Two monochromatic light sources

PHYSICS · Single correct · A Level · Physics · Waves · Waves

Problem

Two monochromatic light sources, one red and one blue, are used separately with the same double-slit apparatus. The apparatus has a slit separation of 0.30 mm and a screen distance of 1.8 m. For the red source the first-order bright fringe is observed 5.5 mm from the central maximum, whereas for the blue source it is 3.0 mm away. Which statement correctly identifies which measurement corresponds to which color? (a) The

3.0\,\text{mm}

fringe is produced by the red light, and the

5.5\,\text{mm}

fringe by the blue light. (b) The

5.5\,\text{mm}

fringe is produced by the blue light’s second-order maximum, while the

3.0\,\text{mm}

fringe is the first-order maximum for red light. (c) The

5.5\,\text{mm}

fringe is produced by the red light, and the

3.0\,\text{mm}

fringe by the blue light. (d) Either color could produce either spacing because fringe spacing depends only on the slit separation and screen distance, not on wavelength.

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