9702 · 8.4
The diffraction grating flashcards
Revision flashcards for Cambridge 9702 The diffraction grating (syllabus 8.4). Flip, recall, then mark a real past-paper question.
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What is the primary advantage of a diffraction grating over a double slit?
Diffraction gratings produce sharper and brighter interference patterns, leading to more accurate measurements.
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Write down the formula for a diffraction grating.
d \sin \theta = n\lambda
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In the diffraction grating formula, what does n represent?
n represents the order of the maximum (e.g., n=0 for the central maximum, n=1 for the first order).
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How does increasing the wavelength of light affect the angular separation of maxima produced by a grating?
A larger wavelength results in a larger diffraction angle (\theta), spreading out the pattern and increasing angular separation.
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Name two practical applications of diffraction gratings.
Analyzing starlight for elemental composition (spectroscopy) and X-ray crystallography for determining atomic spacing.
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What does 'd' signify in the grating equation?
'd' is the spacing between adjacent slits on the diffraction grating, also known as the grating element.
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Why must the incident light be coherent for a stable interference pattern?
Coherent light has a fixed phase difference and the same frequency, which is essential for stable, observable interference fringes.
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What is the condition for the maximum possible order of interference (n_max)?
The maximum order occurs when \sin \theta = 1, meaning n_max \lambda / d \leq 1, or n_max \leq d/\lambda. The result must be an integer.
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Describe the central maximum (n=0) produced by a diffraction grating.
The central maximum is the brightest fringe, located directly along the normal to the grating, where the angle \theta = 0.
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What is the combined phenomenon responsible for the pattern from a diffraction grating?
The pattern arises from both diffraction (spreading of waves as they pass through slits) and interference (superposition of these waves).
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How is the grating spacing 'd' calculated from the number of lines per unit length, N?
d = 1/N. For example, if N = 500 lines/mm, then d = 1/500 mm/line, which must be converted to metres for calculations.
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What happens to the central maximum (n=0) when white light is used with a diffraction grating?
The central maximum remains white. Since θ=0 for n=0, all wavelengths interfere constructively at the same central point, without being separated.
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Why is red light diffracted more than blue light by a grating?
According to d sin θ = nλ, for a given order n, sin θ is proportional to the wavelength λ. Since red light has a longer wavelength than blue light, its angle of diffraction θ is larger.
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What is a transmission grating?
A transmission grating is a transparent plate with a large number of opaque parallel lines drawn on it. Light passes through the transparent gaps between the lines.