Light marching in step
What Makes a Laser Different From a Torch?
A laser's light is one colour, all travelling the same way, with every wave perfectly in step.
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Step 1 of 5
A torch throws out a jumble of light.
Its bulb makes every visible colour at once, heading in all directions, with the waves in no particular relationship to each other.
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Step 2 of 5
A laser makes just one colour.
Every wave leaving a laser is the same length, so the beam is a single pure colour rather than a mixture that looks white.
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Step 3 of 5
All of it travels the same direction.
Torch light spreads out immediately. A laser's waves are all lined up along one path, so the beam stays narrow over long distances.
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Step 4 of 5
And every wave is in step with the rest.
The peaks all line up, so the waves reinforce instead of partly cancelling. That is what makes the beam so intense for its power.
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Step 5 of 5
Mirrors and a trigger atom do the organising.
Thousands of round trips. Only the light that matches perfectly survives them. Light bounces between two mirrors through a material that copies each passing wave exactly, so only perfectly matched light survives.
A laser pointed at the Moon spreads to a few kilometres wide after 380,000 km — a torch would cover the whole sky.
The short version
A torch throws every colour in every direction with the waves unrelated. A laser is the opposite on all three counts, and it gets that way by bouncing light thousands of times between mirrors until only the matching waves survive.
Try it yourself
Shine a torch and a laser pointer at a distant wall. The torch patch grows hugely with distance; the laser dot barely does.