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Study Notes: Transmission and Intensity

Transmission
Light travels without change through a vacuum. Some materials can block light and prevent it from passing through. Solids, liquids and some gases can absorb some or all of the light that strikes them.

A material that transmits light is called transparent. Glass is transparent to visible light but it can block some forms of ultraviolet and infrared light. You can see through a dilute solution, so it is transparent.

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Materials that absorb most but not all of the light that strikes them are called translucent. If you hold paper up to a bright light with your hand behind the page you can count how many fingers there are but you cannot see if they are clean.

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Materials that completely block all of the light that shines on them are called opaque. A sheet of cardboard is opaque to visible light but it is transparent to X-rays.

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Light travels in straight lines until it either strikes an opaque surface or travels into a different material. To help us understand the behaviour of light we draw it as a set of lines call rays. The direction in which the light moves is shown as an arrow on each light ray. A beam of light is made up of many light rays. A single arrow representing a light ray and a set of parallel arrows representing a beam of light.

Intensity
The intensity of a light beam is also called the radiant power of the light or its brightness. In a bright beam of light there are many photons close together. In a less intense light beam, the photons are more spread out.

The light rays from a light globe spread out as they radiate away from the glowing metal filament in the bulb.

A shining light globe with rays of light radiating outwards in all directions - the rays spread out as they move away from the globe.

The light rays near the bulb are close together. This makes the light very intense near the bulb. Further from the bulb the light rays have spread out. This weakens the intensity of the light away from the bulb.

You can see this effect for yourself quite easily. When you view a light bulb from up close it is very bright and may hurt your eyes but if you stand far away where the light is less intense, the light appears weak and dim.

Some light beams are made up of parallel rays of light that do not spread out. The intensity of these light beams is the same when viewed up close and far away.

A set of parallel arrows representing a beam of light.

Special reflectors and lenses can be used to produce light beams made up of parallel rays. This type of light is usually of better quality in a measuring instrument. Some reflectors and lenses will make a light beam become narrower or spread out.
Two beams of light - one with the individual rays moving towards each other (converging) and the other with the rays moving away (diverging).

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