3.4 - Diffraction

Question 1

What is diffraction?

Answer 1

The spreading out of waves when they pass an obstruction.

Question 2

What is an obstruction in the scenario of laser light?

Answer 2

Typically a narrow slit known as an aperture.

Question 3

What effects the extent of diffraction?

Answer 3

The width of the aperture gap compared to the wavelength of the wave.

Question 4

When is diffraction most prominent?

Answer 4

When the width of the slit is approximately equal to the wavelength.

Question 5

What property of a wave changes when it diffracts and why?

Answer 5

Its amplitude because some energy is dissipated when a wave is diffracted through a gap.

Question 6

What are 2 features of the single slit diffration?

Answer 6

• A central maximum with a high intensity
• Subsidiary maxima equally space, successively smaller in intensity and half the width of the central maximum.

Question 7

What are 3 things that would happen to the diffraction pattern if white light was diffracted through a single slit?

Answer 7

• the central maximum would be white
• All maxima would be composed of a spectrum (violet to red)
• The fringe spacing would be smaller and the maxima would be wider

Question 8

What happens when the gap size in single slit diffraction is much wider than the wavelength? (single slit)

Answer 8

The waves are no longer spread out.

Question 9

What are 2 differences in diffraction pattern between a blue and red laser? (single slit)

Answer 9

• The wavelength of red light is longer so the light would diffract more
• The intensity fringes would therefore be wider

Question 10

What happens if the slit in a single slit experiment is made narrower?

Answer 10

• The intensity would decrease
• The fringe spacing would be wider

Question 11

What is a diffraction grating?

Answer 11

A plate on which there is a very large number of parallel, identical, closely spaced slits.

Question 12

What happens when monochromatic light is incident on a diffraction grating?

Answer 12

A pattern of narrow bright fringes is produced on a screen.

Question 13

What is the diffraction grating equation?

Answer 13

nλ = dsinθ
n= order of maxima
λ = wavelength
d = spacing between adjacent slits
sinθ = angular separation between the order of maxima

Question 14

How do you calculate d when given lines per metre/mm?

Answer 14

d = 1/N

Question 15

How do you calculate the angular separation?

Answer 15

θ in the diffraction equation. Rearrange it to make θ the subject.

Question 16

What is the maximum angle to see orders of maxima?

Answer 16

When the beam is at right angles to the diffraction grating.
θ = 90 degrees
sinθ = 1

Question 17

How is the highest order of maxima visible calculated?

Answer 17

n = d/λ

Question 18

How is the highest order of maxima visible calculated?

Answer 18

n = d/λ
(always round n down)

Question 19

What is the path difference at n=0 when light passes through the slits of the diffraction grating?

Answer 19

The path difference is 0.

Question 20

What is the path difference at n=1 when light passes through the slits of the diffraction grating?

Answer 20

The path difference is λ.
There is therefore constructive interference.

Question 21

What is the path difference at any order above n=1 when light passes through the slits of the diffraction grating?

Answer 21

the path difference will always be equal to nλ.

Question 22

What are 3 uses of diffraction gratings?

Answer 22

• separating light of different wavelengths
• spectrometers
• x-ray crystallography

Question 23

What is x-ray crystallography and how is it done?

Answer 23

Diffraction pattern is then used to measure the atomic spacing in certain materials.
X-rays are directed at a thin crystal sheet which acts as a diffraction grating to form a diffraction pattern (due to the wavelength of x-ray being a similar size to the gaps between the atoms).