Two slits produce light that interferes constructively at a point. What does this mean about the light at that location?

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Question restatement: Two slits produce light that interferes constructively at a point. What does this mean about the light at that location?

Option a: The path difference equals a whole number of wavelengths.
- Constructive interference occurs when the waves arrive in phase, meaning their peaks line up with peaks and troughs with troughs. This happens when the path difference between the two waves is an integer multiple of the wavelength, i.e., 0, λ, 2λ, etc. In this case, the amplitudes add in phase, producing a brighter intensity.

Option b: The waves arrive with a path difference equal to half a wavelength.
- A half-wavelength path difference corresponds to destructive interference for light of the same wavelength (out of phase by 180 degrees). The peaks of one wave align with the troughs of the other, reducing or canceling the intensity. This describes destructive interference, not constructive.

Option c: The waves cancel out.
- Cancellation occurs when the waves are out of phase by 180 degrees, producing destructive interference. Since the scenario specifies constructive interference, this option contradicts the given condition and describes a different case.

Option d: The light is absorbed.
- Absorption is a material property that removes light energy by converting it to other forms (like heat). Constructive interference is a geometric/phase phenomenon of superposed waves in space, not a statement about absorption. Therefore, this option misattributes the observed effect to a lossy process rather than interference.

Summary of reasoning: Constructive interference requires the two contributing waves to be in phase at the point of observation, which occurs when their path difference is an integer multiple of the wavelength. A half-wavelength difference yields destructive interference, not constructive. Destructive interference would imply cancellation, not addition. Absorption is unrelated to the phase relationship causing constructive interference in the two-slit setup.

SPH4U - Physics 12 (2025-26) - A

Two slits produce light that interferes constructively at a point. What does this mean about the light at that location?

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