Particles and Waves Part 1 Flashcards
Give an example of a piece of evidence that light behaves as a particle, not as a wave.
Photoelectric effect (radiation / photons incident on a metal surface removing excess electrons).
What is meant by waves which are coherent?
Coherent means that waves have the same frequency, wavelength and velocity, and have a constant phase relationship.
The examples shown are both of coherent waves.
What is the definition of the photoelectric effect?
The photoelectric effect occurs when electromagnetic radiation (in the form of photons) is incident on a metal surface and ejects electrons from the metal.
In an experiment to demonstrate the photoelectric effect, a gold leaf electroscope is negatively charged, and an ultraviolet lamp is directed at a steel cap.
The electroscope does not discharge.
Suggest a change that could be made to the experiment which would cause the electroscope to discharge.
The metal cap of the electroscope could be changed to one with a lower work function than steel (for example, zinc).
When light passes from a material into one with a lower refractive index (e.g. glass to air), what happens to its speed?
When light passes from a material into one with a lower refractive index (e.g. glass to air), its speed increases.
When light passes from a material into one with a lower refractive index (e.g. glass to air), what happens to its frequency?
When light passes from a material into one with a lower refractive index (e.g. glass to air), its frequency remains constant.
What does the symbol f0 represent?
f0 represents the threshold frequency - the minimum frequency of radiation required to eject an electron from the surface of a metal.
Waves from two coherent sources meet at point P, perfectly in phase.
What name is given to point P?
Point P is the central maximum.
What is the unit for energy?
The unit for energy is joules (J).
What is the definition of threshold frequency?
Threshold frequency is the minimum frequency of radiation required to eject an electron from the surface of a metal.
In an experiment to demonstrate the photoelectric effect, a gold leaf electroscope is negatively charged, and an ultraviolet lamp is directed at the metal. The electroscope discharges (the gold leaf slowly falls).
A UV lamp with the same frequency, but greater intensity, is now used.
What difference would be observed in the experiment?
Explain your answer.
The gold leaf (in the electroscope) would fall faster.
A UV lamp of greater intensity will emit more photons per second, meaning more electrons per second are ejected from the metal.
Explain what is shown in this picture.
This is constructive interference.
Two waves meet at a point exactly in phase (peak meets peak, and trough meets trough) to form a wave of increased amplitude.
Name the three quantities shown in this formula, and give the unit for each one.
I is irradiance (in W m-2)
P is power (in W)
A is area (in m2)
What characteristic of waves is shown in this diagram?
Diffraction
Define all symbols in the formulae shown.
n = refractive index
ϴ = angle (in air or in a material)
λ = wavelength (in air or in a material)
v = speed of light (in air or in a material)
In the diagram, a ray of light undergoes refraction as it enters a glass block.
How would the refractive index of the block be calculated?
The refractive index can be calculated by using the formula
Waves from two coherent sources arrive at point P.
Point P is exactly the same distance from sources S1 and S2.
Explain what is happening at P.
Point P is a maximum.
Constructive interference is occurring at P because path difference = zero.
The waves are meeting at P in phase.
In an experiment to demonstrate the photoelectric effect, a gold leaf electroscope is negatively charged, and a red laser is directed at the metal. The electroscope does not discharge.
When the experiment is repeated with a violet laser, the electroscope does discharge.
Explain why this happens.
Violet light has a shorter wavelength / higher frequency than red light.
The photons from the violet laser will have a greater energy than red.
The energy of the photons is given by E = hf.
The diagram shows refraction of different colours (frequencies).
What conclusion can be drawn about the relationship between frequency and refractive index?
Violet light has a higher frequency than red.
Violet light is refracted by a greater angle than red.
The higher the frequency, the greater the refractive index.
A ray of white light is refracted by a prism.
Describe the pattern produced.
A spectrum would be produced.
Red is refracted the least, violet refracted the most.
When does total internal reflection to occur?
Total internal reflection occurs when the angle of incidence is greater than the critical angle.
What is the definition of critical angle?
Critical angle is the angle of incidence that results in an angle of refraction of 90° to the normal.
Waves from two coherent sources arrive at point P1.
The distance from S1 to P1 is one whole wavelength less than the distance from S2 to P1.
Explain what is happening at P1.
Point P1 is a maximum.
Constructive interference is occurring at P1 because path difference = λ.
The waves from S1 and S2 meet in phase.
What characteristic of waves is shown in this diagram?
Interference
A ray of white light is incident on a diffraction grating.
Describe the pattern that would be produced.
There are several maxima observed.
There is a white central maximum.
There are spectra either side of the central maximum.
In each spectrum, violet is diffracted the least, red is diffracted the most.
The relationship between irradiance and distance is an example of an inverse square law.
What is meant by an ‘inverse square law’ - in this context?
An inverse square law, in this context, means that if the distance is multiplied by 2, the irradiance is divided by 4 (2 squared).
If the distance is multiplied by 10, the irradiance is divided by 100 (10 squared) - and so on.
Explain, in terms of waves, what is shown in this picture.
This is destructive interference.
Two waves meet at a point exactly out-of-phase (crest meets trough, and trough meets crest) to form a wave of reduced amplitude.
(if both waves are the same amplitude then the new amplitude will be zero)
What characteristic of waves is shown in this diagram?
Reflection
What characteristic of waves is shown in this diagram?
Refraction
