Compare the blue and red sine-wave cards. Notice the difference in length between the blue and red waves. The red light waves will be longer than the blue. You have made a good model of red and blue light waves.
Look at any sine-wave card and notice three important points: the maximum (highest point), the zero crossings (where the wave crosses the middle line of the card, including at the leading and trailing edges), and the minimum (lowest point). Scientists describe these points using the term phase. When two waves add up “out of phase,” this means the highest point of one wave lines up with the lowest point of the other, canceling out the light. When waves add up “in phase,” this means the highest point of one wave lines up with the highest point of the other, strengthening the light.
Consider a light source like a laser shining into the two slits. The waves come into the slits in phase, oscillating together.
Take one strip of blue sine-wave cards and line it up with the center of one slit. Take the other strip of blue sine-wave cards and line it up with the center of the other slit. Make sure the two strips begin in phase at the slits. Unfurl both card strips towards the “screen” (second tape line), angling them so they cross the screen together at a point opposite the midway point between the two slits. Notice that both waves have the same phase at the screen. They add together in phase, producing a bright spot where they meet on the screen. Make a blue mark at this point on the tape (see photo below).
Investigate other points on the screen, by stretching the strips from each slit to intersect at different places along the screen. Try to find a point where the two strips arrive out of phase. Here the light will be canceled, creating a dark spot. Make a black mark at the intersection point on the tape and draw a blue circle around it to note this is where blue light cancels out (click to enlarge the photos below).
Repeat the experiment using the red sine-wave cards. Notice and mark the locations of the bright regions and the dark regions (see below).
After you’re done investigating both the blue and red sine-wave cards, change the distance between the slits, and repeat the experiment with fresh pieces of tape.