Question

Answer the following question:
An object is placed at a distance of 30 cm from a concave lens of focal length 30 cm.
(i) Use the lens formula to determine the distance of the image from the lens.
(ii) List four characteristics of the image (nature, position, size, erect/inverted) in this case.
(iii) Draw a labelled diagram to justify your answer of the part (ii)

Answer 1

(i)
Object distance, u = −30 cm
Focal length, f = −30 cm

`(1)/("v") - (1)/("u") = (1)/(f)`

⇒ `(1)/("v") = (1)/(f) + (1)/("u")`

⇒ `"v" = ("u"f)/("u"+f) = ((-30)(-30))/(-30 -30) = -15 "cm"`

(ii)
Characteristics of the image:
Nature: virtual
Position: 15 cm from the lens on the same side as the object
Size of the image formed is diminished
Image formed is erect

(iii)
Ray diagram: