Practice portal › Ray Optics and Optical Instruments › Refraction and Total Internal Reflection
Asked in CBSE AIPMT 1989 · Snell's law, slabs and apparent depth
Idea: the ray model says light travels in straight lines, which holds only while diffraction can be neglected.
The angular spread caused by diffraction is set by the ratio λ/a, where a is the characteristic dimension of the aperture or obstacle.
When a is much larger than λ this ratio is tiny, the spreading is negligible, and reflection and refraction can be drawn with rays.
So ray optics is valid when the characteristic dimensions are much larger than the wavelength of light.
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