A-LevelPhysicsSuperposition9702/21 · May/June 2018 · Q55 Marks
When monochromatic light is incident normally on a diffraction grating, the emergent light waves have been diffracted and are coherent. Explain what is meant by (i) diffracted waves,
- (a)(i)When monochromatic light is incident normally on a diffraction grating, the emergent light waves have been diffracted and are coherent. Explain what is meant by (i) diffracted waves,[1]
- (a)(ii)(ii) coherent waves.[1]
- (b)(i)Light consisting of only two wavelengths λ₁ and λ₂ is incident normally on a diffraction grating. The third order diffraction maximum of the light of wavelength λ₁ and the fourth order diffraction maximum of the light of wavelength λ₂ are at the same angle θ to the direction of the incident light. (i) Show that the ratio λ₂/λ₁ is 0.75. Explain your working.[2]
- (b)(ii)(ii) The difference between the two wavelengths is 170nm. Determine wavelength λ₁. λ₁ =[1]
Mark scheme
Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.
- [B1] waves spread at (each) slit/gap
- [B1] constant phase difference (between (each of) the waves)
- [B1] nλ = dsin θ
- [A1] dsin θ is the same and 3λ₁ = 4λ₂ so λ₂/λ₁ = 0.75
- [A1] λ₁ = 680 nm
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