Light Energy and Sound Energy
- Light energy and sound energy are various forms of energy.
- Light does not require a medium to travel, whereas sound needs a medium to travel.
- The speed of light is faster than the speed of sound. For the same reason, we hear the sound of thunder after seeing a lightning strike.
- The particles in the solid medium are situated very closely that is why sound travels faster in solids and slowest in gases.
Sources and Properties of Light
How are Shadows Formed?
- A shadow is a dark figure formed when an opaque object blocks the path of light.
- The source of the light, an opaque object and a screen are three basic requirements for the formation of a shadow.
- A shadow is always black in colour. Its size depends on the distance between the object and the light source.
Explain How Shadows Change Throughout the Day?
- Shadows are longer in the morning and evening, and shorter during the day.
- The size of a shadow depends on the sun's position in the sky.
- The sun is near the horizon during sunrise and sunset, which makes the shadows longer.
- In the afternoon, the sun is over our heads; hence the shadows are smaller.
Why Does the Sky Appear Blue?
- When sunlight passes through the atmosphere, tiny air particles of the atmosphere scatter the light waves.
- These particles scatter blue light more than other colours, making the sky appear blue.
What Is a Rainbow, and How Is It Formed?
- A multi-coloured arc that appears in the sky due to the dispersion of light through water droplets is called a rainbow.
- Dispersion of light is the splitting of white light into seven constituent colours.
- Rainbow is formed after the rain due to the dispersion of sunlight by the water particles.
- The order of seven colours in the rainbow are violet, indigo, blue, green, yellow, orange, and red.
How Does a Magnifying Glass Burn Things?
- A magnifying glass is made using a convex lens.
- A convex lens is thicker in the middle and thinner at the edges and has the ability to converge the light rays passing through it at one point.
- Due to the high concentration of light rays at a point, sufficient heat is produced, which can burn things.
Sources and Properties of Sound
How Do Sound Waves Travel Through the Air?
- Air is made up of molecules which have space between them.
- Soundwaves, while travelling through air, strike these molecules to produce vibrations which are transferred to the adjacent molecules.
- This phenomenon allows sound waves to travel through the air gradually.
- This process is quick and is completed in fractions of a second.
Why Do Sound Waves Travel Faster in Water Than in the Air?
- The lesser the space between the atoms or molecules, the faster propagation of sound.
- The atoms in water are comparatively closer than the atoms in the air. Hence, sound travels faster in water.
Which Property of Sound Determines Its Loudness?
- Amplitude is the property of sound that determines its loudness.
- It is the maximum limit of the vibration produced by an object.
- Higher the amplitude, the higher the loudness.
- Categorise the Following Into Noise and Music—
- The Sound of Firecrackers.
- The Sound of a Guitar.
- The Sound of Television at High Volumes.
- The Sound of Water Coolers.
- The Sound of a Whistle.
- The Sound Produced by a Mouth Organ.
The sound which is loud and unpleasant is categorised as noise.
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Why Is a Female Voice Shriller Than a Male Voice?
- Voice is produced from vocal cords that are located in our throat region.
- These vocal cords are thin flaps of cartilage.
- These flaps of cartilage are thinner in females than in males.
- A thinner object can vibrate faster than a thicker object and is able to produce a high-pitched sound.
- Thus, the sound made by a female is shriller than that of a male.