Security and visibility
Light diffusion experiment
Author:
Title 4
Learning objectives :
This activity allows students to objectively measure the effectiveness of the retro-reflective strips on safety vests. It raises awareness of road safety issues by applying concrete physical principles.
Concepts covered
Reflection and diffusion of light; Retro-reflection; Road safety; Illuminance measurement; Luminance
What students will do :
The student compares the visibility of different clothing items in the dark using FizziQ's light sensors. By successively illuminating a dark outfit, a light outfit and a vest equipped with retro-reflective strips with a flashlight, the student measures and compares the light returned by each material then uses this data to construct a scientific argument on the importance of nighttime visibility.
What is required :
Smartphone with the FizziQ application; A flashlight; A vest with retro-reflective strips; A light piece of clothing; A dark garment; A space that can be darkened; FizziQ experience notebook
Scientific background :
Nighttime visibility relies on how materials interact with light. There are three main phenomena to distinguish: 1) Diffuse reflection: matte surfaces (like most clothing) scatter light in all directions, returning only 10-20% of the incident light; 2) Specular reflection: shiny surfaces reflect light mainly in one direction (like a mirror), but rarely towards the source; 3) Retro-reflection: particular property which reflects light preferentially towards its source, whatever the angle of incidence. The strips of safety vests exploit this latter phenomenon thanks to glass microbeads or microprisms which act as catadioptric reflectors. The effectiveness of these materials is remarkable: they can reflect up to 80% of incident light back to its source, making the wearer visible several hundred meters away in the dark. The FizziQ lux meter or luminance sensor allows you to quantify this difference. For optimal measurement, you must: 1) Calibrate the sensor on a reference surface; 2) Maintain a constant distance between the light source and the different items of clothing; 3) Align the sensor with the source to capture the retro-reflected light. Typical results show that retro-reflective strips reflect 50 to 100 times more light than dark clothing and 10 to 20 times more than light clothing. This experiment scientifically demonstrates the usefulness of safety vests: they significantly increase the distance at which a driver can perceive a pedestrian or cyclist in the dark, thus reducing the risk of an accident. At 50 km/h, this increased distance can represent several seconds of additional reaction time, often enough to avoid a collision.