Today on How it's Made: Lenses - Trey, Philip, Brooke

Working with Lenses - Lab Report

AST 115 H

Due April 23, 2016

Trey, Philip, Brooke

Purpose:

The purpose of this lab is to get a better understanding of lenses, and to use that understanding to create a basic telescope with given supplies.

Materials Needed:

Lense kit #4:

• Two convex ‘()’ lenses of differing focal lengths
• One concave ‘ ) ‘ lens
• One concave ‘ )‘ mirror
• One plane ‘ || ’ mirror
• One plane ‘ || ’  lense

Meter Stick

Special Science Clamps

Assorted Tubes

Paper
Light Source

Introduction:

Astronomers have used telescopes to study the Universe for over 400 years. By now, the science behind it is basically  commonplace. There are two main types of telescopes: refraction and reflection; the main difference between the two would be that refractors use a lens as the main optical component, while a reflector uses a mirror.

The type of telescope we are going to experiment with building today, is a refractor.

Refracting telescopes rely on a convex shaped lense at the end that is struck by the light first. The lense that sits near the eye however can alternate between either a convex or a concave lense.

Important Things to Note:

The materials used in this lab are made of glass, and they can/will break if you drop them.

HANDLE WITH CARE.

Procedure:

1. Setting up the equipment, getting the optics and light source. A meter stick, and a lens kit will be distributed, and a light source will be set up across the room.
2. Finding Focal lengths: A PARTNER OR TWO WILL BE REQUIRED
1. The lens kit number was recorded and the concave lenses were separated from the convex lenses. The types of lenses and mirrors in the lens kit were recorded.
2. The focal length of a convex lens can be best measured by forming the image of an infinitely distant object, and measuring the distance between the lens and the object. But this is classroom science, practicality exists, and the decision was made to settle for an object on the other side of the room.
3. The light source was used as the distant object. A sharp image of the object (light) was formed on a note card. The lens -> image distance was recorded in centimeters. That number is the Focal Length.  The size of the image on the notecard was measured in millimeters and recorded as well as the relative brightness in terms of: bright, brighter, brightest.
4. This process was repeated for the remainder of the convex lenses in the set.
1. It was discovered that the focal length of the concave lens could not be determined this way.
5. Another process was used to determine the same factors for the mirrors present in the kit.
6. The lens was half-covered with an index card, and results were recorded.
3. Astronomical Refractor: **see important things to note**
1. In a refractor, an image is formed by the objective lens at a distance of one focal length behind it. This distance was called fO. This image is viewed through the eyepiece lens by placing it at a distance equal to the focal length of the eyepiece, fE, beyond the image formed by the objective. The total separation of the two lenses is therefore fO + fE. A tube is used as the mode of telescope. This is done to keep out excess light.
1. The convex lens with the longest focal length was mounted at the end of the tube using special tube rubber.
2. The other convex lens became the eyepiece.
3. Objects across the hall were viewed.
4. Our experience was recorded.
2. The eye piece was changed, and the entire process repeated for the other convex lense.
3. The magnification of a telescope is given by the formula M = fO/fE.
1. The magnification of the telescope built during parts a and b were recorded.
4. Galilean Refractor:
1. The above process was repeated but with the concave lense as the eyepiece. Use the same lense as before for the objective lens.  
2. Objects across the room were observed.  Record the orientation and estimate the magnification of the telescope. Record any similarities and differences compared to the astronomical refractor.

Results and Discussion

B)

a) Our group had lens kit number 4. In our kit, we had two convex lenses, a concave lens, a plane lens, a concave mirror, and a plane mirror. The convex and concave lenses were of similar size and shape. The plane lens was larger, but nearly the same thickness. The concave and plane mirror were much larger in both diameter and thickness than all the lenses respectively.

c-e) results are listed below

Type of Lens/Mirror	Focal Length (cm)	Image Size (mm)	Image Orientation	Image Apparent Brightness (1-5)
Convex (a) Lens	3	1	Inverted	5
Convex (b) Lens	6	1	Inverted	3
Concave Lens	1	4	Upright	2
Plane Lens	44	5	Upright	3
Concave Mirror	11	1	Upright	5
Plane Mirror	52	5	Upright	4

For our “Image Apparent Brightness” column, the 1-5 scale denotes the brightness based on 1 being very faint, and 5 being extremely bright.

f) Using Convex (b) Lens, when the lens was half covered, the entire image was still visible. The brightness diminished very slightly, but made the image clearer and sharper.

