Which of the following is responsible for production of shadow ?
Rectilinear propagation of light
Shadows form because light travels in straight lines — option 2.
The reasoning. A shadow is simply the region an opaque object prevents light from reaching. That region has a sharp outline — and roughly the shape of the object — only because light travels in straight lines from the source and cannot bend round the obstacle. If light curved appreciably around edges, there would be no sharp shadow at all.
| Phenomenon | What it explains |
|---|---|
| Rectilinear propagation | Shadows, eclipses, the pinhole camera, the formation of images by rays |
| Polarisation | Proves light is a transverse wave; Polaroid sunglasses, LCD screens, glare reduction |
| Diffraction | Bending of light round obstacles and through narrow slits — it blurs the shadow edge rather than creating it |
| Interference | Superposition of waves — the colours of soap films and oil slicks; Young’s double slit |
The two parts of a shadow. A point source gives a single sharp shadow. An extended source gives two zones :
| Region | Description |
|---|---|
| Umbra | Complete shadow — no part of the source is visible from it |
| Penumbra | Partial shadow — part of the source is visible |
Solar and lunar eclipses are the same geometry on a large scale: a total solar eclipse is seen from within the Moon’s umbra, a partial one from the penumbra.
The qualification worth knowing. Rectilinear propagation is an approximation, and it holds when the obstacle is much larger than the wavelength of light — which for visible light, at a few hundred nanometres, is true of every everyday object. Make the obstacle or slit comparable to the wavelength and diffraction takes over: the shadow edge softens, and fringes appear. So option 3 names the very effect that limits the sharpness of shadows rather than producing them.
Hence, the answer is rectilinear propagation of light.
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