You hold a pair of reading glasses in sunlight, and after moving it up and down

ID: 2124606 • Letter: Y

Question

You hold a pair of reading glasses in sunlight, and after moving it up and down a bit, you

?nd that a small dot of light appears on the sidewalk 75cm below one of the lenses, and 50cm

below the other.

a. Sketch a ray trace (as accurately as you can) for the lens that makes a dot at 50cm with

the object placed 25cm from the lens. Label the image and focal point(s), and include

arrows on the rays to indicate the direction that the light is traveling. Each side of a

square in the grid below represents a distance of 5cm.

b. You remove the lenses from the frame and place them together to essentially form a

single lens. Use the thin lens equation to determine the ?nal image distance from the lens

for an object placed at a distance 25cm from this con?guration. Do the calculation in

steps, treating each lens in turn as if it is at the same place (these are thin lenses, after

all). [It doesn

Can't post a picture for the ray diagram for part A, but here is the solution to the rest.

Part B)

For the first lens...

Apply 1/f = 1/p + 1/q

1/75 = 1/25 + 1/q

q = -37.5 cm

For the second lens

1/50 = 1/37.5 + 1/q

q = -150cm

Final image is 150 cm in front of the lens combination

Part C)

Overall...

1/f = 1/f1 + 1/f2

1/f = 1/75 + 1/50

f = 30 cm

D = 1/f (in meters)

D = 1/.3

D = 3.33 diopters

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