• Wax blocks

• A sharp knife

• The ripple tank from the Ripple Tank experiment

• A pencil or dowel, or a six inch ruler, to create straight line waves

1. Cut a piece of wax to make a block that is one inch long.
2. Cut the corners to make a wedge on each end.
3. Move the two wax blocks from the Diffraction Activity so they are about 1 ½ inches apart.
4. Then place this block in between.
5. This will create a barrier with two quarter-inch openings that are about one inch apart. Again, the barrier should be about 4 inches from the end of the ripple tank where the waves are generated.
6. Generate waves and watch the screen to see how they diffract through both openings.
7. Look for dark lines that are perpendicular to the waves, they may be difficult to see.

When the straight-line waves diffract through the two openings, two sources of semicircular waves are created. When the crest of a wave from one opening meets the trough of a wave from the other opening, they cancel each other out, producing a dark spot on the screen. When a crest of a wave from one opening meets the crest of a wave from the other opening, they add together to make a bigger crest, producing a bright spot. When there are lots of waves from two sources, there are lots of crest-trough and crest-crest meetings, producing dark and light patterns called interference bands or fringes.

We demonstrated diffraction in our ripple tank because it is an easy way to produce two sources of identical waves, and identical waves work best for showing the way waves interfere with one another. Interference patterns between waves can happen no matter how the waves are generated.

Want to Learn More? Read an article related to this activity:

• The Refraction of Light

To find this experiment and many more please read Light Action! Amazing Experiments with Optics by Vicki Cobb and Josh Cobb, illustrated by Theo Cobb. To purchase a copy click here.