Question

Which of the following is a unique property of laser?

• A. directional
• B. speed
• C. coherence
• D. wavelength

Answer 1

Answer: (A), (C)

coherence

The explanation of the correct options:

Option C:

1. When the two waves have the same frequency and wavelength, the phase difference is constant. These waves are called coherent waves.
2. The source at which the coherent wave is generated is called the coherent source. This phenomenon is known as coherence.
3. The laser is commonly used as a coherent light source.
4. Lasers are unique because they are coherent, which essentially means that their waves are all in phase with one another, or, synchronized—the peaks and troughs of the waves all align and never intersect.
5. The laser beams are monochromatic and their wavelengths are homeomorphic in space and time.

Option A:

1. Unidirectional emission is highly desirable since most of the laser power will be in the desired direction.
2. Thus, Laser radiation contains a narrow band of wavelengths and can be produced closer to monochromatic than light from other sources.
3. Laser radiation is highly directional.

The explanation of the incorrect option(s):

Option B:

1. The rate of change of position of an object with respect to some reference is called speed and it is not the characteristic property of lasers.
2. It is used for speed or distance measurement, laser devices are also known by the name LIDAR (an acronym for Light Detection and Ranging) technology.
3. Speed measuring lasers are also sometimes referred to as laser speed measurement devices (LSMD).
4. LIDAR devices are often used in high-traffic areas, since the laser beam can easily focus on individual vehicles, allowing pinpoint accuracy.
5. Thus, measuring the speed of moving objects is one of the applications of lasers.

Option D:

1. Monochromatic light contains only single color or wavelength. The light emitted from ordinary light sources have different energies, frequencies, wavelengths, or colors. But laser light has a single wavelength or color.
2. Laser light covers a very narrow range of frequencies or wavelengths. Hence, the wavelength is not the unique property of lasers.

Hence, options (A) and (C) are correct.