Question

The table shows the object distance, u, and the corresponding image distance v, for an object placed in front of a concave mirror.

u (cm)	20	25	30	40	50	70
v(cm)	20	16.7	15	13.3	12.5	11.6
1/u (cm−1)
1/v (cm−1)

1. Complete the table by filling in the values of ¹/_u and ¹/_v 
2. plot a graph of ¹/_u (y-axis) against 1/_v (x-axis)
3. From the graph, determine the focal length of the mirror. 

Answer 1

1.  

   u (cm)	20	25	30	40	50	70
   v(cm)	20	16.7	15	13.3	12.5	11.6
   1/u (cm−1)	0.05	0.04	0.033	0.025	0.020	0.014
   1/v (cm−1)	0.05	0.06	0.067	0.075	0.08	0.086

2. 
   
3. ¹/_f = ¹/_u + ¹/_v intercept = ¹/_f  
    0.1 = ¹/_f 
    f = 10cm