Rectilinear propagation
Beginner
Light travels in straight lines in uniform media.
In a uniform medium light travels in straight lines, explaining shadows, eclipses and the pinhole camera. It breaks down when apertures are comparable to the wavelength.
Sharp shadow of a tree at noon.
Diffraction shows the limit of this idea.
In more detail
In a uniform medium, light travels in straight lines, which is the basis of the ray model. Shadows and pinhole images show rectilinear propagation.
Real-world uses
- Pinhole cameras form images by straight-line propagation.
- Surveyors use straight light paths to align structures.
- A pinhole camera forms a sharp image because light from each point travels in a straight line through the small hole.
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Frequently asked questions
Why do shadows have sharp edges?
Light travels in straight lines, so an opaque object blocks the light cleanly unless the source is large.
When does light bend?
When it passes through regions of different refractive index or near edges, as in diffraction.
Why does a shadow have sharp edges?
Light travels in straight lines from a small source, so the edge of an object casts a sharp shadow.
Related topics in Optics
Nature of light · Reflection of light · Plane mirrors · Spherical mirrors