Refraction of light
Age Group
12 13 14 15 16 17 18
Expected time
3 periods
The curved light.PDF
beaker symbol

The curved light

What’s the mystery? Light rays propagate rectilinearly, as can be seen with micro-particles of powder, talcum powder, or smoke in a medium with a laser beam.Why is the laser light in the Plexiglas pan filled with water be curved?
Domain(s) Physics
Subdomain keywords Refraction of light.
Age group 12 to 18 years old.
Expected time for the mystery Approximate time for teacher preparation: two hours.Approximate time in classroom: three or four individual 50-min lessons.
Safety/supervision Laser pointers have to be handled with care and never directed at people’s eyes.Disclaimer: the authors of this teaching material will not be held responsible for any injury or damage to persons or properties that might occur in its use.
Preparation and list of materials Laser pointers, talcum powder, plexiglass pans, glass prisms, plexiglass prisms, glycerol, water, sugar syrup, milk, mineral oil, alcohol, water tanks.To prepare the mystery, put 3 to 4cm of glycerol (better) or sugar syrup at the bottom of the Plexiglas pan and then add water without stirring. After about one hour of rest (it can work better after some hours or even after days), a density gradient of glycerol will be present in the pan. Throw the light of a green laser beam longitudinally through the pan near the interface between the glycerol and the water; adjusting the direction of the beam, you will see it curved towards the bottom. To better visualise the trajectory of the light, add a touch of skimmed milk to the two liquids.
Learning objectives - Qualitative behaviour of light when the medium in which it propagates changes its refraction index.- Quantitative evaluation of the behaviour of light in refraction by means of Snell’s law.
  • A very clear tutorial about the refraction of light can be found on YouTube:

  • In order to face the intriguing gravitational lensing, you can use the following videos: