Reasons why concave mirrors cannot be used as rear view mirrors in vehicles:

1. Image Formation:
- Concave mirrors produce either real or virtual images depending on the position of the object. Real images are formed in front of the mirror, making it unsuitable for use as a rear view mirror in vehicles where the driver needs to see a virtual image behind them.

2. Size of the Image:
- Concave mirrors magnify the size of the image, which can distort the perception of distance and size of objects behind the vehicle. This can lead to misjudgments while driving.

3. Field of View:
- Concave mirrors have a limited field of view due to their curved surface, which may not provide a wide enough view of the surroundings for the driver. This can be dangerous while changing lanes or parking.

4. Focal Length:
- The focal length of a concave mirror is fixed, which means that it cannot be adjusted to suit the driver's preference or to accommodate different driving conditions. This lack of flexibility makes it impractical for use in vehicles.

5. Safety Concerns:
- Using a concave mirror as a rear view mirror may lead to distractions and visual discomfort for the driver, as the mirror can create glare and distortions that hinder clear visibility of the road behind.
In conclusion, while concave mirrors are useful for applications such as reflecting light or focusing images, they are not suitable for use as rear view mirrors in vehicles due to their image formation, size, limited field of view, fixed focal length, and safety concerns.