A specific magnifying glass is used to achieve a magnification of -3 when an obj

Question

A specific magnifying glass is used to achieve a magnification of -3 when an object is placed only 15 cm away from it.

a) What is the focal length of the magnifying glass?

cm

b) What type of lens is the magnifying glass?

planeconcave    convexunknown

c) The image would be  ---Select--- real virtual unknown ,  ---Select--- upright inverted unknown , and  ---Select--- enlarged reduced unknown .

d) It is possible to use this magnifying glass to burn a hole through a piece of paper. If you were to use light rays from the sun to do this, how far away from the paper would you need to hold the magnifying glass?

answer choices:

a) between the lens and the focal point b) at the focal point c) between the focal point and the center of curvature d)beyond the center of curvature

Explanation / Answer

Distance of object, do = 15 cm
M = -3
M = -di/do
-3 = -di/15
di = 45 cm

(a)
Using thin lens equation,
1/f = 1/di + 1/do
1/f = 1/45 + 1/15
f = 11.25 cm

(b)
As the focal length is positive,
The lens is Convex !!

(c)
Image is Real.
Image is Inverted.
Image is Enlarged.

(d)
Yes, Convex lenses are Converging lenses !!
This would be possible when the lens is held at the focal point.

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