Diffraction

Question 1

When does diffraction occur?

Answer 1

Diffraction occurs when waves spread out after passing through a gap or around an obstacle.

Question 2

What are some features of diffraction?

Answer 2

• The narrower the gap, the more the waves spread out.

* The longer the wavelength the more the waves spread out.

Question 3

What is diffraction?

Answer 3

The way that waves spread out as they come through a narrow gap or go round obstacles.

Question 4

What does the amount of diffraction depend on?wavelengths wide.
• Most diffraction occurs when the gap is the same size as the wavelength

Answer 4

The amount of diffraction depends on the wavelength of the wave compared with the size of the gap.
• When the gap is bigger than the wavelength, diffraction is unnoticeable.
• Noticeable diffraction occurs through a gap several wavelengths wide.
• Most diffraction occurs when the gap is the same size as the wavelength
• If the gap is smaller than the wavelength, the waves are mostly reflected back.

Question 5

What happens when the gap is bigger than the wavelength?

Answer 5

Diffraction is unnoticeable.

Question 6

When do you get noticeable diffraction?

Answer 6

• Noticeable diffraction occurs through a gap several wavelengths wide.

Question 7

When does the most diffraction occur?

Answer 7

• Most diffraction occurs when the gap is the same size as the wavelength

Question 8

What happens if the gap is smaller than the wavelength?

Answer 8

• If the gap is smaller than the wavelength, the waves are mostly reflected back.

Question 9

What is required to view a clear diffraction pattern for light?

Answer 9

You need a monochromatic, coherent light source.

Question 10

What does monochromatic mean?

Answer 10

All the light has the same wavelength and frequency, and also therefore is the same colour.

Question 11

Why do waves diffract?

Answer 11

• Each point on a wavefront is a secondary emitted of wavelets.
• The wavelets from the point along a wavefront travel only in the direction in which the wave is travelling, not in the reverse direction.
• They then combine to form a new wavefront spreading beyond the gap.

Question 12

What is a wavefront?

Answer 12

A wavefront is a line or surface, in the path of a wave motion, where all the displacements at any point have the same phase

Question 13

What is the central maximum?

Answer 13

• The central maximum in a single slit light diffraction pattern is the brightest part of the pattern.
• This is because the intensity of light is highest in the centre.

Question 14

What is the formula for the width of the central fringe?

Answer 14

Width of the central fringe = ( Wavelength of the light / Width of the Single Slit ) x 2 D
• D = Distance from the slit

Question 15

What happens if the single slit pattern is observed using different sources of monochromatic light?

Answer 15

Using different sources of monochromatic light:

The observations show that the greater the wavelength the wider the fringes

Question 16

What happens if the single slit pattern is observed using an adjustable slit?

Answer 16

observations show that making the slit narrower makes the fringes wider.

Question 17

What happens if the slits are too wide and too far apart in young’s experiment?

Answer 17

No interference pattern is observed, this is because interference can only occur if the light from the two slits overlaps, for this to be the case:
• Each Slit must be narrow enough to make the light passing through it diffract sufficiently
• Two slits must be close enough so the diffracted waves overlap on the screen.

Question 18

What is required for interference to occur?

Answer 18

interference can only occur if the light from the two slits overlaps, for this to be the case:
• Each Slit must be narrow enough to make the light passing through it diffract sufficiently
• Two slits must be close enough so the diffracted waves overlap on the screen.

Question 19

For monochromatic light, incident on two slits, what is the formula for for fringe spacing of the interference fringes?

Answer 19

W = wavelength x screen distance from the slit / s 
S = slit separation
W = λ x D / S

Question 20

What is the formula for the width of the central diffraction fringe?

Answer 20

W = 2 x λ x D / a
D = slit-screen distance

Question 21

What is intensity?

Answer 21

Power per unit area

Question 22

What does increasing the light do for monochromatic light and why?

Answer 22

• For monochromatic light, all photons have the same energy
• An increase in intensity means an increase in the number of photons per second.
• There are more photons per unit area hitting the central maximum per second than the other bright fringes.

Question 23

What two factors affect the width of the central maximum?

Answer 23

• Slit width

* Wavelengeth

Question 24

How does slit width affect width of the central maximum?

Answer 24

• Increasing the slit width decreases the amount of diffraction.
• This means the central maximum is narrower, and the intensity of the central maximum is higher

Question 25

how does wavelength affect width of the central maximum?

Answer 25

• Increasing the wavelength increases the amount of diffraction.
• This means the central maximum is wider, and the intensity of the central maximum is lower.