How do we know in which materials light can travel?

Light can travel through certain materials, such as glass, water, clear plastic. Such materials are called transparent materials. Translucent materials, such as greaseproof paper, and frosted glass let some but not all light through. They scatter or diffuse the light.

Most materials do not allow light through at all and these are called opaque. When light from a source cannot travel through a material then a shadow is formed.

So by checking the shadow, we can check if light passes through a given material or not. If shadow is clearly visible behind an object, that means that material does not allow light. If shadow is not formed by an object, that means it will allow light to pass through it.


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