1. multiple reflections
A periscope is used to observe something that is at a different height level. This is mostly used to avoid detection.
A simple periscope uses 2 mirrors tilted at an angle so that, the light from the object is reflected multiple times (in this case, twice) to reach the observer.
We see an object (here, a fruit) when the light reflecting off the object reaches our eye. Here, the fruit is in a glass of water, and we are observing from outside (air).
Since there is a change in the medium, the light rays would bend. This phenomenon is called refraction. As a result, the light rays coming from the fruit bend, and reach our eyes, but we think that the light rays are coming straight into our eyes. This results in the magnification of the image. To understand clearly, see the following figure.
Vacuum means “empty space”. It does not contain any form of matter. Sound requires a medium (like air, water, solids etc) to travel. As vacuum is devoid of everything, sound cannot travel in it. One can verify using the vacuum pump experiment mentioned in the textbook.
A bell when tapped using a rod, starts to vibrate, and we know that vibrating bodies produce sound. That is why, when we touch the bell, we feel a sensation of vibration.
5. 20-20000 Hz
Audible sound means the sound which can be heard by the human ear. The human ear is capable of hearing sounds of frequencies 20 Hz – 20000 Hz. Humans cannot hear the sounds beyond the given frequency range.
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