QUICK QUIZ 23.4
A plastic sandwich bag filled with water can act as a crude
converging lens in air. If the bag is filled with air and placed under
water, is the effective lens (a) converging or (b) diverging?
QUICK QUIZ 23.4 ANSWER
(b). In this case, the index of refraction of the lens material is less than that of the surrounding medium.
Under these conditions, a biconvex lens will be divergent.
Image Example Between F’ and O
F’ FO
Behind lens, virtual, uprightlarger than object
Magnifying glass
F’ FO
At F’
No image Lighthouse
Between F’ and 2F’
F’ FO2F’
Beyond 2F, real, inverted, larger
Projector
F’ FO2F’
At 2F, real, inverted, same as object
Office copier
At 2F’
F’ FO2F’Between F and 2F,Real, inverted, smaller
Camera
Beyond 2F’
F’ O2F’
At F, real, inverted,smaller
F CameraAt infinity
Example: (a) An object 31.5 cm in front of a certain lens is imaged 8.20 cm in front of that lens (on the same side as the object). What type of lens is this and what is its focal length? Is the image real or virtual? (b) If the image were located, instead, 38.0 cm in front of the lens, what type of lens would it be and what focal length would it have?
Solution: (a) 1/do+1/di=1/f where do=31.5 cm, di=-8.20 cm
1/31.5+1/(-8.20)=1/f, which yields f=-11.1 cm, thus
diverging lens. The image is in front of the lens, so it is virtual
(b) Similarly, we have 1/31.5+1/(-38.0)=1/f. which gives
f=+184 cm, thus, converging lens.
F’ FO
(b)
F ImageF’
(a)
32
Question: An object infinitely far from a converging lens has an image that is
(a) real
(b) virtual
(c) upright
(d) larger than the object
Answer: a
33
Question: An object farther from a converging lens than its focal point always has an image that is
(a) inverted
(b) virtual
(c) the same in size
(d) smaller in size
Answer: a
34
Quiz: An object closer to a converging lens than its focal point always has an image that is
(a) inverted
(b) virtual
(c) the same in size
(d) smaller in size
Answer: b
Sign Conventions for Refracting Surfaces/Lenes
These are the same sign conventions – so just remember them for thin lenses.
1 2
1 1 1( 1)n
f R R
1
2
' n qhM
h n p
n1 is the source mediumn2 is the observing medium
Rnn
qn
pn 1221
'
1 1 1
h qM
h p
p q f
Combinations of Thin Lenses The image produced by the first lens is calculated as though the second lens
were not present The light then approaches the second lens as if it had come from the image of
the first lens The image of the first lens is treated as the object of the second lens The image formed by the second lens is the final image of the system If the image formed by the first lens lies on the back side of the second lens,
then the image is treated at a virtual object for the second lens p will be negative
The overall magnification is the product of the magnification of the separate lenses