Ray optics is valid, when characteristic dimensions are (a) much smaller than the wavelength of light (b) of the same order as the wavelength of light (c) of the order of one millimetre (d) much larger than the wavelength of light

Answer: (d). Diffraction spreads light by an angle θ ≈ λ/a. When the size a λ, this spreading is negligible and light travels in straight lines (rays). So the answer is (d) much larger than the wavelength.

Ray optics is valid, when characteristic dimensions are | NEET 1989 (AIPMT)

Q28NEET 1989 · AIPMTEasy

Wave Theory, Wavefronts & Doppler Effect

Ray optics is valid, when characteristic dimensions are

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Answer: (d)

Diffraction spreads light by an angle $\theta \approx \lambda/a$ .
When the size $a \gg \lambda$ , this spreading is negligible and light travels in straight lines (rays).
So the answer is (d) much larger than the wavelength.

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