{"id":54624,"date":"2025-10-19T05:24:11","date_gmt":"2025-10-18T20:24:11","guid":{"rendered":"https:\/\/phys-edu.net\/wp\/?p=54624"},"modified":"2026-05-08T04:33:25","modified_gmt":"2026-05-07T19:33:25","slug":"the-20-science-breakthrough-prove-light-is-a-wave-with-a-paper-cup","status":"publish","type":"post","link":"https:\/\/phys-edu.net\/wp\/?p=54624&lang=en","title":{"rendered":"Recreate a 200-Year-Old Scientific Breakthrough with Paper Cups! See Proof That Light Is a Wave at Home (Young\u2019s Experiment)"},"content":{"rendered":"

This is Kuwako Lab, your science trainer. Every day is an experiment.<\/strong><\/span><\/p>\n

\u3010You can also listen to this article as a radio episode!<\/a>\u3011<\/p>\n

\"\u30b9\u30af\u30ea\u30fc\u30f3\u30b7\u30e7\u30c3\u30c8<\/a><\/p>\n

\u201cLight travels in straight lines.\u201d
\nSunlight, flashlight beams\u2026 it certainly looks that way, doesn\u2019t it?<\/p>\n

But here\u2019s a bigger question:<\/p>\n

Is light made of particles? Or is it actually a wave?<\/p>\n

This mystery sparked one of the greatest scientific debates of all time between Newton and Huygens back in the 17th century. And believe it or not, you can explore the answer yourself at home using nothing more than a paper cup, a utility knife, and a cheap laser pointer.<\/p>\n

Today\u2019s experiment lets you actually see proof that light behaves like a wave through a phenomenon called \u201cinterference.\u201d It may sound advanced, but it\u2019s simple enough to enjoy with elementary school kids and perfect for helping students experience what waves really are.<\/p>\n

Let\u2019s dive in!<\/p>\n

Why Does This Prove Light Is a Wave?<\/h3>\n

The heart of this experiment is a single thin slit cut into a paper cup.<\/p>\n

Waves have a special property: when they pass through a narrow opening, they spread out behind it. This phenomenon is called diffraction.<\/p>\n

You may have seen ocean waves fan out after passing through a gap in a breakwater. Light does the exact same thing.<\/p>\n

What\u2019s even more interesting is that the tiny cut in the paper cup actually behaves like two extremely close slits rather than one. The thickness of the blade and the roughness of the paper edges create this effect naturally.<\/p>\n

The light waves spreading from these two tiny openings overlap when they hit the wall.<\/p>\n

When wave peaks line up with other peaks, the light becomes brighter.
\nWhen peaks meet valleys, they cancel each other out and become darker.<\/p>\n

That alternating bright-and-dark stripe pattern is called an interference pattern.<\/p>\n

If light were only particles, you\u2019d simply see the shape of the slit projected onto the wall. The appearance of stripes is direct evidence that light behaves like a wave.<\/p>\n

A Simple Home Experiment: Seeing the \u201cWaves\u201d of Light<\/h3>\n

What You\u2019ll Need<\/b><\/p>\n

    \n
  • A laser pointer (presentation lasers work perfectly fine \u2014 inexpensive ones around $15 are enough)<\/li>\n
  • A paper cup<\/li>\n
  • A utility knife or box cutter<\/li>\n<\/ul>\n

    \"\u30b9\u30af\u30ea\u30fc\u30f3\u30b7\u30e7\u30c3\u30c8 <\/p>\n

    Laser chalk line tools are actually excellent for this kind of experiment.<\/a><\/p>\n

    \"\"<\/p>\n

    Steps<\/b><\/p>\n

      \n
    1. Cut one straight slit into the bottom of the paper cup using the utility knife. (This acts as the experimental slit.) \"\u30b9\u30af\u30ea\u30fc\u30f3\u30b7\u30e7\u30c3\u30c8<\/li>\n
    2. Darken the room and shine the laser from inside the cup toward the slit.<\/li>\n
    3. Observe the light pattern projected onto a wall.\n

      (A distance of about 50 cm to 1 meter between the cup and the wall works well.)<\/li>\n<\/ol>\n

      So\u2026 What Did You See?<\/h3>\n

      \"\"<\/p>\n

      Instead of a simple laser dot, you should see the light spreading sideways into a fuzzy pattern of alternating bright and dark stripes.<\/p>\n

      Congratulations!
      \nThat pattern is the interference pattern \u2014 clear evidence that light behaves as a wave.<\/p>\n

      This experiment is fundamentally the same as the famous \u201cYoung\u2019s Double-Slit Experiment\u201d performed over 200 years ago by the British scientist Thomas Young.<\/p>\n

      Yes \u2014 you just recreated one of the most important experiments in physics using a paper cup.<\/p>\n

      At the time, Young\u2019s claim that \u201clight is a wave\u201d shocked the scientific world. Many scientists simply couldn\u2019t accept it. The idea went completely against human intuition.<\/p>\n

      Try moving the wall farther away from the paper cup. What happens to the spacing between the stripes?<\/p>\n

      You\u2019ll probably notice the stripes spread farther apart and become easier to see.<\/p>\n

      And here\u2019s the really amazing part: if you measure the stripe spacing and the distance to the wall, you can actually calculate the wavelength of light itself.<\/p>\n

      Most Important Safety Rule<\/b><\/p>\n

      This experiment is fascinating, but safety comes first.<\/p>\n

      Never look directly into the laser beam or shine it into anyone\u2019s eyes. Laser light is extremely powerful and can seriously damage vision.<\/p>\n

      Only observe the reflected light on the wall.<\/p>\n

      Calculating the Wavelength of Light from the Stripes<\/h3>\n

      Pretty amazing, right?<\/p>\n

      \"\"<\/p>\n

      In this setup, the spacing between the stripes is 1.5 cm, and the distance to the screen is 5.863 meters.<\/p>\n

      Using these values, you can calculate the wavelength of the laser light.<\/p>\n

      For a red laser pointer, the wavelength is usually around 600\u2013700 nanometers.
      \n(One nanometer is one-millionth of a millimeter.)<\/p>\n

      It\u2019s incredible to think that such precise information is hidden inside those tiny light stripes.<\/p>\n

      I also created an interactive Scratch simulation to help people explore Young\u2019s experiment hands-on.<\/p>\n

      Try the Scratch simulation here<\/a><\/p>\n

      Here\u2019s the accompanying video:<\/p>\n