Lab 2 Part B Hint:
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To answer the first section of Part B of Lab 2, there are several things we need to take
into consideration. One major factor we need to consider is the field of view of an
eyeball.
As long as any object (or image) is in our field of view of our eyeball, we can “see” it. If
the object (or image) is outside our field of view of our eyeball, we cannot “see” it (or it
will appear blurry).
Take the following examples:
Bair 1
The bungee cord jumper hanging from the bungee cord after jumping off the cliff is in
our field of view of our eye. We would be able to clearly see this object since it is
included in our field of view in its entirety.
Bair 2
In this case, the bungee cord jumper hanging from the bungee cord after jumping off the
cliff is NOT in our field of view of our eye, so we would not be able to “see” him in his
perilous pursuits – but rather, the aerobics trainer is.  - Typical guy example…
Don’t believe me … try it. Look at a picture on the wall in your room or something. Now
turn around so the picture is no longer in your field of view … can you see it? Not unless
you have eyes on the back of your head - freak! Now get a mirror and hold the mirror in
your field of view – no the image of the picture behind you is IN your field of view, and
guess what, YOU CAN SEE IT!
The same thing applies for images produced by thin lenses. If the lines of light emitted
from an object all coincide at a specific point in space, they form a sharp image (this
image is called a “real” image). If that image is in our field of view of our eye, we can
“see” the image.
Bair 3
CASE 1
Notice that the convex lens above (according to the ray-diagram) creates a “real” image if
the object being viewed through the lens is outside the focal point of the convex lens. The
object (located on the far left – and designated by the arrow of height h-not) can be any
“tangible” object – like a bungee cord jumper, an aerobics instructor, a periodic chart on
the wall, or a letter on a piece of paper. As long as our eyeball is far enough away from
the focal point (on the right side of the lens) then a sharp, real image will be formed
(located on the right side and designated by the arrow of height h-sub-i). This is the case
where we held the convex lens at arms length and looked at an object far away – like
something on the wall. Notice that the image is IN the field of view of the eyeball, so we
can literally “see” the object (bungee cord jumper, an aerobics instructor, a periodic chart
on the wall) inverted and shrunk down (negative magnification).
Original object
Image
Bair 4
Now consider the case where we held the convex lens close to our eye and looked at an
object through the lens.
Bair 5
CASE 2
Notice that the convex lens above (according to the ray-diagram) still creates a “real”
image if the object being viewed through the lens is outside the focal point of the convex
lens. This is the case where we held the convex lens close to our eyeball. Notice that the
image is no longer in the field of view of the eyeball, so we cannot “see” the object. Also
notice what IS in the field of view. There are light rays that we can “see” but they don’t
all arrive at the same point in space. The light ray designated as blue is higher then the
light ray designated as green – but they all were emitted from the same point (the top of
the arrow on the left). What we will “see” in this case is a very blurry image. In fact, you
probably couldn’t even tell what the original object was. This is the same problem people
with myopia experience ALL the time – when they are not wearing corrective lenses
(wait a minute – so glasses are just thin lenses that have “very specific” focal points to
move the location of where the image if formed into the field of view of the eyeball!)
Bair 6
Original object
Image
Bair 7
CASE 3
Look at the picture above. This is the case where we are looking though a convex lens
held at approximately arms distance at an object that is INSIDE the focal length of the
convex lens. Will we be able to “see” the image? YES. It is in the field of view of the
eyeball – and will appear larger then the original object. This is called a magnifying
glass. You can use it to look at small things like a bug – and make it look larger. Notice
that even though the light rays emitted from the object do not intersect to make a “real”
object, we can STILL see the image (do not let the word “virtual image” make you think
it is not there – it IS there – just the light rays emitted from the object are not intersecting
to make the image).
Original object
Image
Bair 8
Bair 9
CASE 4
Notice that if we hold the convex lens close to our eye and have the object inside the
focal length, the object is STILL in the field of view of the eyeball and hence we can still
“see” the object. If you get a magnifying glass and hold it near your eye or far from your
eye, you will always be able to see the virtual image created by the lens. This is why they
can freely sell magnifying glasses – it’s hard to get a blurry image as long as you keep
your objects within the focal length of the lens. How many people do you know that use a
magnifying lens to do bird watching? NONE! Why? See the examples above where the
object is located outside the focal point … that’s kinda boring!
Original object
Image
Bair 10
Bair 11
CASE 5
Think you got it now? This a concave lens. Is the image real or virtual? It’s virtual. Can
we “see” the image? Yes, it’s in the field of view of our eye, and it looks like it will
appear smaller then the actual object. Is the image erect or inverted? Erect. Got it? This is
the case where we were using the concave lens at arms length to view an object like the
periodic table on the wall.
Bair 12
CASE 6
Here’s the concave lens held close to our eye looking at a distant object. Notice that all
the same stuff we just discussed still applies. Can we “see” the image? You betcha! It’s
virtual, small and erect.
Original object
Image
Bair 13
How about looking at something inside the focal point of a concave lens with the lens far
from our eye? Guess what … it’s the same.
Original object
Image
Bair 14
How about looking at something inside the focal point of a concave lens with the lens
close to our eye? Guess what … it’s STILL the same.
Original object
Image
Got it now? See how this stuff relates to what you saw from part A of this experiment?
Bair 15
If you still have questions on this stuff, PLEASE don’t hesitate to ask more questions. I’ll
be glad to help where I can. 
Bair 16