In an ordinary passenger car, which type of mirror is used as the external rear view mirror and why is that type preferred for drivers?

Introduction / Context:
Rear view mirrors in cars are a very common safety feature, yet many learners are not fully aware of the type of mirror used and the reason for choosing that particular type. This question tests basic understanding of image formation by spherical mirrors in geometrical optics and connects it directly to a real life application in road safety and driving comfort.

Given Data / Assumptions:

• We are talking about the external rear view mirrors fitted on the sides of ordinary passenger cars.
• The driver needs to see vehicles and objects behind and to the side of the car.
• The mirror should provide a reasonably large field of view so that blind spots are reduced.
• The options include convex mirror, concave mirror, plane mirror and parabolic mirror.

Concept / Approach:
In geometrical optics, different types of mirrors form images with different characteristics. A convex mirror forms a virtual, erect and diminished image of objects placed in front of it and also provides a wider field of view. A concave mirror may form magnified or inverted images depending on the object position and has a narrow useful field of view for distant objects. A plane mirror gives images of the same size as the object and offers a moderate field of view. For safety, a driver wants to see a large area behind the vehicle even if the images are slightly smaller. Therefore, convex mirrors are preferred for car rear view mirrors because they give a wide angle view with all images erect, even though the objects appear smaller and farther than they actually are.

Step-by-Step Solution:
Step 1: Recall that a convex mirror always forms virtual, erect and diminished images of objects placed in front of it. Step 2: Note that a convex mirror has the property of covering a larger field of view compared to a plane mirror of the same size. Step 3: For a driver, a larger field of view is more important than exact image size, because it helps in seeing more vehicles behind and to the side. Step 4: Concave mirrors may produce inverted images for distant objects and are not suitable when an erect image is required over a large range of distances. Step 5: Plane mirrors give correct sized images but cover a smaller field of view, which is not ideal for minimising blind spots. Step 6: Parabolic mirrors are mainly used in applications like headlights and searchlights to produce parallel beams and are not typically used as car rear view mirrors. Step 7: Therefore, the external rear view mirrors of cars are designed as convex mirrors.

Verification / Alternative check:
If you look carefully at a typical car rear view mirror, you will notice that the image appears slightly smaller and that much more of the road behind is visible compared to looking through a flat glass. Many mirrors even have a printed warning such as Objects in mirror are closer than they appear, which directly indicates that the image is diminished in size, a characteristic of convex mirrors. Concave and plane mirrors would not give this combination of wide viewing angle and diminished but erect images. This real world observation confirms that convex mirrors are used.

Why Other Options Are Wrong:
Concave mirror is unsuitable because it can form inverted images for distant objects and has a narrow effective field of view for traffic behind the car. Plane mirror produces images of the same size as the object but covers a smaller field of view, which leaves larger blind spots compared to a convex mirror. Parabolic mirror is primarily used for concentrating or projecting light in devices like headlights and is not the standard choice for rear view mirrors in passenger cars.

Common Pitfalls:
A common misconception is that plane mirrors are used everywhere in daily life. While plane mirrors are standard in bathrooms, automotive designers select mirror shapes based on safety needs. Learners also sometimes confuse concave and convex mirrors because both are curved. An easy memory aid is that the outer bulging surface of a spoon acts like a convex mirror and shows a smaller, upright image, similar to a car rear view mirror. Remembering that safety mirrors in parking lots and at road intersections are also convex helps reinforce this concept.

Final Answer:
In an ordinary passenger car, the external rear view mirrors are convex mirrors, because they provide a wide field of view with erect but diminished images for better safety.