incidence

Everyday

1. The incidence of something, like an illness or a crime, is how often it happens. tree →
2. (science) The angle of incidence is the angle at which a ray of light hits a surface, measured from the normal line. tree →

In the lessons

In science lessons on light and mirrors, the angle of incidence equals the angle of reflection. In health and news, «the incidence of flu» means how many people catch it. In maths it is linked to rates and statistics. Phrases: «angle of incidence», «a high incidence of», «the incidence of disease».

Examples

• The angle of incidence equals the angle of reflection. tree →
• We measured the angle of incidence with a protractor. tree →
• There is a high incidence of colds in winter. tree →
• Washing hands lowers the incidence of tummy bugs. tree →
• The report shows the incidence of road accidents has fallen. tree →
• Draw the normal before you mark the angle of incidence. tree →

Note

Not the same as «incidents» (events), though they sound alike. «Incidence» is usually uncountable and means a rate. In maths and science problems, the angle of incidence is measured from the normal (the line at right angles to the mirror), not from the mirror surface; if a ray hits at 30° to the normal, it bounces off at 30° to the normal.