NIS – PHYSICAL SCIENCE

1. NIS – PHYSICAL SCIENCE Lecture 42 – Lecture 43 The Behavior of Waves OzgurUnal

2. Reflection • Everyday you look at yourself in the mirror • You probably heard the echo of your voice somewhere • How are these possible? • Reflection occurs when a wave strikes an object and bounces off it. • Light waves are reflected twice (from your face and from the mirror) when you see your face in the mirror. • Echoes form when sound • waves reflect off a surface.

3. Reflection • According to the law of reflection, the angle of incidence is equal to the angle of reflection.

4. Refraction • Why does the pen in the glass look broken? • The speed of a wave depends on the medium it is moving through. • When a wave passes from one medium to another it changes its speed. • If the wave is traveling at an angle when it passes from one medium to another, it changes direction, or bends, as it changes its speed. • Refraction is the bending of a wave caused by a change in its speed as it moves fro mone medium to another.

5. Refraction • Example: Light waves travel more • slowly in water that in air. • This causes light waves to change • direction when they move from water • to air or air to water. • Objects that are under water seem closer to the surface than they really are. • This is because of refraction of light rays as • they pass from water to air.

6. Diffraction • How can you describe the change in • these waves as they pass around the rocks? • Diffraction occurs when an object causes a wave to change direction and bend around it. • Diffraction and refraction both causes waves to bend. • The difference is that refraction occurs when waves • pass through an object, while diffraction occurs when • waves pass around an object (or, through an opening).

7. Diffraction • The amount of diffraction that occurs depends on how big the obstacle or opening is compared to the wavelength. • If the obstacle is smaller than wavelength, the waves bend around it. • If the obstacle is larger than the wavelength, the waves do not diffract much. • Explain why we cannot see a person talking around a corner although we can hear her.

8. Interference • When two or more waves overlap and combine to form a new wave, the process is called interference. • This new wave exists only while • the two original wave continue to • overlap. • There are two ways that the waves can combine: • Constructive interference • vs • Destructive interference

9. Interference • Constructive interference: • In constructive interference • the waves add together. • Destructive interference: • In destructive interference • the waves subtract from each • other as they overlap. • Can compressional waves, such as sound, have interference?

10. Standing Waves • www.walter-fendt.de/ph14e/stwaverefl.htm • A standing wave is a special type of wave pattern that forms when waves equal in wavelength and amplitude, but traveling in opposite directions, continuously interfere with each other. • The places where the two waves always cancel are called nodes. • The nodes always stay in the same place on the rope. Meanwhile, the wave pattern vibrates between the nodes. • Example: Standing waves in a violin string.

11. Resonance • The tuning forks of different type vibrate at certain frequencies called natural frequencies. • All objects have their own natural frequencies. • Natural frequencies depend on the object’s size, shape and the material it is made from. • An object can be made to vibrate at its natural frequency. • Example: A tuning fork with natural frequency of 512 Hz starts vibrating by a sound wave with a frequency of 512 Hz. • The process by which an object is made to vibrate by absorbing energy at its natural frequency is called resonance.

12. Resonance • Sometimes, resonance can cause disasters  see the handout