Lab 27: More refraction activities!
- Reflected and refracted light beams
- Prisms
- Refraction and shadows
15) Two laser pointers in different places are aimed at a screen, so that their beams
converge at a single point on the screen (that is, only one doubly bright spot shows). A
wide, thick plate of glass is inserted so that both laser beams have to pass through it
toward the screen. Will the beams still converge at a single point (assuming the laser
pointers remain in the same positions)? Draw a beam diagram of this situation to
explain your answer.
The dotted lines show the path the
lasers take without the glass present (a
straight line). When a glass plane is
inserted, the refraction for either laser
is not equal to the other, because they
both strike the glass surface at different
angles. Therefore, no matter how the
glass is angled, the two laser beams
will no longer meet at exactly the same
point.
16) You viewed the shadows produced by rounded, transparent water bottles on a
viewing screen. Imagine you are given a strange-shaped bottle that curves inward, as
shown from a top view, below. Describe the shadow it will produce on the screen, and
justify your answer by showing the refraction paths that light beams will follow through
the bottle.
Instead of being brightest at the center, like a rounded water bottle’s
shadow is, this water bottle will produce a wider pattern of light that is
brightest at the edges, where the beams bent outward by the water bottle
combine with rays coming straight from the source outside the edges.
Lab 28: Characterizing Lenses ; Model Camera
- Types of lenses
- Focal length of lenses
- Image formation by lenses
17) Below are shown the side views (or “cross-sections”) of 4 different lenses, shown
with parallel lines approaching from a “far-away” object.
a) For each lens, use the Law of Refraction to trace the approximate path each beam
will take passing through the lens. (Ignore reflected beams here.)
b) After tracing the beams, label the approximate focal point for each lens. One of the 4
will not have a focal point on the paper at all - why not?
The third lens is just flat glass - the beams never converge and remain
parallel, so there is no focal point. (As the glass becomes less rounded, the
focal point moves further and further away, until the glass is flat and there
is no focal point at all any more.)
c) One of these lenses is called a “negative lens”. Which is it, and why do we call it
that?
The bottom lens is a negative lens. The beams never converge, but if you
extend the exiting beam backwards with an imaginary dotted (red) line, you
see that they converge at a focal point behind the lens. Note that the actual
light doesn’t converge at this focal point; the location of the focal point for
a lens with an inward curvature is behind lens, so it is called a negative
lens.
d) We show the incoming light rays from a distant object to be parallel - an object would
have to be INFINITELY far away for the rays to be exactly parallel. Obviously, this can
never happen. Is it possible that an object can be far enough away that parallel beams
are a good approximation? For a given lens, what rule of thumb can be used to say an
object is “far enough” away?
If the object’s distance is long compared to the focal length, then it is far
enough away to use the approximation of parallel lines. The lenses you
used in class had focal lengths of ~20 cm or less, so a few meters was far
enough away to approximate “infinitely far away”. If the focal lengths were
longer (say 2 meters), you would need to get much further away from an
object to find the focal length.
18) You observed and used 2 types of lenses in class, Fresnel lenses and spherical
lenses. You saw that a Fresnel lens can be used to focus a light beam from a
lighthouse, but if you look at a nice camera, you won’t find Fresnel lenses; you will find
small, shiny spherical lenses. Study these 2 types of lenses in your notes; briefly
describe some advantages and disadvantages of each.
Fresnel lens have the advantage of being cheap and compact and flat because they are made of several “fringes”, they can be flattened
considerably more. However, light rays that hit the undersides of the
“ridges” tend to get distorted, so they are not high-precision lenses.
Rounded glass spherical lenses do not have this problem, as the surface is
smooth - however, a large spherical lens will tend to be bulky, as it is
rounded out. Thus, a Fresnel lens is perfect for focusing a lighthouse beam
- it keeps the lens “compact”, and a little bit of distorted light is okay, since
it’s not being used to make an image, just to focus light.
Lab 29: Image size
- Location of projected images
- Size of images formed by lenses
19) You have studied different types of images in class - you located images of objects
in a flat mirror, and you projected images on to a screen using lenses.
a) Could you see the images a the flat mirror just by observing the mirror with your
eyes, without using a “viewing screen”? Why or why not?
Yes - the light was reflected directly into your eyes from the mirror, where
they were seen to converge at a point behind the mirror.
b) If you placed a viewing screen behind a flat mirror, would you be able to project the
mirror image on the screen so you could see it? Why or why not?
No - the mirror is opaque - no actual light reaches behind the mirror - so
you could not place a screen behind the mirror and see the image, even
though it appears behind the mirror to your eyes. This kind of image is
called a virtual image.
c) Could you see the images made by lenses just by observing with the lenses with your
eyes, without using a “viewing screen”? Why or why not?
You can see images through lenses with your eye (think about a
magnifying glass), but these are not the same as the images you project on
a screen. A magnifying glass looks at objects closer than the focal length;
these cannot be projected on a screen - they are also virtual images. To
get a real image you can actually project (like you did in class), you need to
move a screen past the focal length or further, and actually focus the light
on the screen.
d) Look at the light beam diagrams in your notes used to locate images for the flat
mirrors and the lenses. Comparing these diagrams, briefly explain how and why the
images formed by these objects differ.
Virtual images (like those made by a magnifying glass or mirror) are not
located where light rays are actually focused - you have to extended
existing lines to see where the object appears (like you did to locate
images in the mirror images lab). A real image (like the projection of the
“object” on a screen) reflects real light from the screen where it is focused.
e) You saw in a lab exercise that refraction makes an object underwater produce an
image closer to the surface than it really is. If you placed a “viewing screen” underwater,
could you project this image on to it? Explain.
You had to extend existing light lines to find this image (see also Problem
14); real light beams do not reach the image formed underwater. Therefore,
it is a virtual image; a viewing screen placed underwater would not show
the image.
20) Jack wishes to observe the sun during a solar eclipse, but knows it’s very
dangerous to look directly at the sun. So, Jack finds a large positive lens with a focal
length of 25 cm, and a viewing screen, and wishes to project an image of the sun on the
screen.
a) Draw a sketch showing the orientation and distances between the objects that will
make this work.
(“lesson” changed to “lessen” below - typo fixed)
b) Before the eclipse has started, Jack finds his setup works TOO well, and the light
from the sun focused through the lens starts to burn a hole in his viewing screen. Based
on your experience with lenses, what can Jack do to lessen the intensity of the image
on his screen? As the eclipse progresses, and the observed sunlight lessons, what
must Jack do to maintain an observable image?
Covering part of the lens (maybe even MOST of the lens) will reduce the
intensity of the image without removing any of the sun’s image. As the
eclipse progresses, the amount of light reduces - he could then slowly
uncover more of the lens as it darkens to keep the brightness constant.