3B.3 Diffraction

Question 1

What is diffraction?

Answer 1

The spreading of waves when they pass through a gap or around an obstacle.

Question 2

When is diffraction most noticeable?

Answer 2

When the gap size is similar to the wavelength of the wave.

Question 3

How does gap size affect diffraction?

Answer 3

Very small gap: little wave passes; gap ≈ wavelength: maximum diffraction; very large gap: little spreading.

Question 4

Why can sound be heard around corners but light usually cannot?

Answer 4

Sound has longer wavelengths, so it diffracts more easily around obstacles.

Question 5

What pattern is produced in single-slit diffraction?

Answer 5

A central bright maximum (widest) with alternating dark minima and bright maxima, with intensity decreasing away from the centre.

Question 6

What happens to the diffraction pattern when the slit becomes narrower?

Answer 6

The central maximum becomes wider.

Question 7

What is a diffraction grating?

Answer 7

A surface containing many closely spaced parallel slits that produce multiple diffraction patterns.

Question 8

What is the diffraction grating equation?

Answer 8

nλ = d sinθ, where n is order, λ is wavelength, d is slit spacing, and θ is the diffraction angle.

Question 9

What does “order” mean in diffraction gratings?

Answer 9

The number of the bright fringe measured from the central maximum.

Question 10

What is the order of the central maximum?

Answer 10

n = 0.

Question 11

How is slit spacing calculated from grating lines?

Answer 11

d = 1 / (number of lines per metre).

Question 12

How can slit spacing be calculated if the grating is given in lines per mm?

Answer 12

d = 1×10⁻³ / (lines per mm).

Question 13

How is wavelength measured using a diffraction grating?

Answer 13

Shine monochromatic light through the grating, measure the angle of bright maxima, and use nλ = d sinθ.

Question 14

What everyday object behaves like a diffraction grating?

Answer 14

A CD or DVD.

Question 15

What are maxima and minima in diffraction patterns?

Answer 15

Maxima are bright regions and minima are dark regions.