Question
(a) Define optical centre of a spherical lens.
(b) A divergent lens has a focal length of 20 cm. At what distance should an object of height 4 cm from the optical centre of the lens be placed so that its image is formed 10 cm away from the lens. Find the size of the image also.
(c) Draw a ray diagram to show the formation of image in above situation.
Solution
a) Optical centre is the point lying on the principal axis of a lens through which light passes through without undergoing any deviation.
b) Given,
Focal length, f = -20 cm
Height of the object, h1 = 4 cm
Image distance, v = -10 cm
To find: Object distance = ?
Size of the image, h2 = ?
Now, using the lens formula,
Therefore, object is at a distance of 20 cm on the left side of the lens.
Size of the image is given by,
c) The ray diagram is as given below:
b) Given,
Focal length, f = -20 cm
Height of the object, h1 = 4 cm
Image distance, v = -10 cm
To find: Object distance = ?
Size of the image, h2 = ?
Now, using the lens formula,
Therefore, object is at a distance of 20 cm on the left side of the lens.
Size of the image is given by,
c) The ray diagram is as given below:
