Bending light

How can you bend a beam of light?

Safe­ty pre­cau­tions

At­ten­tion! Do not point the laser at your eyes! It may cause in­jury! Use eye pro­tec­tion!


  • salt;
  • sy­ringe;
  • aquar­i­um;
  • laser point­er;
  • veg­etable oil;
  • bot­tle;
  • glass.

Step-by-step in­struc­tions

Use a spray gun or smoke to high­light the beam of light and make the re­frac­tion eas­i­er to ob­serve. To achieve com­plete in­ter­nal re­flec­tion, gen­tly pour some veg­etable oil over some wa­ter. In or­der to vis­i­bly bend the light, make a sat­u­rat­ed salt so­lu­tion (rec­om­mend­ed ra­tio 350g salt : 1L wa­ter. Pour fresh wa­ter into the aquar­i­um, then use a sy­ringe with a tube to care­ful­ly add the salt so­lu­tion to the bot­tom. Point the laser beam at an an­gle to the side wall of the ves­sel and you will see some­thing amaz­ing! By ad­just­ing the an­gle of in­ci­dence of light, you can bend the beam down com­plete­ly.

Process de­scrip­tion

Dif­fer­ent light-trans­mit­ting me­dia have dif­fer­ent re­frac­tive in­dices. A medi­um’s re­frac­tive in­dex in­di­cates how the speed of light in vac­u­um re­lates to the speed of light in said medi­um. The high­er the medi­um's re­frac­tive in­dex, the slow­er light trav­els through it. When a ray of light cross­es the bound­ary be­tween two me­dia, it is par­tial­ly re­fract­ed and re­flect­ed. More­over, the an­gle of in­ci­dence is al­ways equal to the an­gle of re­flec­tion. The re­frac­tion of light is a phe­nom­e­non in which a ray changes di­rec­tions when tran­si­tion­ing from one medi­um to an­oth­er. If there is no re­fract­ed beam, then the light is re­flect­ed com­plete­ly. A medi­um is called “op­ti­cal­ly ho­mo­ge­neous” if its re­frac­tive in­dex is the same across all points, which means that light prop­a­gates lin­ear­ly through it. Ac­cord­ing­ly, a medi­um is “op­ti­cal­ly in­ho­mo­ge­neous” if the re­frac­tive in­dex varies from point to point, which caus­es non­lin­ear prop­a­ga­tion of light. A sat­u­rat­ed salt so­lu­tion and fresh­wa­ter have dif­fer­ent re­frac­tive in­dices: the re­frac­tive in­dex of a salt so­lu­tion is greater than that of fresh­wa­ter. Due to mix­ing and dif­fu­sion, af­ter a while, a lay­er of op­ti­cal­ly in­ho­mo­ge­neous liq­uid forms at the bound­ary be­tween them. When en­ter­ing the op­ti­cal­ly in­ho­mo­ge­neous liq­uid lay­er, the light beam is bent, bend­ing to­wards in­creas­ing val­ues in re­frac­tive in­dex.