How light travels

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 table summarises some similarities and differences between light waves and sound 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

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.

The law of reflection

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

• the incident ray is the light going towards the mirror
• the reflected ray is the light coming away from the mirror

A ray diagram for reflection at a mirror

In the ray diagram:

• the hatched vertical line on the right represents the mirror
• the dashed line is called the normal, drawn at 90° to the surface of the mirror
• the angle of incidence, i, is the angle between the normal and incident ray
• the angle of reflection, r, is the angle between the normal and reflected ray

The law of reflection states that the angle of incidence equals the angle of reflection, i = r. It works for any angle. For example:

• the angle of reflection is 30° if the angle of incidence is 30°
• the angle of reflection is 90° if the angle of incidence is 90°

In the second example, if a light ray travelling along the normal hits a mirror, it is reflected straight back the way it came. The reflection of light from a flat surface such as a mirror is called specular reflection – light meeting the surface in one direction is all reflected in one direction.

Scattering

If light meets a rough surface, each ray obeys the law of reflection. However, the different…