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.
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
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:
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
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)