DIY optical illusions

Safe­ty pre­cau­tions

None.

Equip­ment

• glass;
• wa­ter;
• veg­etable oil;
• draw­ings.

Step-by-step in­struc­tions

Place a coin in a cup so that it isn't in your line of sight. Just add wa­ter, and the coin will ap­pear! Draw two hor­i­zon­tal ar­rows point­ing in op­po­site di­rec­tions. Place an emp­ty glass in front of them. Fill the glass with wa­ter – the ar­rows change di­rec­tions! You can achieve an equal­ly amaz­ing ef­fect by draw­ing di­ag­o­nal stripes or a chess­board. Wow! Op­tics is awe­some!

Process de­scrip­tion

Re­frac­tion is a phe­nom­e­non by which a wave, such as light, changes di­rec­tions when tran­si­tion­ing from one medi­um to an­oth­er. Dif­fer­ent light-trans­mit­ting me­dia have dif­fer­ent op­ti­cal den­si­ties, and there­fore light trav­els through them at dif­fer­ent speeds. The high­er a medi­um’s op­ti­cal den­si­ty, the slow­er light trav­els through it. Re­frac­tion is ev­ery­where, and we per­ceive it as com­plete­ly nor­mal: you can see how a straw in a glass of oil “breaks” at the bor­der be­tween the liq­uid and the air. Due to its shape, a glass of wa­ter acts as a lens ca­pa­ble of flip­ping an im­age. This cre­ates stun­ning op­ti­cal il­lu­sions. The re­frac­tion of light at the bound­ary of two me­dia cre­ates a para­dox­i­cal vis­ual ef­fect: ob­jects over­lap­ping both me­dia ap­pear "re­fract­ed up­ward" in the denser medi­um, while a ray of light en­ter­ing a denser medi­um “re­fracts down­ward.” This ef­fect caus­es bod­ies of wa­ter to seem shal­low­er than they ac­tu­al­ly are. The re­frac­tion of light through glass or wa­ter is per­haps the sim­plest and most ob­vi­ous ex­am­ple of beam dis­tor­tion, but the laws of re­frac­tion ap­ply to any waves – elec­tro­mag­net­ic, acous­tic, and even ocean waves.