The ripple tank is used to generate water waves in laboratory. It is useful in demonstrating wave properties such as reflection and refraction. It consists of a shallow tray of water with a transparent base, a light source directly above the tray and a white screen beneath the tray to capture the image of the shadows formed when water waves spread across the tank as shown above. Straight waves can be set up by using a straight dipper, while circular waves can be formed by using a spherical dipper. Both dipper are vibrated up and down by a motor.
The waves will be seen in bright and dark patches on the screen below the tray. These patches show the position of the crests and troughs of the waves. The dark patches will correspond to the crests and bright patches will be the troughs.
Why does the dark patches correspond to the crests and bright patches correspond to the troughs?
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Video of a ripple tank in action