Light Waves Travel in Straight Lines Light waves travel in straight lines. They do not change direction unless they are reflected or refracted.
Shadows
When light from the sun hits a tree, it cannot pass through it. It cannot go around the tree. The light is reflected and absorbed by the tree, and an area without light is made—a shadow. Early Egyptians understood shadows. They knew the sun travels the same path in the sky everyday. They knew that shadows change as the sun moves across the sky. They used this knowledge to develop a tool to tell time—a sundial. They placed a stick in the ground. The position and length of the stick’s shadow told the people the time.
An Eclipse Is a Shadow Sometimes the moon moves between the Earth and the sun. The sun’s light cannot pass through the moon to reach the Earth. A large shadow covers the Earth. This is called a solar eclipse. During an eclipse, some parts of the Earth get no light at all. They become as dark as night. This area is called the umbra. Some parts of the Earth get some light. They are in part shadow, part light. This area is called the penumbra.
When Light Waves Hit Substances Light waves travel in straight lines. When light waves hit something, three things can happen. The light wave can pass through transparent substances. This may cause the light wave to speed up or slow down. If a light wave changes speed, it will bend where the two substances meet. The bending of light waves as they travel from one substance to another is called refraction. Light waves can enter a substance and be absorbed. Plants absorb some light waves and convert them into sugars. Light waves can also bounce off a substance. When light bounces off a substance, this is called reflection. A mirror reflects light waves. Many substances absorb some light waves and reflect others.
Eclipse PENUMBRA UMBRA the sun is the sun is partly hidden totally hidden
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Earth 23