A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Fringes and Huygens' Wave Theory of Light — mark scheme explained
The short answer
In the realm of physics, particularly within the study of wave phenomena, understanding the behavior of light is crucial. One significant aspect of this is the explanation for fringes in general terms, which ties into the delayed acceptance of Huygens’ wave theory of light.
The question
Explain how Huygens' principle can be used to describe the formation of fringes in a double-slit experiment. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
According to Huygens' principle, every point on a wavefront acts as a source of secondary spherical wavelets.
- S2
In a double-slit experiment, light passing through each slit acts as a new source of these wavelets.
- S3
The wavelets from the two slits overlap and interfere with each other on a screen placed some distance away.
- S4
Constructive interference occurs when the path difference between the waves from the two slits is an integer multiple of the wavelength (nλ), resulting in bright fringes.
- S5
Destructive interference happens when the path difference is an odd multiple of half a wavelength ((2n+1)λ/2), leading to dark fringes.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
According to Huygens' principle, every point on a wavefront acts as a source of secondary spherical wavelets.
- S2
In a double-slit experiment, light passing through each slit acts as a new source of these wavelets.
- S3
The wavelets from the two slits overlap and interfere with each other on a screen placed some distance away.
- S4
Constructive interference occurs when the path difference between the waves from the two slits is an integer multiple of the wavelength (nλ), resulting in bright fringes.
- S5
Destructive interference happens when the path difference is an odd multiple of half a wavelength ((2n+1)λ/2), leading to dark fringes.
Final answer: Huygens' principle explains that light waves from each slit interfere constructively and destructively, creating bright and dark fringes on the screen.
Common mistakes
- Confusing constructive and destructive interference — Reinforce that constructive interference occurs when the path difference is an integer multiple of the wavelength (nλ), while destructive interference happens when the path difference is an odd multiple of half a wavelength ((2n+1)λ/2).
- Failing to mention Huygens' principle in explanations — Emphasize the importance of including Huygens' principle in explanations of interference patterns and fringe formation.
- Incorrectly stating that Newton's corpuscular theory was immediately rejected — Clarify that Newton's corpuscular theory had a significant following and was only gradually replaced by wave theory due to experimental evidence and mathematical developments.
- Overlooking the role of Thomas Young in validating Huygens' theory — Highlight the contributions of Thomas Young and his double-slit experiment in validating Huygens' wave theory.
- Failing to explain why Huygens' theory was initially rejected — Encourage students to discuss multiple factors, such as the lack of experimental evidence and the strong influence of Newton's corpuscular theory.
- Confusing the formation of fringes with other optical phenomena — Clearly differentiate between these phenomena and focus on how interference specifically leads to the formation of bright and dark fringes in a double-slit experiment.
Where the marks go
- Full worked solution (all marking points)5 marks