Linear expansivity of steel is 0.000012/K, The forth bridge has a steel span of 1000m between the main piers. calculate in centimetres the change in length of the span when the temperature changes from -10k to 30k.

12. ritu obtained a distinct image of a distant tree on the screen with the of a concave mirror.to find the focal length of the mirror she has to measure the distance between: (a) mirror and the tree. (b) screen and the tree.(c) mirror and the screen. (d) mirror and tree as well as between screen and the tree.