Question

A physics student mounts two thin lenses along a single optical axis (the lenses are at right angles to the line connecting them, and they appear concentric when viewed from either end). The lenses are identical, each with a positive (converging) focal length of 14.8 cm. They are separated by a distance of 39.4 cm. Lens 1 is to the left of Lens 2.

Required:
a. What is the final image's distance (in cm) from Lens 2?
b. Where is the final image located?
c. What is the overall magnification of the lens pair, considered as a single optical instrument?

Answer 1

A) q₂ = 75.98 cm, B) q₂' = 115.38 cm, C)

Step-by-step explanation:

A) This is an exercise in geometric optics, as the two lenses are separated by a greater distance than their focal lengths from each lens, they must be worked as independent lenses.

Lens 1. More to the left

let's use the constructor equation

`(1)/(f) = (1)/(p) + (1)/(q)`

where f is the focal length, p and q are the distance to the object and the image, respectively,

We must assume a distance to the object to perform the calculation, suppose that the object is 50 cm from lens 1 that is further to the left of the system.

`(1)/(q_1) = (1)/(f) - (1)/(p)`

`(1)/(q_1) = (1)/(14.8) - (1)/(50)`

1 / q₁ = 0.04756

q₁ = 21.0227 cm

this image is the object for the second lens that has f₂ = 14.8 cm

the distance must be measured from the second lens

p₂ = 39.4 -q₁

p₂ = 39.4 -21.0227

p₂ = 18.38 cm

let's use the constructor equation

1 / q₂ = 1 / f - 1 / p2

`(1)/(q_2) = (1)/(14.8) - (1)/(18.38)`

`(1)/(q_2)` = 0.01316

q₂ = 75.98 cm

measured from the second lens

B) the position of the final image with respect to the first lens is

q₂’= q₂ + 39.4

q₂'= 75.98 +39.4

q₂' = 115.38 cm

C) the magnification of a lens is

m = - q / p

in this case the image measured from lens 2 is q2 = 75.98 cm

the distance to the object from the first lens is p1 = 50cm

m = - 75.98 / 50

m = -1.5 X

the negative sign indicates that the image is inverted