Retrieval Practice
Interference & Diffraction — AQA A-Level Physics
Q1. State the two conditions for coherence.
Same frequency and a constant phase difference.
Q2. Define path difference.
The difference in distance travelled by two waves from their sources to the point where they meet.
Q3. State the condition for constructive interference in terms of path difference.
Path difference = n lambda (a whole number of wavelengths), where n = 0, 1, 2, 3...
Q4. State the condition for destructive interference in terms of path difference.
Path difference = (n + 1/2) lambda (an odd number of half wavelengths), where n = 0, 1, 2, 3...
Q5. Write the Young's double slit fringe spacing equation and define all terms.
w = lambda D / s. w = fringe spacing (m), lambda = wavelength (m), D = slit-to-screen distance (m), s = slit separation (m).
Q6. How does the single slit diffraction pattern differ from the double slit pattern?
- Single slit: wide bright central maximum, side maxima decrease in intensity.
- Double slit: equally spaced fringes of equal brightness.
Q7. Write the diffraction grating equation.
d sin theta = n lambda, where d = slit spacing (m), theta = diffraction angle, n = order number, lambda = wavelength (m).
Q8. How do you calculate slit spacing d from the number of lines per metre N?
- d = 1/N.
- If N is in lines per mm, convert to lines per m first.
Q9. What is the maximum visible order from a diffraction grating?
n_max = d / lambda, rounded down to the nearest integer.
Q10. What happens to the diffraction pattern when the wavelength increases?
- Fringes become wider and more spread out.
- More diffraction occurs.
Q11. State three laser safety precautions.
Never look directly at the beam, wear laser goggles, place a 'laser on' warning sign outside the room.
Q12. Name two applications of diffraction gratings.
Analysing light from stars in spectrometers, and X-ray crystallography to determine atomic spacing.
Q13. What experimental evidence showed light is a wave?
Young's double slit experiment (1801) showed constructive and destructive interference, which only waves can produce.