Stimulated Emission and How a Laser Works · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

How a laser builds its beam

Science · Waves, Light & Sound · ages 21-22
Name ______________________   Date ____________

  1. What is spontaneous emission like?

    • A clone of a passing photon
    • Random in direction and timing
    • Always aimed at the mirrors

  2. What makes a stimulated photon identical to its trigger?

    • It copies the frequency, direction, and phase
    • It leaves at a random time and angle
    • It carries twice the trigger energy

  3. Laser light is coherent while ordinary lamp glow is not.

    Circle one:   True   False

  4. How does a three level scheme reach inversion?

    • Pump to a short lived band that drains into a long lived upper level
    • Heat the gas until every atom ionizes
    • Cool the gas until all atoms drop together

  5. Why must more atoms sit excited than grounded?

    • Excited atoms live forever
    • Otherwise absorption beats emission
    • Ground atoms block the mirrors

  6. What do the cavity mirrors do for the beam?

    • They change the photon color
    • They pump atoms into excited states
    • They recycle photons for many passes, then leak a fraction

  7. A student says brightness is what separates a laser from a lamp. What is the better answer?

    • Coherence is the real difference
    • Cost is the real difference
    • Size is the real difference

  8. A builder skips the mirrors and just pumps the gas hard. What comes out?

    • A coherent beam, because pumping is enough
    • Bright but random glow, because no recycling builds a cascade
    • No light at all, because atoms refuse to emit
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Answer key

For grown-ups. Fold this page away before handing over the rest.

How a laser builds its beam W1-mt_xgjD0mzEAV-s1

  1. Random in direction and timing · No trigger organizes it, so it fires randomly.
  2. It copies the frequency, direction, and phase · The emitted photon is a clone of the passing one.
  3. True · Cloned photons march in step; lamp photons do not.
  4. Pump to a short lived band that drains into a long lived upper level · Fast draining piles atoms into the long lived upper laser level.
  5. Otherwise absorption beats emission · Inversion tips the balance from soaking up photons to cloning them.
  6. They recycle photons for many passes, then leak a fraction · Many passes build the cascade; the leak is the output.
  7. Coherence is the real difference · A dim laser is still coherent; a bright lamp is still random.
  8. Bright but random glow, because no recycling builds a cascade · Without passes through the gas, cloning never snowballs.
Worksheet · LightMySky