To understand this technique of propulsion let us first see what are lasers and their properties, which enable them to be used to propel crafts.
Laser is an acronym for light amplification by simulated emission of radiation. It is an intense ray consisting of photons.
Laser light is monochromatic, directional and coherent
Monochromatic - means of one wavelength. In contrast, ordinary white light is a combination of different wavelengths.
Directional- laser light is emitted as a narrow beam and in a specific direction. This property is referred to as directionality.
Coherent - The light from a laser is said to be coherent. This means that the wavelengths of the laser light are in phase.
Let us see the principle of production of laser light
Laser is a device, which helps in controlling the release of photons by energized atoms. Laser uses lasing medium to excite the atoms from their normal state. For eg- the ruby rod in the ruby laser acts as a lasing medium to excite the atoms. For the lasers to work efficiently it is necessary to have a large collection of atoms in the excited state.
When the lasing medium is pumped, absorption of energy excites some electrons present in atoms. For the atoms to come back to its normal state it has to release energy. This release in energy is accompanied by the release of photons. The photons emitted have a specific waveleng