Diffraction

Question 1

What is a wavefront?

Answer 1

an imaginary line that cuts the waves, joining up points that are in phase with each other

Question 2

What is diffraction?

Answer 2

when a wave passes through an obstacle or goes into a gap it spreads out and changes shape

Question 3

What wave properties change during diffraction?

Answer 3

none, frequency wavelength and wavespeed remain the same

Question 4

What does narrowing the gap do to a wave passing through it?

Answer 4

the wavefronts become more circular - there is more diffraction and hence more spreading

Question 5

when do we get the most diffraction?

Answer 5

when the gap is 1 wavelength long of the wave passing through it

Question 6

What happens when the gap is too small?

Answer 6

the wave is reflected back

Question 7

What does diffraction depend upon?

Answer 7

the wavelength and width of gap (in comparison)

Question 8

When does diffraction become more noticeable?

Answer 8

as the gap is decreased to the magnitude of the wavelength of the wave

Question 9

How can we investigate diffraction?

Answer 9

an oscillating paddle generating waves with paralllel wavefronts in water. we also have an object in the ripple tank which acts as an obstacle

Question 10

Is diffraction noticeable with a huge gap?

Answer 10

no

Question 11

What is HUYGEN’S CONSTRUCTION?

Answer 11

it states that every point on a wavefront may be considered to be a source of secondary wavelets that spread out in the forward direction at the speed of the wav. The every wavefront is an envelope of secondary wavelets

Question 12

What happens when an obstacle stands in front of a wave?

Answer 12

we get diffraction around it but behind we get an area wth a shadow, as waves don’t combine straight away

Question 13

How can we make the shadow longer?

Answer 13

by increasing object width compared to wavelength to cause less diffraction

Question 14

What interference pattern do we get when white light is passed through a single slit, with the slit having a similar width to the wavelength

Answer 14

consecutive bright and dark fringes.

• this pattern has a central bright fringe with alternating bright and dark fringes either side
• dark fringe is a minima
• bright fringe is a maxima
• intensity decreases the further a fringe is from the central maxima

Question 15

What does diffraction of light through more slits cause?

Answer 15

narrower maxima and minimas but of greater intensity, this is because the pattern is reinforced

Question 16

Why does having narrower maximas and minimas help (PAPER 3)?

Answer 16

increased precision of measurements decreasing uncertainty -

Question 17

What is a diffraction grating?

Answer 17

surface with loads of parallel gaps

Question 18

Why are maximas produced?

Answer 18

when coherent waves superpose causing constructive interference. The waves arriving at the point of the maxima must be in phase.

Question 19

Why are minimas produced

Answer 19

when coherent waves superpose causing destructive interference. The waves arriving at this point must be out of phase

Question 20

Equation for maximum amount of orders?

Answer 20

D/lambda = max orders

Question 21

Equation for diffraction?

Answer 21

n(lambda) = dsinx 
where 
n = order 
lamdba = wavelength 
d = slit seperation or slit width
sinx = spread

Question 22

How can we create sources of coherent light?

Answer 22

passing it through a single slit

Question 23

Larger wavelength means…

Answer 23

more spread

Question 24

greater slit seperation means…

Answer 24

less spread

Question 25

What happens when we use white light with a diffraction grating?

Answer 25

since white light is a mixture of colours we get a spectra pattern. Each order in the pattern becomes a spectrum, with red on the outside and violet on the inside. The zero order maximum is just white as all wavelengths just pass straight through.

Question 26

monochromatic light?

Answer 26

light of one singular wavelength