Question 1a

Marks: 3

Monochromatic light from a single source is incident on two thin parallel slits.

9-3-ib-hl-hsq1-a-q

The following data are available:

Distance from slits to screen = 4.5 m
Wavelength = 690 nm
Slit separation = 0.14 mm

The intensity, I of the light on the screen from each slit separately is I₀.

9-3-ib-hl-hsq1-a-q2

(a)

Sketch, on the axis, a graph to show variation with distance p on the screen against the intensity of light detected on the screen for this arrangement.

[3]

Question 1b

Marks: 3

(b)

Calculate the angle of diffraction of the central and subsequent two bright fringes that would appear on the screen.

Give your answer in degrees to one significant figure.

[3]

Question 1c

Marks: 4

The relative intensity I₁ for the first bright fringe is 0.75I₀ and for the second bright fringe I₂ is 0.25I₀.

$angle-of-diffraction$

(c)

Plot, on the axis, a graph to show this diffraction pattern.

[4]

Question 1d

Marks: 4

(d)

State and explain the changes that will occur to the diffraction pattern when the number of slits is increased from two to three.

[4]

Question 2a

Marks: 6

Students in a laboratory have created the following set-up. Parallel rays of monochromatic light from two adjacent slits A and B of wavelength $λ$ are incident normally on a diffraction grating with a slit separation $d.$

9-3-ib-hl-hsq2a-q

(a)

Use the diagram to derive the equation nλ = dsinθ where θ is the angle of diffraction of a maxima order n visible on a screen.

[6]

Question 2b

Marks: 5

The monochromatic light in the set-up in part (a) has a wavelength of 545 nm. The graph shows the variation of sinθ with the order n of the maximum. The central order corresponds to n = 0.

9-4-ib-hl-hsq2b-q

(b)

Determine a mean value for the number of slits per mm of the grating.

[5]

Question 2c

Marks: 3

The grating is 40 mm wide.

(c)

Determine the number of slits required to obtain a maximum of six bright fringes on the screen.

[3]

Question 2d

Marks: 2

The students claim that they observed the following diffraction pattern on the screen for the grating from part (c).

9-3-2

(d)

State two reasons why the interference pattern obtained cannot be correct.

[2]

Question 3a

Marks: 3

An internet company is looking to improve the amount of light transmitted through its optical fibres. The fibres are made of glass and have a refractive index n_g.

The proposed solution has been to spread a thin film of oil on the inside surface with a refractive index n_o. The air inside the fibre has a refractive index n_aand the air outside has a refractive index n.

n < n_a < n_o < n_g

9-3-ib-hl-hsq3a-q

(a)

Complete the ray diagram to show the path of the incident light ray along the optical fibre.

You do not need to indicate any angles of refraction.

[3]

Question 3b

Marks: 3

(b)

Outline the conditions for constructive interference to occur as two light rays travel down in an optical fibre with oil on the inside.

You may include a diagram in your answer.

[3]

Question 3c

Marks: 2

The oil has a thickness of 100 nm and a refractive index of 1.4.

(c)

Determine the longest possible wavelength of light that can be incident on the oil to obtain constructive interference.

[2]

Question 3d

Marks: 4

(d)

Analyse whether the oil improves the amount of light transmitted through the optical fibres.

(i)

Describe the path of the ray with the presence of oil.

[1]

(ii)

Describe the path of the ray without the presence of oil.

[1]

(iii)

State and explain whether the oil improves the amount of light transmitted through the optical fibres.

[2]

DP IB Physics: HL

Topic Questions

9.3 Interference

Question 1a

Question 1b

Question 1c

Question 1d

Question 2a

Question 2b

Question 2c

Question 2d

Question 3a

Question 3b

Question 3c

Question 3d

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