UNIT:	Waves
PURPOSE:	Demonstrate: Huygens Principle, Young's Experiment interference pattern, focus by a concave                     "water mirror", etc.
EQUIPMENT:	1. PASCO Ripple tank apparatus (square tank on four legs, wave generator, halogen light source 2. Assorted implements such as barriers, plungers, flat edge, etc. 3. A white cardboard screen OR 4. PASCO translucent screen attachment 5. A pitcher with water
SUGGESTIONS:	To obtain regular wave patterns it is necessary to keep the vibrating elements clean of grease so that they are properly wetted by the water. The water depth for interference experiments should be about 1cm (deeper for refraction experiments). Best results with an amplitude setting of 2-4. Explore the role of phase difference and frequency.
DISCUSSION:	Regarding the concave water mirror (Pic. 12 below), the focus point at half a radius is manifested  as a "Huygens source."

Instructions-1

Instructions-2

Instructions-3
 

1. Set-up
2-6. Two Huygens sources with increasing separation
7. Planar wave with a disturbance to be avoided
8. Correctly formed planar wave
9. Huygens source created by a planar wave impacting a 1.5cm slit
10-11. Two Huygens sources created by two 1.5cm slits and the interference pattern
12. A planar wave collides with a concave plate causing the incoming waves to reflect and converge at a focal point