BPSC 70th CCE Re-exam Prelims General Studies · Question 129 of 142

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.

• (a)Diffraction — 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 — 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 — 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.

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.

The habit that answers this question is to ask which model of light the phenomenon needs. If the phenomenon can be drawn with straight lines and a blocked path, it belongs to ray optics; if it needs waves adding and cancelling, or a field with an orientation, it belongs to wave optics. Shadows, eclipses, the pinhole camera, the working of a sundial and the similar-triangle relation between an object's height and its shadow's length all sit in the first group. Interference fringes, diffraction patterns, polarised glare and optical activity sit in the second. Two related refinements are worth carrying: the umbra-and-penumbra distinction, which explains why the shadow of the same object is sharp in bright point-like light and diffuse under a broad sky, and the fact that shadow length varies with the source's angle — the basis of the sundial and of the classic shadow-length problem set in reasoning papers.

• Light travels in straight lines in a homogeneous medium; a shadow is the region behind an opaque body from which those straight-line paths are excluded
• A point source produces only an umbra with a sharp outline; an extended source produces an umbra surrounded by a partly lit penumbra
• Solar and lunar eclipses are shadows at astronomical scale — the Moon's shadow on the Earth and the Earth's shadow on the Moon
• Diffraction, interference and polarisation are wave-optics phenomena; diffraction slightly blurs the edge of a shadow but does not produce the shadow
• The pinhole camera works on the same straight-line principle and forms an inverted image because the rays cross at the aperture
• Visible light has wavelengths of roughly 400 to 700 nanometres, which is why diffraction is negligible for obstacles of everyday size

• Choosing diffraction because it is the only wave phenomenon that touches a shadow; it blurs the edge, it does not create the shadow
• Confusing interference, which needs two coherent wave trains, with diffraction, which needs one wave and an obstacle
• Forgetting that the ray model is an approximation valid when everything is much larger than a wavelength — which is why the simple answer is the right one here

BPSC asks which named phenomenon explains an everyday observation, and offers three wave-optics terms as decoys, so the mark goes to a candidate who can place each term in the right layer of the subject. UPSC's General Studies paper asks the same optics through applications and observations — the diamond ring at an eclipse, why an endoscope works, why a CD shows colours — so the terminology has to be attached to real situations.

• practice — not a real PYQ

  The partially shaded outer region of a shadow cast by an extended source is called the

  • (a)umbra
  • (b)penumbra
  • (c)corona
  • (d)aperture

  Answer(b) penumbra — the region from which the source is only partly blocked; the fully dark central region is the umbra.

• practice — not a real PYQ

  Which phenomenon establishes that light is a transverse wave ?

  • (a)Rectilinear propagation
  • (b)Interference
  • (c)Polarisation
  • (d)Refraction

  Answer(c) Polarisation — only transverse waves can be polarised, since the property concerns the orientation of the oscillation perpendicular to the direction of travel.