Think about being completely in the dark. Can you see anything? You cannot. Why? You can only see objects in the light. We need light energy to see. Light is a form of energy. It travels in straight lines. Light energy helps make objects visible to us. Light bounces off an object and lights it up (illuminates it), or an object gives off its own light energy. Light can also cause an object to cast a shadow depending on the way it is focused on an object.
The sun is the most important light source we have here on Earth. There are many other sources of natural light as well. Stars, lightning, fireflies, and the northern lights are all sources of natural light. However, we also have many sources of man-made light. Light bulbs, candles, flashlights, and televisions are just a few examples of man-made light.
Sometimes objects do not actually generate their own light, but they reflect the light of other objects instead. The moon, planets, mirrors, and water surfaces are some examples of objects that reflect light.
Sound is a type of energy created by vibrations that travel through the air or other media and can be heard when they reach the ear. Sound travels as waves. Sound waves need a medium through which to travel. Without a medium, sound waves cannot continue to travel. For example, if there is no air, such as in space, there is no sound. Sound waves travel when matter is present in any of the solid, liquid, or gas phases. As it travels, sound causes the medium to vibrate. Sometimes the vibrations of sound can be felt as they travel through the medium. For example, vibrations from guitar strings create sound waves that travel to our ears.
There are many different types of sounds. Sounds can be loud, such as a siren blaring, or quiet, such as a whisper. The more air that is moved, the louder the sound is. Sounds can be high pitched, such as an alarm, or low pitched, such as a tuba; these are examples of high-frequency sound waves and low-frequency sound waves, respectively.