e. ionisation can take place). Electrons in all other energy levels are still bound to the atom so would need energy added to escape i.e. they have less than zero potential energy.
Explain how a continuous emission spectrum is produced.
When atoms are packed closely together (e.g. in a solid or high pressure gas) electrons can make transitions to neighbouring atoms, meaning all energy transitions are possible and all frequencies of light are produced.
Explain how a line emission spectrum is produced.
- Each line in a spectrum is produced when an electron falls between two specific energy levels.
- The energy levels, and therefore line spectrum, are unique to each element.
Explain how an absorption spectrum is formed.
When an atom absorbs a photon with the specific energy equal to the difference in energy levels to excite electrons in a lower energy level to a higher energy level. These specific frequencies appear as dark lines in a continuous spectrum.
What type of spectrum is this?
Line emission spectrum
What type of spectrum is this?
Continuous emission spectrum
What type of spectrum is this?
Line absorption spectrum
If a spectral line is bright - what does this tell you?
More electrons are making that energy level transition producing more photons of light with the same frequency.
Are all photons produced by energy level transitions visible?
No, some frequencies of photons may be in in the infrared, ultraviolet or X-ray frequency
What do absorption lines in the Sun’s spectrum show?
Evidence for the composition of the outer atmosphere of the Sun.
How are absorption lines in the Sun produced?
Photons of all frequencies are produced in the hot, dense core of the sun. Certain frequencies are absorbed by the relatively cooler gases in the outer atmosphere of the Sun.
