Introduction to Lasers – SS3 Physics Lesson Note (First Term)

A laser is a device that produces a narrow, coherent beam of light or other electromagnetic waves, all with the same frequency and phase.

Behavioural objectives

By the end of the lesson, students should be able to:

1. explain how a laser produces a coherent beam;
2. state the four major characteristics of laser beams;
3. name the kinds of laser by the material in the cavity;
4. give examples of the uses of lasers in industry, medicine, communication and defence;
5. state the differences between a flashlight and a laser.

What is a laser?

A laser is a device that uses pumped-in energy and a mirrored cavity to produce a coherent beam of electromagnetic waves.

• Name: LASER stands for Light Amplification by Stimulated Emission of Radiation.
• Coherent beam: a beam in which all the waves have the same frequency and are in phase.
• Monochromatic: having a single colour, which means a single frequency.
• Cavity: the chamber between the two mirrors that holds the laser material.
• Pumping: the process of putting energy into the cavity from an external source.

When was the laser invented?

The first practical laser was built by Theodore H. Maiman in 1960.

• Inventor: Theodore H. Maiman built the first practical laser.
• Place: Hughes Research Laboratories.
• Year: 1960.
• Today: lasers are in homes, offices and shopping centres, and many people use them daily without noticing.

How does a laser work?

A laser works by pumping energy into a cavity so that waves reflect between two mirrors, reinforce each other and leave as one coherent beam.

What are the parts of a basic laser?

• Cavity: a chamber designed to reflect infrared, visible or ultraviolet waves so that they reinforce each other.
• Laser material: the solid, liquid or gas that fills the cavity. It decides the wavelength of the output.
• Fully reflective mirror: one end mirror, which lets no energy pass through.
• Partially reflective mirror: the other end mirror, which lets a small part of the energy pass through (about 5 per cent in this note).
• Pump source: an external source that supplies energy to the cavity.

What are the steps in producing a laser beam?

1. Energy is pumped into the cavity from an external source.
2. An electromagnetic field appears in the cavity at the natural frequency of the atoms of the laser material.
3. The waves are reflected back and forth between the two mirrors.
4. The cavity length is such that the reflected waves reinforce each other.
5. Waves that are in phase leave through the partially reflective mirror.
6. The output is a continuous beam or a series of brief, intense pulses.

What are the characteristics of lasers?

There are four major characteristics of a laser beam: monochromatism, directivity, coherence and high output.

• Superior monochromatism: the beam has a single colour and frequency.
• Superior directivity: the beam is very narrow and travels a long distance without spreading much.
• Superior coherence: all the waves have the same frequency and are in phase.
• High output: a large amount of energy is concentrated in a small, narrow beam.

Because of these characteristics, lasers are used in fields such as optical communication and defence.

What are the types of laser?

There are three kinds of laser, named after the material in the cavity: solid, liquid and gas.

• Solid laser: uses a solid material. Maiman’s first laser used a ruby crystal, and semiconductor lasers are also solid.
• Liquid laser: uses a liquid dye as the laser material.
• Gas laser: uses a gas, as in the CO₂ and helium-neon lasers.

What are the uses of lasers?

Lasers are used in industry, medicine, communication and defence.

• History: when first invented, lasers were called “a solution looking for a problem”. Today they are widespread, from consumer electronics to the military.

Industry (tools)

• Cutting tools: CO₂ lasers are widely used in industries. They are precise, easy to automate and need no sharpening, unlike knives.
• Cloth cutting: robot-guided lasers cut cloth for items such as denim jeans. This is faster and more accurate than cutting by hand, and it raises productivity.

Medicine

• Surgery and treatment: doctors use lasers to destroy cancerous tumours.
• Eye care: lasers are used to correct defective eyesight.
• Reason: medicine needs the same precision that makes lasers good cutting tools.

Communication

• Barcode scanner: a laser in a grocery store converts a printed barcode into a number the checkout computer understands.
• CD and DVD players: a semiconductor laser beam bounces off the spinning disc and reads its printed pattern of data as numbers. A computer chip then turns the numbers into movies, music and sound.
• Fibre optic cables: lasers are used in fibre optic cables and in photonics, a technology that uses photons of light to communicate.

Defence

• Weapons: the military uses laser-guided weapons and missiles.

What is the difference between a flashlight and a laser?

A flashlight is a hand-held source that gives out white light, which is a mixture of colours of different frequencies.

A laser is a source that gives out coherent light of a single colour and frequency in a narrow beam.

Evaluation

1. Who built the first practical laser, where, and in what year?
2. What is a coherent beam of electromagnetic energy?
3. What is pumping, and what is the job of the partially reflective mirror?
4. State the four major characteristics of laser beams.
5. State two differences between a flashlight and a laser.

Assignment

Choose any two uses of lasers from the note. For each one, write two or three sentences to explain how the laser is used.