Refraction and Snell's Law

1
3 easy to trace lines can help locate the image
formed by a lens
p
f
f
q
q
M
p
With the magnification,
-
2
ConcepTest
air
water
A
If a glass lens in air focuses a light as shown,
which of the figures best illustrates how the
light will focus in water?
B
C
3
p
q
For a given lens (of fixed focal length, f ), as
the object approaches the focal point ( p ? f ),
the image position, q, approaches

1. 0.
2. f / 2.
3. f.
4. 2f.
5. ?

4
p
q
For a given lens (of fixed focal length, f ),
as the object moves very far back from the lens (
p ? ? ), the image position, q, approaches

1. 0.
2. f / 2.
3. f.
4. 2f.
5. ?

5
Some Answers to Concept Questions
nair 1.0 nwater 1.3
nglass1.5 With water surrounding the glass lens,
the transition across boundaries involves a much
smaller change in density, a smaller shift in
light speed. If the speed of light changes less,
the resulting change in direction will be less
pronounced.
C
(5) ?
For this fixed height object, the yellow line
will still enter the lens at the same height as
the object is moved closer, following the same
path once through the lens. Notice how the red
line, pinned to the objects tip on one end, and
through the focus, rotates as the object moves in
closer. It turns downward intercepting the lens
at a progressively lower pointthat means the
refracted red line meets the yellow line closer
and closer to the axis and closer and closer to
the focus. The white line, pivoting about the
point At the lens center exhibits the same
behavior.
(3) f.
Following a argument similar to above see how
the light rays traced will shift. OR look at the
thin lens equation 1/f 1/p 1/q. With f (and
thus the left-hand-side of the equation) fixed,
as p ??, 1/p ? 0. So 1/f ? 1/q.
MORE
6
B
A
B
C
A light source is held near a lens and its light
focuses at point B, as shown in the top figure.
If the light source is moved closer to the lens,
then at which of the indicated points will the
light focus?

1. A
2. B
3. C

7
In the figure at top, a simple single lens camera
is focused on a distant mountain. The camera is
then focused on a nearby person. Which of the
figures at left best illustrates the lens
position to focus on the person?
1
2
3
8
B
A
B
C
A light source is held near a lens and its light
focuses at point B, as shown in the top figure.
If the light source is moved closer to the lens,
then at which of the indicated points will the
light focus?
Conceptually Bending power of the lens is the
same in both cases. For a closer object, the
rays diverge more, and so it takes longer for
them to be brought to a convergence. Mathematicall
y 1/i 1/o 1/f If o gets shorter (1/o
bigger), then i must get longer (1/i smaller)
since f does not change.
9
In the figure at top, a simple single lens camera
is focused on a distant mountain. The camera is
then focused on a nearby person. Which of the
figures at left best illustrates the lens
position to focus on the person?
1
o becomes smaller (1/o bigger), so i must become
bigger (1/i smaller) so that 1/f 1/i 1/o
2
3
10
A magnifying glass has f 30 cm. If the object
to be viewed is at p 10 cm. Locate the image.
What kind of image is formed? What is the
magnification factor?
virtual image!
If p150 cm where will the image form?
Magnification? Kind of image?
11
p ? dobj
objects distance to lens or mirror
always positive (for real objects)
POSITIVE for CONVERGING NEGATIVE for DIVERGING
f
focal length
R/2 for spherical approximations to parabolas
q ? dimage
distance image comes into focus from lens or
mirror
POSITIVE means REAL IMAGE NEGATIVE means VIRTUAL
IMAGE
M ? -q/p -dimage/dobj
MAGNIFICATION