(a) Determine the ‘effective focal length’ of the combination of the two lenses in Exercise 9.10, if they are placed 8.0cm apart with their principal axes coincident. Does the answer depend on which side of the combination a beam of parallel light is incident? Is the notion of effective focal length of this system useful at all?
(b) An object 1.5 cm in size is placed on the side of the convex lens in the arrangement (a) above. The distance between the object and the convex lens is 40 cm. Determine the magnification produced by the two-lens system, and the size of the image.

(a)
Hint:
Use the lens equation.

Step 1: Find the final image formed when the rays are incident on the convex lens first.
The focal length of the convex lens, = 30 cm
The focal length of the concave lens,  = −20 cm
Distance between the two lenses, d = 8.0 cm
When the parallel beam of light is incident on the convex lens first:
Object distance, u1=
Image distance, v1=?
According to the lens formula,

1v1-1u1=1f1

1v1=130-1=130
v1=30cm

Forconcavelens,
Objectdistance,u2=(30-d)=30-8=22cm
lmagedistance,v2=?
1v2=122-120=10-11220=-1220
v2=-220cm

 The parallel incident beam appears to diverge from a point that is220-d2=220-4=216cm from the centre of the combination of the two lenses.

Step 1: Find the final image formed when the rays are incident on the concave lens first.
 When the parallel beam of light is incident on the concave lens first:

Object distance, u2=

Image distance, v2=?
Using the lens formula,

1v2-1u2=1f2
1v2=1f2+1u2

1v2=1-20+1-=-120
v2=-20cm

For the convex lens:

Object distance, u1=-20+d=-20+8=-28cm

Image distance, v1=?
Applying the lens formula:

1v1-1u1=1f1
1v1=130+1-28=14-15420=-1420
v2=-420cm

Hence, the parallel incident beam appears to diverge from (420-4=416 cm). The diversion happens from the left of the centre of the combination of the two lenses.

Step 3: Compare the effective focal length in both cases.
The answer does depend on the side of the combination at which the parallel beam of light is incident. The notion of effective focal length does not seem to be useful for this combination.

(b)

Hint: Use the lens formula.

Step 1: Find the magnification produced by the convex lens.
For the convex lens:
Object distance, u1=-40 cm

Image distance, v1=?

According to the lens formula:

1v1-1u1=1f1
1v1=130+1-40=4-3120=1120
v1=120cm
Magnification,m1=v1u1=120-40=-3

Hence, the magnification due to the convex lens is 3.

Step 2: Find the magnification produced by the concave lens.
For the concave lens:

Object distance, u1=+120-8=112cm

Image distance, v2=?
According to the lens formula:

1v2-1u2=1f2
1v2=1-20+1112=-112+202240=-922240
v2=-224092cm
Magnification,m2=v2u2=-224092×1112=-2092


Step 3:  Find the magnification produced by the combination.
The magnification produced by the combination:

m=m1m2
m=-3×-2092=6092=0.652

The magnification is given by:

m=heightoftheimageheightoftheobject=h'h=0.652
h'=0.652×h
h=0.652×1.5=0.98cm

Hence, the height of the image is 0.98 cm.