A laser is a device that produces a narrow, intense beam of light by making many atoms emit photons in step with one another. Laser light matters because it can travel long distances with little spreading, focus to tiny spots, and carry precise information. This makes lasers useful in medicine, manufacturing, communication, barcode scanners, and scientific measurement.
The key idea is stimulated emission, where one photon triggers an excited atom to release a second matching photon.
Inside a laser, energy is pumped into a gain medium so more atoms are in excited states than in lower energy states. This condition is called population inversion, and it allows light to be amplified instead of absorbed. Mirrors form an optical cavity that sends photons back and forth through the gain medium, causing repeated stimulated emissions.
One mirror is partially reflective, so a fraction of the coherent, nearly monochromatic light exits as the laser beam.
Understanding Physics: Lasers and Stimulated Emission
Atoms do not store energy in arbitrary amounts. Electrons in an atom or solid can occupy particular energy levels. When an electron drops between two levels, the energy difference leaves as light of a particular frequency.
Real laser materials have small variations in these energies because atoms move, collide, or sit in slightly different surroundings. This broadens the range of light frequencies they can emit. Many lasers use a long lived excited level, often called a metastable state.
It holds energy long enough for a large collection of atoms to build up before they release light. The gain medium may be a gas, a crystal, a liquid dye, or a semiconductor chip.
A few photons usually begin the process through ordinary spontaneous emission. Most of these photons travel in unhelpful directions and leave the device. The optical cavity favours photons moving along its main axis, since they can make repeated trips through the gain material.
For sustained operation, the amplification on each round trip must exceed losses from absorption, scattering, imperfect mirrors, and the useful light leaving the output. This requirement is called the laser threshold. Below threshold, the device may glow like a lamp.
Above threshold, one or more cavity modes grow rapidly. A mode is a light pattern that fits the cavity length, much like a standing wave on a string fits between its ends. Careful cavity design selects stable modes and reduces unwanted ones.
Coherence has more than one meaning. Temporal coherence describes how steadily the light wave keeps its phase over time. It is linked to a narrow spread of frequencies.
Spatial coherence describes how well light across different parts of a beam stays in step. This helps a laser form a clean interference pattern and a small focused spot. A laser is not perfectly parallel, however.
Any beam spreads because of diffraction. A wider starting beam spreads less, while a shorter wavelength can be focused more tightly by a good lens. Some lasers run continuously.
Others release very short pulses. A pulse can contain modest total energy yet have enormous power because its energy is delivered in a tiny time interval.
Students meet lasers in fibre optic links, printers, distance meters, optical discs, alignment tools, and laboratory sensors. In fibre communication, rapid changes in laser light can represent digital information. In manufacturing, a focused beam heats or removes material at a chosen location.
In medicine, the wavelength matters because different tissues absorb different colours by different amounts. Laser safety matters because the eye lens can focus a narrow beam onto a very small area of the retina. When studying diagrams, follow the energy input, the possible electron transitions, and every route by which light is lost.
Keep intensity, power, and total energy separate. Intensity is power spread over an area, so a tiny spot can be intense even when the total power is not extreme.
Key Facts
- Photon energy is E = hf, where h is Planck's constant and f is frequency.
- The wavelength and frequency of light are related by c = λf.
- Stimulated emission produces a photon with the same energy, direction, phase, and polarization as the incoming photon.
- Population inversion means N_excited > N_lower, allowing amplification to dominate absorption.
- Laser gain increases light intensity as photons repeatedly pass through the gain medium inside the optical cavity.
- A laser cavity usually has one highly reflective mirror and one partially reflective output coupler.
Vocabulary
- Stimulated emission
- The process in which an incoming photon causes an excited atom to emit a second identical photon.
- Population inversion
- A condition where more atoms are in an excited energy state than in a lower energy state.
- Gain medium
- The material inside a laser that amplifies light by stimulated emission.
- Optical cavity
- A pair of mirrors that traps light so it passes repeatedly through the gain medium.
- Coherence
- A property of waves that have a constant phase relationship, making them reinforce in an organized way.
Common Mistakes to Avoid
- Thinking a laser makes light by reflection alone, which is wrong because mirrors only provide feedback while stimulated emission in the gain medium creates amplification.
- Forgetting that population inversion is required, which is wrong because without more excited atoms than lower state atoms, absorption can exceed stimulated emission.
- Assuming all bright light is laser light, which is wrong because laser light is special due to its coherence, narrow wavelength range, and low beam divergence.
- Using E = hcλ instead of E = hc/λ, which is wrong because photon energy increases when wavelength decreases.
Practice Questions
- 1 A laser emits light with wavelength 632.8 nm. Calculate the photon energy in joules using E = hc/λ, with h = 6.63 x 10^-34 J s and c = 3.00 x 10^8 m/s.
- 2 A laser cavity is 0.50 m long, and light travels at 3.00 x 10^8 m/s. How long does one round trip between the mirrors take?
- 3 Explain why a laser beam can stay narrow over a long distance while light from a regular lamp spreads out in many directions.