Question

If light waves emitted by an ordinary source, for what period of time, the phase remains constant
(A) $10\sec $
(B) $1\sec $
(C) ${10^{ - 3}}\sec $
(D) ${10^{ - 8}}\sec $

Answer 1

Hint: We use the concept of the active time period of a light pulse in an ordinary light source to solve this question. An ordinary light emits a wavelength of random size and waves don’t have a common phase relationship.

Step by step solution An ordinary light source is a source of light that emits atoms in discrete steps to make light. An atom forms a short light pulse that goes inactive after a certain time. After that time a new light pulse is formed by some interactions between the atoms. This period is called active time. The phase of two different light pulses is never the same. Each light pulse has an active time period of${10^{ - 8}}\sec $.
Hence the time period for which the phase remains constant in an ordinary light source is ${10^{ - 8}}\sec $.

Option (D) ${10^{ - 8}}\sec $ is the correct answer.

Additional information The contrast to ordinary light is a laser. A laser emits light that is coherent in nature. The phase as well as the frequency of the light emitted by the laser will be the same. In the case of ordinary light, the intensity of light decreases rapidly, whereas in the case of a laser the intensity is very high as all the energy is concentrated into a narrow spot.

Note: One must know the functioning of the source of ordinary light and the concept of the light pulse to solve this problem. An easy way to remember an ordinary light source is to remember that ordinary light is the contrast of the properties of laser light.