Lessons · Teaching · light and sound
Both are waves, and only one needs something to travel through
Light travels in straight lines, very fast, and passes through a vacuum. Sound is a vibration passing through a material and cannot travel through a vacuum. We see objects because light reflects off them into the eye, not because the eye sends anything out.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a state certification officer. Practice material, not professional advice. What that means.
What it is for
Two misconceptions live here and both are worth killing: that the eye emits something, and that sound travels in space. The second is worth a minute of honesty about every film a class has seen, and they enjoy it.
How to think about it
Trace the light from its source to the object to the eye, in that order, every time. For sound, name the material it is travelling through, because there always is one. Remember that sound travels faster through solids than through air, which is the opposite of what people expect.
Worked example
We see a book because light from a source reflects off it into the eyeSource, object, eye. The eye emits nothing.
Light travels in straight lines, which is why shadows have sharp edgesAnd why a shadow is the shape of the blocking object.
Sound is a vibration in a material: no material, no sound, so space is silentLight crosses a vacuum and sound cannot.
Sound travels faster in solids than in liquids, and slowest in gasesParticles closer together pass the vibration on sooner.
Your turn
Name what sound needs in order to travel, which light does not.
Sound must travel through a
Solve one, graded on the server
The trap
Teaching that we see because the eye looks at a thing. Nothing leaves the eye. Light leaves a source, bounces, and arrives, and a pupil who has this can explain why a dark room is dark.