An object is placed at a distance of 60 cm from a concave lens of focal length 30 cm.
(i) use lens formula to find the distance of the image from the lens
(ii) List 4 characteristics of the image (nature, position, size, erect/inverted) formed by the lens in this case.
(iii) Draw ray diagram to justify you answer of part (ii).
Given:
The focal length of concave lens = f = -30 cm
The distance of object from the lens = u = -60cm
(-ve due to sign convention)
(i) We have to find the distance of image from lens using lens equation
The lens equation is
Where f is the focal length, u is the object distance and v is the image distance.
Using the lens equation
Substituting values for f and u
∴ v = -20 cm
The image is located 20 cm away from the concave lens and is formed on the same side as that of the object.
(ii) The formula for magnification is given as
Where m is the magnification.
The magnification is
Hence, the image size is 3 times less than the size of the object. As the image distance is –ve, it implies that the image is virtual.
The characteristics of the image are:
1. The image is virtual
2. The image is erect
3. The image is diminished
4. The image placed between the focus and optical center.
(iii) The ray diagram is as follows:
AB is the object
A’B’ is the image
F is the focus
O is the optical center
Ray 1 is striking to the lens parallel to the lens axis
Ray 2 is incident on the optical center
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