Topic 5 Waves and particle nature of light Flashcards
What could cause the first order maxima to be closer to the central maxima? (1)
-Using laser light with a higher frequency.
For two waves of light to be coherent waves must. (1)
-Originate from one source.
Describe how a diffraction grating is used and the measurements that should be taken. (3)
-Set up diffraction grating at right angles to light from laser.
-Measure distance between diffraction grating and screen.
-Measure the distance between first order images on screen.
Explain how the diffraction pattern is created. (3)
-Waves pass through narrow gap and spread out.
-Light reaches wall from each part of slit with differing phase relationship.
-When waves meet superposition occurs, if waves are in phase it results in constructive interference so brighter spots are formed.
Explain how the diffraction pattern would differ if green laser light were used instead of red laser light. (3)
-Green light has a shorter wavelength than red light.
-So green light diffracts less than red light.
-So dark spots closer to centre.
Use Huygens’ construction to describe what happens to light waves after passing through a narrow gap. (3)
-Wave spreads out after passing through gap.
-Each point on wave acts as a source of secondary wavelets.
-That interfere/superpose.
State what is meant by a wavefront. (1)
-Surface joining points on a wave that are in phase.
Explain why a monochromatic light source is important in diffraction experiments. (3)
-Emits a very small range of wavelengths.
-So smaller variation at each diffraction angle.
-Producing clearer/sharper interference pattern.
State what is meant by superposition of waves. (2)
-Two or more waves meet.
-The resultant displacement is the sum of the individual displacements from the individual waves.
Radiation of frequency f and wavelength λ is emitted when an electron falls from energy level E2 to energy level E1.
What is E2 − E1 is equal to? (1)
hc/λ
When the light from a star is dispersed to form a spectrum, dark lines are seen at a number of frequencies. This is known as an absorption spectrum and is caused by the presence of certain elements in the star.
Explain how the absorption spectrum is created. (3)
-If photon energy is equal to energy level difference in elements present.
-Then photon can be absorbed by an electron, electron is then excited and moves to a higher energy level.
-So absorption spectrum is created as frequencies of absorbed photons are missing from continuous spectrum produced by stars.
Describe the particle model of ultraviolet radiation that explains how it can “knock electrons out of atoms”. (3)
-UV radiation consists of photons.
-One photon interacts with one electron.
-Electrons released if energy of photon>work function.
Explain how atoms emit radiation of a particular frequency. (5)
-Atoms contain/have/exist in discrete energy levels.
-Electron loses energy and falls down energy levels emitting a photon.
-With energy equal to difference in energy levels.
-Energy of photon is proportional to frequency, E=hf.
-So emitted frequency of radiation corresponds to difference in energy levels of a particular atoms.
When light is incident on an LDR, electrons move to a higher energy level where they become conduction electrons. This causes the resistance of the LDR to decrease.
A student suggests that this is an example of the photoelectric effect. The student is not correct.
Compare and contrast the photoelectric effect with the effect of radiation incident on an LDR. (6)
Similarities:
-An electron absorbs a photon.
-Photons need minimum amount of energy.
-Light must be above a certain frequency.
-Increasing light intensity increases no. electrons released per second.
-Evidence for particle model of light.
Differences:
-In photoelectric effect electrons are released from surface.
-But electrons remain within LDR.
-Photelectric effect occurs in metals.
Explain what is meant by the particle nature of light. (2)
-Light consists of particles called photons.
-These particles are discrete packets of energy.
