Expert: James Gort - 5/22/2007

Question
What happens to light once its source stops producing it, e.g. when a candle is blown out? Also, does a light beam (say, from a laser light) continue to 'travel on' through space indefinitely, even after it is switched off?  If a light beam is 'stopped' when it is shone onto a wall, what happens to it? Does it cease to exist? How?

Answer
Hi Ron,

When a candle is lit (or a laser is turned on), some of the chemical energy (or electrical energy in the laser example) is converted to photons.  Remember, the energy of each photon is h (Planck's constant) x the frequency of the photon.  So this "photon stream" continues to escape (at the speed of light) until the light source is extinguished.  The photons will continue forever if they were in a vacuum with no walls.  However, in a practical sense, the photons will either hit a wall or be absorbed (or scattered) by an air molecule, dust particle, etc.  We'll assume they're absorbed (since scattering just changes the frequency and prolongs the process).  When a photon is absorbed, it changes the energy state of the absorbing atom.  in other words, it "kicks up" an electron in the absorbing atom to a higher energy state.  This could be re-emitted at a later time (via one or more photons at different frequencies) or simply go into making the absorber "hotter".  The photon's energy can go into thermal energy!

So energy (i.e., photons) don't ever cease to exist, but it does change form quite readily.

Hope that helps.

Prof. James Gort