PHYS 1402 – General Physics
II
Optical Instruments
Materials
Optics Bench with Light source and screen Scissors
100 mm lens Ruler
250 mm lens Masking
tape
Empty mount Post-it
note
2 x index cards Push
pin
Introduction
In
this activity we will examine the ideas behind a few basic optical
instruments. In particular, we will look
at the image formation by a pinhole camera and then a single lens camera. Following that we will construct a Keplerian
(astronomical) telescope and a microscope.
Part I - The Camera
In
simplest terms, a camera is a device that forms a real image that can be
recorded on a photosensitive film. We
have seen in class that we can form a real image with either a converging
mirror or lens. The simplest cameras
however don't even require a lens. An
image can be formed with a small hole, a pinhole. A camera which uses a pinhole to form an
image is called – amazingly enough - a pinhole camera. We will start this lab by constructing a
pinhole camera. Instead of using film,
we will simply view the image on a screen.
Stack
two index cards and fold them in half.
Tape them. Use a thumbtack to
poke a hole in the center of the card.
Make sure that the hole goes all the way through the card and is
clear. Tape the card to the empty mount
and center the hole on the light source with the double arrow. Position the card 3 or 4 inches in font of
the light source and place the screen on the opposite side of the light source
and as close as possible to the pinhole, but so that you can still clearly see
the screen. Use a piece of black cloth
to shield any extraneous light from the light source and view the screen.
Q1) Do you see an image?
Q2) Is the image real or
virtual? Explain how you know.
Q3) Is the image upright
or inverted? You may need to move the
pinhole slightly to be able to see the arrows to answer this question.
Q4) Draw a ray diagram showing the formation of
this image. Use the diagram to explain
why the image is inverted. Remember, all
of the light has to pass through the pinhole.
Q5) Move the screen backwards and forwards at
least several inches. What affect does
moving the screen have on the size of the image?
Q6) What effect does
moving the screen have on the brightness of the image?
Q7) What effect does
moving the screen have on the sharpness of the focus of the image?
Q8) Summarize the magnification and focusing
properties of the pinhole camera by circling the most correct answer below.
i) The pinhole camera
1. is always in focus
2. focuses at only one
point for a given object distance
3. is never in focus.
ii) The magnification of the pinhole camera
1. is fixed by the
location of the object
2. depends on the
object distance
3. depends on the image
distance
4. depends on both the
object and the image distance
Q9) Based on your experience with the pinhole
camera, what seems to be its biggest drawback?
Now
let us look at a camera that uses a lens.
We will refer to such a camera as a single lens camera. Replace the pinhole with a mount containing a
+100 mm lens.
Q10) What is the minimum
distance that the object can be placed at and a real image be formed?
Q10) Place the object at a distance greater than
the minimum distance and find the image.
Describe the image.
Q11) Move the screen forwards and backwards at
least several inches. Does the image
stay sharp?
Q12) Move the object several inches away from the
lens. Does the image stay sharp? Can you move the lens and make the image
sharp on the screen again? What is this
called?
Q13) What is the process
of moving the lens to obtain a sharp image called in photography?
Q14) Explain why in an actual
camera, it is preferable to move the lens to moving the screen.
Q15) Summarize the magnification and focusing
properties of the single lens camera by circling the most correct answer below.
i) The single lens camera
1. is always in focus
2. focuses at only one
point for a given object distance
3. is never in focus.
The
f number of a lens is defined as f# = focal length/lens diameter
Q16) What is the f number
of this lens?
Use
a post-it note and cut out a circular mask to reduce the area of the lens.
Q17) What affect does
placing the mask have on the image?
Q18) Move the object backwards and forwards at
least several tens of
inches and observe the effect on the image. Remove the mask and compare to the effect
without having the mask. Does the mask
seem to have any effect on the focusing properties of the camera?
Q19) How does placing the mask on the lens affect
the f number?
Q20) Does changing the f-number seem to change the
range of object distance over which the focus will remain sharp?
The
range in object distance over which the focus a camera remains sharp is called the depth of field.
Q21) Does going to a larger f number seem to
increase or decrease the depth of field of the camera?
Q22) What value might
this have in photography?
Part II - The Keplerian
Telescope
Take
the 250 mm and the 100 mm lens and construct a simple refracting (Keplerian)
telescope.
Q23) Which lens will you
place forward? Why?
Q24) Calculate the distance between the two
lenses?
25) Calculate the magnification of the telescope?
Observe
a distant object in the classroom, or outside.
Q26) What object are you
observing?
Q27) Describe the difference in appearance of the
object when you view it with the unaided eye, and when you view it through the
telescope. (Include a discussion of whether the image is upright or inverted
and discuss the magnification.)
Q28) Look through the telescope backwards and
describe the image.
Q29) Draw a ray diagram showing the principles of
how the telescope works.
Part III - The Compound Microscope
Take
two 100 mm lenses and construct a simple microscope. The working
distance is the distance from the object to the objective lens. Take as the working distance 15 cm.
Q30) The lens you place
forward is called the objective lens.
The object is placed a distance equal to the working distance of the
microscope from the objective lens.
Calculate the position of the image formed by the objective lens.
Q31) What is the
magnification of the image formed by the objective lens? (Don't include the eyepiece.)
Q32) Where should the
eyepiece be located with respect to the image of the objective lens. (Or you might prefer to answer where should
the eyepiece be placed with respect to the image formed by the objective lens?)
Q33) Calculate the distance between the two
lenses?
Q34) Assuming the eyepiece is used as a simple
magnifier with a relaxed eye, calculate the magnification of the eyepiece.
Q35) Calculate the magnification of the
microscope. Use the idea that each lens
has a magnification and combine the magnifications appropriately. Don't use the formula given in the text since
the approximations it involves don't hold particularly well for our microscope.
Position
the provided slide 15 cm in front of the objective lens. Illuminate the object with the point light
source.
Q36) What object are you
observing?
Q37) Describe the difference in appearance of the
object when you view it with the unaided eye, and when you view it through the
microscope you have made.
Q38) What evidence do you
have that there are aberrations in your microscope? What aberrations do you think you see?