Question
A concave lens has a focal length 15 cm. If the object
is placed at 30 cm from the lens, what is the image distance?Solution
focal length f = -15 cm Distance of object u = -30 cm 1/ v = 1/f +1/u substituting these values in the lens equation we get, 1/v =1/-15 +1/-30 1/v =-3/30 v = -10cm Hence, the distance of the object from the lens is 10 cm and the negative sign signifies a virtual image.
69, 35.5, 39.5, 68.25, 152.5, ?
140, 146, 158, 176, ?, 230
Find the missing number in the given number series.
?, 26, 38, 70, 130, 226
18    ?    20    13     22     11
What will come in place of (?) question mark in the given number series.
11, 6, 7, 12, 26, ?
84, 105, ?, 168, 210, 259
18, 30, 50, 102, 282, ?
31, 60, 118, 234, 466, ?
4, 4, 8, 32, 256, ?
21, 37, 73, 137, ?, 381