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?