Question

Find the distance between two possible positions of a real object in front of a convex lens of focal length 20 cm such that the size of image is triple the size of object.

Answer 1

Step-by-step explanation:

Given Find the distance between two possible positions of a real object in front of a convex lens of focal length 20 cm such that  the size of image is triple the size of object.

• So we need to find for real image and virtual image.
• Real image : f = 20 cm, u = - u1, m = - 3
•                    So m = f / f + u
•                  Or – 3 = 20 / 20 – u1
•          So we get – 60 + 3 u1 = 20
•                Or 3 u1 – 80 = 0
•                  Or 3u1 = 80
•                 Or u1 = 80/3 cm
• Virtual Image: f = 20 cm, u = - u2, m = + 3
•          So 3 = 20 / 20 – u2
• So we get 60 – 3u2 = 20
•            Or – 3u2 = 20 – 60
•          Or – 3u2 = - 40
•           Or u2 = 40 / 3 cm
• Now u1 – u2 = 80/3 – 40/3
•                        = 40/3 cm

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