Part A When we talk about energy from light that a surface receives, we use the word irradiance (E). Irradiance measures the amount of energy per second (P) that arrives on a surface per unit area (A). As an equation, this idea can be expressed in this way: E = P/A.

As the angle of the light source moves from 0° to 45°, the illuminated area (A) increases. You can see this increase in the image. However, although the illuminated area increases, the energy per second (P) from the light is constant. What effect will an increase in the angle of light, and therefore illuminated area (A), have on irradiance (E)?

two flashlights, one with light falling perpendicular to the surface and the other tilted with light falling at an angle of incidence of 45 degrees to the surface