1.2 Analysing Reflection of Waves
Reflection of waves obey the law of reflection, that is the angle of incidence is equal to the angle of reflection. Besides that, the incident wave, the reflected wave and the normal all lie on the same plane. When waves are reflected, the wavelength, frequency and speed do not change but the direction of propagation changes.
Formula for echo: 2s = vt
1.3 Analysing Refraction of Waves
The refraction of water occurs when there is a change of direction of the propagation of waves travelling from different depth due to a change in speed. After refraction, the waves have the same frequency but different speed, wavelength and direction.
Why do sound waves transmit farther at night? Is it because it is quieter at night?
If you go to the beach on vacation, during the night, you will discover that voice of people far away can be heard clearly. You may wonder, it is because it is quieter at night than in the daytime. Therefore it is easy to hear the sound far away. However, it is only one of the reasons. Actually, sound transmits farther at night may be related to refraction of sound waves! First, the sound is the vibration of air, and it is a kind of wave motion. The propagation of the sound wave is faster in hot air and slower in cold air. Therefore regions of air at different temperatures have different refractive indices, just like media with different optical densities. When sound waves propagate in air whose temperature changes with altitude, refraction of air happens. The sound will move towards areas with lower temperatures. In the daytime, when the sun shines the earth, the air near the earth surface is hotter than the air above. Sound waves will be refracted to the sky. On the contrary, in the night time, the air near the surface is cooler and sound waves are refracted to the earth surface.
Reflection of waves obey the law of reflection, that is the angle of incidence is equal to the angle of reflection. Besides that, the incident wave, the reflected wave and the normal all lie on the same plane. When waves are reflected, the wavelength, frequency and speed do not change but the direction of propagation changes.
Formula for echo: 2s = vt
1.3 Analysing Refraction of Waves
The refraction of water occurs when there is a change of direction of the propagation of waves travelling from different depth due to a change in speed. After refraction, the waves have the same frequency but different speed, wavelength and direction.
Why do sound waves transmit farther at night? Is it because it is quieter at night?
If you go to the beach on vacation, during the night, you will discover that voice of people far away can be heard clearly. You may wonder, it is because it is quieter at night than in the daytime. Therefore it is easy to hear the sound far away. However, it is only one of the reasons. Actually, sound transmits farther at night may be related to refraction of sound waves! First, the sound is the vibration of air, and it is a kind of wave motion. The propagation of the sound wave is faster in hot air and slower in cold air. Therefore regions of air at different temperatures have different refractive indices, just like media with different optical densities. When sound waves propagate in air whose temperature changes with altitude, refraction of air happens. The sound will move towards areas with lower temperatures. In the daytime, when the sun shines the earth, the air near the earth surface is hotter than the air above. Sound waves will be refracted to the sky. On the contrary, in the night time, the air near the surface is cooler and sound waves are refracted to the earth surface.
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