C) Assembling the Telescope

1. In this part of the lab, we were put to the task of putting together our telescope with one of the two convex lenses.  Our group chose the Convex B lens as our first eyepiece.  The orientation of images with Convex B through the telescope was inverted.  The magnification of objects with Convex B was bigger than the image with the concave lens acting as our eyepiece.  What I mean by this is that when we viewed object with Convex B lens as the eyepiece, our viewing circle had a bigger viewing area than the other.  With this telescope, the objects looked ten to fifteen times bigger through the telescope than in real life.  
2. We were asked to change the eyepiece lens with the other convex lens, which was Convex A.  The similarities between using Convex A and B were that both views we saw with these lenses were inverted and both convex lenses had a bigger viewing image than the concave lens that we tested later.  The difference between the two was that Convex A had a smaller image than Convex B.  Objects in this telescope seemed at least twenty times bigger than they actually were in real life.  
3. In this part of the lab, we were asked to use the formula M=fO/fE to discover part a and b’s magnification.  
1. In part a of section C, we use Convex B for the eyepiece, so this is the magnification formula for that telescope.  Here, fO is the objective plane lens, and its focal length was 44cm.  fE is the focal length of the eyepiece, and the eyepiece was Convex B, which had a focal length of 6cm

M = fO/fE

M = 44cm / 6cm

Therefore, Magnification for our telescope with Convex B as the eyepiece is 7.3cm

2. In part a of section C, we used Convex A as the eyepiece.  This is how we found out how much this telescope magnified images.  fO is the same as above, it is 44cm for our plane lens, and fE is now 3cm because that is the focal length of the eyepiece, Convex A.  

M = fO/fE

M = 44cm / 3cm

Therefore, Magnification for our telescope with Convex A as the eyepiece is 14.6cm.  

This means that the telescope with Convex A as the eyepiece had a higher amount of magnification than the telescope with Convex B as the eyepiece.  This makes sense because objects in the Convex A telescope did look closer to us than the objects in the Convex B telescope.  When we were viewing a piece of paper down the hall with the Convex B telescope, we could barely see the details on the paper.  With Convex A we could see many more details on the object we were viewing, which was a clock down the hall.  So our predictions on how well the magnification was for these two telescopes were spot on.  

In the beginning of the lab, it said to describe the image orientation with the words right-side-up or inverted, AND normal or reversed.  What does this mean?  

D)  Galilean refractor:

1. Use the concave lens as the eyepiece in your telescope with the same lense as before for the objective lens.  You will find a focus at a much closer separation of the lenses.  
2. Observe the objects around the room.  Record the orientation and estimate the magnification of this telescope.  Record any similarities and differences compared to the astronomical refractor (which was section C above).    
1. The image orientation was right-side-up and normal.  There were many differences between this telescope and the astronomical refractor that we used before.  The concave lens as the eyepiece made the viewing circle shrink in size.  Therefore, out of Convex A and Convex B, the concave lens was the smallest of them all.  We took this to mean that magnification was better for this lens than the others, but our calculations truly surprised us as to how high the magnification was.  Another difference between this Galilean refractor and the astronomical refractors was that the astronomical refractors had inverted image orientations while the Galilean had a right-side-up and normal orientation.

We calculated the amount of magnification of this telescope.  Here the fO = 44cm because we used the plane lens as the objective one, and the concave lens’s focal length meant that the fE = 1cm.  

M = fO / fE

M = 44cm/1cm

Magnification for the concave lens is 44cm.  This means that out of all the lenses we used for eyepieces, the concave lens had the highest degree of magnification.

Conclusion:

The purpose of this lab was to acclimate students to the way lens and mirrors operated independently and together. This was achieved through different experiments that obtained information about a single lens or mirror, whether it be the focal length, image size, image orientation, or the apparent brightness of that image, and also through the experience of building a telescope using multiple lenses to bend light into a focus for the eye to make out clearly. While on the telescope portion of this lab, an appreciation can be gathered for those who only had telescopes like the ones we made to use for astronomical observances. The math behind the reason why lenses and mirrors function the way they do was also a goal of this lab, and was achieved through calculation of the magnification the two lenses in our telescope provided.