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.
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.