waves and the particle nature of light

Question 1

wave velocity

Answer 1

v=λf

Question 2

longitudinal waves

Answer 2

oscillations are parallel to the direction of energy transfer, made up of compressions and rarefactions. sound is the best example

Question 3

rarefaction stage of longitudinal wave

Answer 3

pressure is decreased, neighbouring particles move away from each other

Question 4

compression stage of transverse wave

Answer 4

pressure is increased, neighbouring particles move towards a point

Question 5

interference

Answer 5

occurs during superposition

Question 6

constructive interference

Answer 6

occurs when two waves are in phase and so their displacements are added

Question 7

destructive interference

Answer 7

occurs when two waves are completely out of phase and so displacements are subtracted

Question 8

phase difference

Answer 8

two waves are in phase if they are both at the same point in the wave cycle, so they have the same frequency, wavelength and their phase difference is an integer of 360°. the waves do not need the same amplitude

Question 9

path difference equation

Answer 9

∆x=∆ φx λ/2π

Question 10

stationary waves

Answer 10

formed by the superposition of two waves, travelling in opposite directions in the same plane, with the same frequency, wavelength and amplitude. no energy is transferred in a stationary wave

Question 11

stationary wave in phase

Answer 11

constructive interference, where an antinode forms at a point of maximum displacement

Question 12

stationary wave out if phase

Answer 12

destructive interference, a side forms at a point of minimum displacements

Question 13

speed of transverse wave on string equation

Answer 13

v=rootT/μ

Question 14

refractive index

Answer 14

a property of a material which measures how much it slows down loath passing through it, n=c/v. higher refractive index means the material is more optically dense

Question 15

refraction

Answer 15

when a wave enters a different medium, causing it tot change direction, either towards or away from the normal depending on the materials refractive index. as the light moves across the boundary, its speed changes which causes its direction to change.
n1sinθ1=n2sinθ2

Question 16

n2>n1, which way does light bend

Answer 16

towards the normal

Question 17

critical angle

Answer 17

as the angle of incidence is increased, the angle of refraction also increases until it gets closer to 90°. when the angle of refraction is exactly 90° and the light is refracted along the boundary, the angle of incidence has reached the critical angle (C)

Question 18

critical angle equation

Answer 18

if n2 is air: sinC=1/n
if otherwise: sinC=n2/n1

Question 19

total internal reflection

Answer 19

occurs when the angle of incidence is greater than the critical angle, and the incident refractive index (n1) is greater than the refractive index material at the boundary (n2)

Question 20

converging lenses

Answer 20

cause parallel rays to move closer

Question 21

diverging lens

Answer 21

cause parallel rays to move apart

Question 22

3 features of lenses

Answer 22

-principle focus:
converging-where light rays focus
diverging- where rays appear to come from
-focal length: distance from lens to principle focus
-power: measure of lens’ ability to bend light. in a converging lens it is positive. diverging it is negative

Question 23

power of a lens

Answer 23

the reciprocal of the focal length
P=1/f

Question 24

power of several thin lessons in combination

Answer 24

acts as a single lens with a power equal to the sum of the powers of the individual lenses
P=P1+P2+P3

Question 25

real vs virtual image

Answer 25

real image can be projected onto a screen, a virtual image cannot

Question 26

magnification

Answer 26

the ratio of the size of the image it creates with respect to the size of the object
image height/object height

Question 27

plane polarisation

Answer 27

allowing oscillation of waves in one plane. a polarised wave only oscillates in one plane. only transverse waves can be polarised.

Question 28

why does light diffract less through a door than sound

Answer 28

the wavelength of light is so much smaller, and the greatest amount of diffraction occurs when the gap is the same size as the wavelength

Question 29

diffraction grating

Answer 29

a slide containing many equally spaced slits very close together. when light is passed through, it forms an interference pattern composed of light and dark fringes

Question 30

diffraction grating equation

Answer 30

nλ=dsinθ
d= distance between slits

Question 31

what is the pattern produced when blue and red light pass through a diffraction grating

Answer 31

lines seen where constructive interference occurs. red wavelength longer than blue so diffracted greater and so blue lines on the inside and red lines on the outside of the light fringes.

