Diffraction and diffraction gratingsEdexcel A-Level Physics: Subtopic test

1. 1
   In a ripple tank, a vibrating dipper produces plane water waves of wavelength 1.5 cm that travel towards two barriers with an adjustable gap between them. The speed of the waves in the tank is constant, and the frequency of the dipper can be changed.

   (a)

   For which gap width is the diffraction of the water waves most pronounced?

   [1 mark]

   • A1.5 cm
   • B150 cm
   • C15 cm
   • D6.0 cm

   (b)

   The frequency of the dipper is doubled with the gap unchanged. What happens to the diffraction at the gap?

   [1 mark]

   • AThe waves no longer pass through the gap
   • BDiffraction is more pronounced because the frequency is higher
   • CDiffraction is less pronounced because the wavelength is shorter
   • DDiffraction is unchanged because the wave speed is constant

   (c)

   Use Huygens' construction to explain why the waves spread into the region behind the edges of the barriers.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student shines a red laser of wavelength 633 nm at normal incidence onto a diffraction grating with 600 lines per millimetre. A series of bright spots is seen on a distant screen, either side of a central bright spot.

   (a)

   What is the spacing between adjacent lines of the grating?

   [1 mark]

   • A1.7 × 10⁻³ m
   • B6.0 × 10⁵ m
   • C1.7 × 10⁻⁹ m
   • D1.7 × 10⁻⁶ m

   (b)

   What is the highest order of maximum that can be observed?

   [1 mark]

   • A1
   • B2
   • C3
   • D4

   (c)

   Calculate the angle between the central maximum and the first-order maximum.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a core practical, a student determines the wavelength of light from a laser using a diffraction grating with 300 lines per millimetre. The grating is held perpendicular to the beam and the screen is 1.00 m from it. The student measures the distance between the two second-order maxima, one on each side of the central maximum, as 0.850 m.

   (a)

   Calculate the wavelength of the laser light.

   [3 marks]

   (b)

   Explain why the student measures the distance between the second-order maxima on both sides of the central maximum rather than from the central maximum to one second-order maximum. State and explain how the maxima would change if a laser of shorter wavelength were used.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student has a filament lamp and a laser pointer, and a diffraction grating with 500 lines per millimetre. She directs light from each source in turn at normal incidence onto the grating and observes the pattern on a distant white screen.

   (a)

   Describe the pattern the student sees on the screen with the filament lamp, and explain how it is formed.

   [6 marks]

   (b)

   She replaces the filament lamp with the laser pointer of wavelength 532 nm. Determine the first-order angle and the total number of maxima seen on the screen. She then replaces the grating with one of 100 lines per millimetre. Evaluate which grating would give the more precise value of wavelength from measurements of the angles of the maxima.

   [6 marks]

   Total for question 4: 12 marks

End of questions