**Diagram and Rays of Light**

To trace the diagram, draw a concave mirror and label the principle focus (F) and the center of curvature (C). Place the car headlight bulb in front of the mirror with the filaments A and B at the same distance from the mirror.

To draw the rays of light, follow these steps:

1. Draw a ray from filament A parallel to the principal axis. This ray will reflect off the mirror and pass through the focal point (F).
2. Draw a ray from filament A through the center of curvature (C). This ray will reflect back on itself.
3. Draw a ray from filament A that strikes the mirror at the focal point (F). This ray will reflect parallel to the principal axis.
4. Draw a ray from filament B through the focal point (F). This ray will reflect parallel to the principal axis.
5. Draw a ray from filament B through the center of curvature (C). This ray will reflect back on itself.

The intersection of the reflected rays from filaments A and B will create a bright spot on the road.

**Why is this Arrangement Used in Car Headlights?**

The arrangement of using two filaments with one at the principal focus of the concave mirror is used in car headlights for several reasons:

1. Brightness: The two filaments produce a brighter light than a single filament. This is because the light from two filaments is focused on a smaller area, creating a brighter spot on the road.
2. Distance: The arrangement allows the light to be focused on a farther distance. This is because the light from filament B is reflected parallel to the principal axis, increasing the distance the light can travel.
3. Safety: The brighter light and increased distance make it easier for drivers to see the road ahead, increasing safety while driving at night or in low light conditions.
4. Cost: The arrangement is cost-effective as it uses only one mirror to focus the light from two filaments, reducing the cost of the headlight assembly.