(a) Explanation with reason: 3 Marks
(b) Solution: 2 Marks
(a) The mirrors commonly used for back and side mirrors are convex in nature. This is done to take advantage of the wider angle that can be reflected by a Convex mirror. A convex mirror, also known as converging mirror, is one which bulges at the middle and is thinner at the edges. While this captures a wider object field, it does so at the cost of image accuracy. The result is compression of the reflected image, which makes objects appear smaller and farther away than they really are.
 
The warning is mentioned in the mirrors so that the driver can accordingly make adjustments and take a well-calculated decision.
(b) The speed of car is 2 m/s which means the car is approaching the truck with a speed of 2 m per second.
The distance between the image of truck and truck will decrease at a double rate. This happens because for each meter the car moves, the mirror becomes closer to the object by 1 meter and also the image becomes 1 meter closer.
This is because the image of the truck will travel a distance twice the distance travelled by the car in equal time.
Hence, the image of the truck will appear to approach the driver with the speed of 2 × 2 = 4 m/s.
As the distance between the truck and car is 20m and the car is going back at a speed of 2m per second, the car will hit the truck in 10 seconds.