Science; Light Waves

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Light Waves

Light Waves

Light travels as waves. These are transverse waves, like the ripples in a tank of water. The direction of vibration in the waves is at 90° to the direction that the light travels. Light travels in straight lines, so if you have to represent a ray of light in a drawing, always use a ruler. Unlike sound waves, light waves can travel through a vacuum (empty space). They do not need a substance to travel through, but they can travel through transparent and translucent substances.

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Light Waves

The Speed of Light

Light travels extremely quickly. Its maximum speed is approximately 300,000,000 m/s, when it travels through a vacuum. The very large difference between the speed of light in air (almost 300,000,000 m/s) and the speed of sound in air (343 m/s) explains why you:

• see lightning before you hear it
• see a firework explode before you hear it
• see a distant door slam before you hear it
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Light Waves

Reflection

A ray diagram shows how light travels, including what happens when it reaches a surface. In a ray diagram, you draw each ray as:

• a straight line
• with an arrowhead pointing in the direction that the light travels

Remember to use a ruler and a sharp pencil.

When light reaches a mirror, it reflects off the surface of the mirror:

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Light Waves

Refraction

Light waves change speed when they pass across the boundary between two substances with a different density, such as air and glass. This causes them to change direction, an effect called refraction.

At the boundary between two transparent substances:

• the light slows down going into a denser substance, and the ray bends towards the normal
• the light speeds up going into a less dense substance, and the ray bends away from the normal
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Light Waves

Convex Lens

A convex lens is made from a transparent material that bulges outwards in the middle on both sides. It can focus light so that appears to meet at a single point, called the focal point. Light is refracted as it passes into, then out of, the lens.

Convex lenses are found in:

• magnifying glasses
• spectacles for people with long-sight (who can see distant objects clearly but not nearby ones)
• telescopes
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