Give reasons for the following: a) A beam of light is of a higher intensity than a ray of light.
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Answer:
It is important to understand that ray and beam are artificial constructs. Ray is usually associated with geometric optics and helps us think of light propagation and so it helps to visualize how light propagates through an optical system. In this picture the ray is one dimensional and has zero cross-section and is therefore not physical. Sometimes we associate a ray with an infinite plane wave where the ray is perpendicular to the plane of the wave (however an infinite plane wave is also not physical). Beams are have finite cross-section and are typically associated with lasers, for example. A good approximation of many laser system outputs are Gaussian beams – beams of light whose intensity across the cross-section follows a Gaussian distribution; at the center of the beam you have the highest intensity and it falls off as you go out radially from the center of the beam (as e^(-r^2)). Such a beam can be decomposed (via Fourier transform) into an “angular spectrum of plane waves”. The smaller the beam the larger the angular spectrum, which is at the heart of diffraction – “tighter” laser beams diffract faster. In this sense you can think of the beam as being “made up of rays” pointing in different directions – so beams can be thought of as “bundles of rays”. Note that even though rays are an artificial construct (and not physical) they may be really useful in many scenarios and applications where geometric optics is sufficient, such as ray tracing applications.