Which of the following is responsible for production of shadow ?

Question

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Correct — C, Rectilinear propagation of light. In a uniform medium light travels in straight lines, and that single property is what makes a shadow possible. Put an opaque body in the path of the light and the straight-line paths that would have reached the surface behind it are blocked, leaving an unlit region shaped like the projection of the object. Everything else about shadows follows from the same fact. A point source gives one sharp shadow, the umbra, because from a single point there is exactly one straight line to each part of the screen. An extended source gives an umbra surrounded by a partly lit penumbra, because different points of the source are blocked for some parts of the screen and not for others — which is why a shadow softens as the source grows or as the object moves away from the screen. Eclipses are the same geometry at astronomical scale: the Moon's shadow falling on the Earth in a solar eclipse, the Earth's shadow falling on the Moon in a lunar one. The pinhole camera is the demonstration in reverse, forming an inverted image because rays cross at the aperture and travel straight on either side. If light bent easily around obstacles, none of this would happen — and the fact that it does not bend appreciably around everyday objects is precisely why a shadow has a recognisable outline.

Why the other options are wrong — (a) The closest thing to a genuine competitor, and worth understanding rather than dismissing. Diffraction is the bending of light around an obstacle or through an aperture, and it does affect a shadow — the edge is never mathematically sharp, and with a small enough obstacle the fringes become obvious. But diffraction disturbs the boundary of a shadow; it does not create the shadow, and because the wavelength of light is tiny compared with everyday objects the effect is negligible for the shadow of a hand or a tree.  (b) Interference is the superposition of two or more coherent waves producing alternating bright and dark bands, as in Young's double-slit experiment or the colours of a thin oil film. It needs two or more wave trains from coherent sources; a shadow needs only one source and one obstacle, and involves no superposition at all.  (d) Polarisation concerns the orientation of the oscillating electric field of a light wave, and it is the property that proves light is a transverse wave — it is what sunglasses and photographic filters exploit. It has nothing to do with whether light gets past an obstacle, so it cannot account for a dark region behind an opaque body.

Concept: Optics is taught in two layers, and each of the four options in this question belongs to one of them. Ray or geometrical optics treats light as travelling in straight lines and explains shadows, eclipses, the pinhole camera, reflection from mirrors and refraction through lenses; it is the layer that governs image formation in a concave mirror or a telescope. Wave optics treats light as a wave and explains interference, diffraction and polarisation — the phenomena that appear when the obstacles or apertures involved approach the wavelength of light, about 400 to 700 nanometres for visible light. The ray model is not wrong; it is the limit of the wave model when everything in the experiment is very much larger than a wavelength, which is exactly the situation of a person standing in sunlight. That is why the correct explanation of an everyday shadow is the simplest one, and why diffraction becomes visible only at the fine scale of a shadow's edge or in a laboratory arrangement built for it.

State PCS 2025 Q129 — Science & Technology PYQ with Answer

State PCS 2025 Q129 — Science & Technology PYQ with Answer