Physics · Ray Optics And Optical Instruments · NEET
It is a model, not a real object. Light actually spreads out as waves, but in Ray Optics we pretend light moves along thin straight lines called rays because that makes reflection and refraction easy to draw and calculate. NCERT calls this the ray picture (or ray/geometric optics). You will never see a single ray in the lab; the thinnest laser beam is still a very narrow bundle of rays. For NEET, treat a ray as a straight line with an arrow showing the direction light travels.
A ray is a single straight-line path of light with a direction arrow. A beam is a bundle of many rays travelling together. So a beam is made of rays, the same way a rope is made of threads. NCERT says it directly: 'The path is called a ray of light, and a bundle of such rays constitutes a beam of light.' In diagrams you draw one arrow for a ray and several arrows for a beam.
Rectilinear propagation means light travels in a straight line in a single uniform (same everywhere) medium. This is the reason we can draw light as straight rays. Everyday proof: sharp shadows, the straight beam of a torch, and the pinhole camera all show light going straight. This rule only lets us use straight rays as long as the medium does not change; at a boundary (like air to glass) the ray bends, which is refraction.
Because the ray model is simple and accurate enough when the objects (mirrors, lenses, slits) are much larger than the wavelength of light (light's wavelength is about 400 to 750 nm, very tiny). In that case the wave nature can be ignored and straight-line rays give correct answers for image position, size and direction. NEET's Ray Optics chapter is built entirely on this model. Wave effects (interference, diffraction) come later in Wave Optics, where object sizes get close to the wavelength.
Parallel beam: all rays travel parallel to each other (like sunlight reaching Earth, or light from a very far source). Converging beam: rays come closer and meet at a point (light heading toward the focus of a convex lens). Diverging beam: rays spread out from a point (light coming from a small bulb or from the focus of a concave lens). NEET loves 'a parallel beam is incident on a lens/mirror' phrasing, so link parallel beam with object at infinity.
The ray model fails when light meets an obstacle or opening that is about the same size as its wavelength (extremely small, a few hundred nanometres). Then light bends around edges and spreads (diffraction) and shows interference, which straight rays cannot explain. So rays work for mirrors, lenses and prisms (Ray Optics) but not for narrow slits and thin films (Wave Optics). For this concept page, just remember: big objects = ray model works, tiny objects near wavelength = ray model breaks.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A ray of light is the straight-line path along which light travels, drawn as a line with an arrow to show the direction of travel.
A ray is a single path of light; a beam is a bundle of many such rays travelling together. A beam is made up of rays.
The ray model treats light as thin straight lines (rays) that travel in straight lines and bend only at surfaces. It is used in Ray Optics because it accurately predicts images from mirrors and lenses when objects are much bigger than light's wavelength.
In a single uniform medium light moves in straight lines because there is nothing to bend it. This is called rectilinear propagation and is shown by sharp shadows, torch beams and pinhole cameras.
Parallel beam: rays stay parallel (light from a very far source). Converging beam: rays meet at a point. Diverging beam: rays spread out from a point. NEET often uses a 'parallel beam' to mean an object at infinity.
Yes as a foundation. Every ray diagram, the laws of reflection and refraction, and all optical instruments in the NEET Ray Optics chapter are built on the ray model, so the terms ray, beam and rectilinear propagation must be clear before you solve numericals.
