Video Transcript
The diagram represents the beams emitted by a red-laser-light source and an
incandescent white-light source. Which light source produces a more collimated beam? (A) The incandescent white-light source. (B) The red-laser-light source. (C) Both sources produce equally collimated beams.
We’re being asked here about collimated light. And our task is to work out which of two light sources produces a more collimated
beam. First, we should recall what it means for light to be collimated. A perfectly collimated beam of light consists of parallel rays of light. Because all the light rays in the beam are parallel, such a light beam will not
spread out, or diverge, as it propagates. The less well collimated a beam is, the more the light rays spread out as the beam
propagates. This divergence means that the light will get dimmer the further away from the source
it is observed.
In the diagram we’re given, these dashed lines represent the edges of the beam as it
propagates from the source. The circular cross sections represent the area of the beam at different distances
from the source. In the first case, the red laser light, the area of the beam does not increase with
distance from the source. This tells us that the rays are parallel, and the beam is perfectly collimated. In the second case, the incandescent white-light source, the area of the beam does
increase as the beam travels further from the source. This means that the rays spread out as they propagate. So the light is not well collimated.
Our answer then is option (B). The light source that produces a more collimated beam is the red-laser-light
source.