Question 32

electron diffraction

Answer 32

using an electron gun to accelerate electrons through a vacuum towards a crystal lattice, they interact with the small gaps between atoms and form an interference pattern on a fluorescent screen behind the crystal. the pattern looks like concentric rings

Question 33

deBroglie hypothesis

Answer 33

λ=h/p
this shows all particles have a wave nature

Question 33

what does electron diffraction show

Answer 33

if electrons had particle nature, the pattern would be a singular point. this is nit the case as the electrons are diffracted which proves evidence of their wave nature

Question 34

wave behaviour at an interface

Answer 34

transmitted- pass into next material
reflected- bounce off surface

Question 35

pulse echo technique using ultra sound to form an image

Answer 35

1) short pulse of ultra sound waves are transmitted into the target
2) pulse travels until it reaches a boundary between two mediums where some of the pulse is reflected back. the amount of reflection depends on the difference in densities. the greater the difference the greater the reflection
3) reflected waves are deflected as they leave
4) the intensities of the reflected waves are used to determine the structure and the time taken is used to determine the position

Question 36

issues with pulse echo technique

Answer 36

-if the duration is too long, they will likely overlap, meaning the amount of information you obtain will decrease
-the larger the wavelength, the less fine details can be resolved, meaning less information

Question 37

wave model of light

Answer 37

EM radiation can be described as a transverse wave

Question 37

the work function

Answer 37

minimum energy required for electrons to be emitted from the surface of a metal

Question 38

photon model of light

Answer 38

EM waves travel in discrete packets called photons, which have energy directly proportional to their frequency (E=hf)

Question 39

photoelectric effect as evidence of the particle nature of EM radiation

Answer 39

-wave theory suggests any frequency of light should be able to cause photoelectric emission
-photoelectric effect is immediate
-increasing the intensity does nit increase the speed of emission, but increases the amount of electrons emitted
-electrons released with a range of kinetic energies

Question 39

photoelectric effects

Answer 39

photoelectrons are emitted from the surface of a metal after light above a certain frequency (threshold frequency) is shine on it.
photoelectrons are emitted because electrons near the surface of the metal absorb a photon and gain enough energy to leave the surface

Question 40

why must light be a particle, not a wave?

Answer 40

-all photon energy is transferred to the electron, one photon interacts with one electron
-energy transfer ins immediate
-intensity is equal to the number of photons released per second, an increase would increase the number of interactions with electrons per second
-all electrons receive the same amount of energy, but those deeper electrons loose energy through collisions and so have less kinetic energy

Question 41

evidence that electrons in atoms can only transition between discrete wavelengths

Answer 41

inside a fluorescent tube, electrons are accelerated, causing gas atoms to become excited and then de-excite, releasing a photon. by passing the light through a diffraction grating or prism, you get a line spectrum. Each line in the spectrum represents a different wavelength of light emitted. spectrum contains discrete values of wavelength , the photon energies correspond to these wavelengths.

Question 41

the difference in energy levels

Answer 41

it is equal to a specific photon energy emitted or absorbed, therefore the energy of emitted photon:
∆E=E1-E2
hf=E1-E2
f=(E1-E2)/h

Question 41

atomic line spectra

Answer 41

electrons in atoms exist in discrete energy levels, if an electron gains enough energy it can move up in energy level, however t will quickly return to its original level and release the energy it gained in the form of a photon of light

Question 42

superposition definition

Answer 42

when two waves meet, their resultant displacement is the sum of their individual displacements

Question 43

visible light wavelengths

Answer 43

400-750nm

Question 44

intensity relation to amplitude

Answer 44

I is proportional to A^2, and so if intensity is doubled, the amplitude is multiplied by four

Question 45

phase difference calculation

Answer 45

∆d/λ= ∆t/T then multiply by 2π or 360

Question 46

how to calculate how many order will be visible

Answer 46

using dsinx=nλ, make x=90 and then you should find a value for n e.g 4.7 and so 4 is the maximum order found and 9 is how many light fringes will be seen

Question 47

order in relation to wavelength

Answer 47

as d is proportional to nλ, λ and n are inversely proportional and so a smaller wavelength means more visible orders

Question 48

number of slits

Answer 48

you will be told how many slits per mm which should be converted into metres and then find the reciprocal to fid the distance between slits which =d