Science Lab
Telescopes
Associated Lessons:
Choosing Wisely I Don't See the Light and It's Blurry
Objective:
After this lab, students will be able to: 1) describe the effect of changing the eyepiece on a telescope's magnification and field of view (Module Objective 3) 2) describe methods of circumventing the blurring effects of the atmosphere on a ground-based telescope's resolution (Module Objective 4)
In the lessons, you learned about refracting and refracting telescopes and the parts that are required to make construct them. In this lab, you will simulate light rays as they interact with various lenses and mirrors and how changes to a lens or mirror will affect the focal length.
Part A: Lenses and Mirrors
As you aren't expected to have your own lenses and mirrors to use, we'll have to suffice with observing some simulations. Start by going to the following website: http://www.jnoodle.com/ps_2/lab16.html.
On the page, you'll find a Java applet designed to let you simulate the effects from various lenses and mirrors. If you don't already have Java installed, you can download a free copy here.
Note: If you cannot get the applet to work, you can watch a video that contains the same information on Youtube: Telescopes Applet Video.
Once the applet is running, start by clicking on "Beam" and clicking near the left edge of the applet window to place a light source there. You should end up with a picture like the following:
Click on the "Lens" button and click on the center of the applet window to place a lens. If you want to adjust the placement of the lens, you can click on it and move it to another location. If you want to remove the lens, you can click on the "Clear Active" button.
Once you have the lens where you want it, you should see parallel light rays from the beam striking the lens. The lens bends the path of the light rays. For parallel rays, the point where the rays cross is called the "focus" and the distance from the lens to the focus is the focal length. With the lens, you'll see two entries, one that says "x = ..." and fl = ...", with the dots replaced by numbers for each entry. The top number is the location of the lens in the applet and the second number is the focal length of the lens. It is the second number that you will record.
1) Describe the path of the light rays after leaving the lens. What is the focal length of the lens?
If you click on the focus, you can move it. Start by moving the focus farther from the lens.
2) What is the new focal length for the lens? Has the lens become thinner or thicker?
Take the focus and move it between the beam and the lens.
3) Describe the path of the light rays after leaving the lens. What is the new focal length?
4) Describe the new appearance of the lens. How does its shape differ from the original lens you used?
Now, click on the "Clear Active" button to get rid of the lens and click on "Mirror". Place a mirror near the center of the picture.
5) Describe the path of the light rays after striking the mirror. What is the focal length of the mirror?
Move the focus so that it is closer to the mirror.
6) What is the new focal length? Has the mirror become flatter or more curved?
Part B: Building a Refracting Telescope
For this part of the lab, you will observe images of distant objects produced by several lenses and you will use those images to determine the focal lengths of the lenses. Then, you will consider combining the lenses to create a make-shift refracting telescope.
In this part of the experiment, light from the window passes through the lens and is projected on to a screen. When the image of objects outside the window is clear, the distance between the lens and the screen is equal to the focal length. Watch the video in the link below and listen to the distance values between the lens and the screen as they are called out. Record the distance value at which the image is the clearest - the focal length of the lens.
https://www.youtube.com/watch?v=34bRaOVeyek&feature=youtu.be
7) What is the focal length of the first lens? Is the image upright or inverted?
Watch the second video to determine the focal length of the second lens.
https://www.youtube.com/watch?v=dHTnvUfAQM4&feature=youtu.be
8) What is the focal length of the second lens? Is the image upright or inverted?
Part C: Observing With a Reflecting Telescope
In the class lessons, you saw an animation that showed how a reflecting telescope works. The reflector we'll be using for this lab is a Cassegrain-type: the light bounces off of the objective, travels to the secondary mirror that is located near the telescope opening, then returns through a hole in the primary mirror to the eyepiece. Therefore, the focal length of the objective is roughly twice the length of the telescope tube. In the picture, you will see the telescope with a meter stick. The numbered markings are centimeters. Read off the length of the tube (to the nearest tenth of a centimeter) and then calculate the focal length of the mirror by doubling the length of the tube.
9) Length of tube and focal length of the objective mirror in centimeters:
The telescope was then pointed at a building across the street from the lab room. Using a 25 mm eyepiece, the following image was obtained: Image with 25 mm eyepiece
10) What words are found on this building across the street? How is the writing oriented (upright/inverted; left to right or backward)?
Next different eyepieces are used with the telescope - a 10 mm eyepiece and a 6 mm eyepiece. Links to images taken through those eyepieces are found in the table below. Compare the pictures through the three eyepieces. Rank them "1" to "3" in the areas of magnification and field of view (how large is the area that you can see at one time), giving a "1" to the largest in each category.
11) Rank the eyepieces "1" to "3" in each category to fill in the following chart:
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Eyepiece |
Magnification |
Field of View |
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Analysis 1 (2 Points): In Part A, you looked at the effect that the thickness of a lens has on its focal length. What is the relationship between the thickness of the lens and the focal length of the lens? For a research telescope (with a long focal length), would the thickness of the lens be greater or lesser than for a backyard telescope (with a short focal length)? Explain your answer.
Analysis 2 (2 Points): In Part A, you also observed the effect of the curvature of a mirror on its focal length. Based on your observations, what is the relationship between the curvature of a mirror and its focal length? For a research telescope (with a long focal length), would the mirror be more flat or more strongly curved than for a backyard telescope (with a short focal length)? Explain your answer.
Analysis 3 (3 Points): It is a bit hard to get an accurate set of pictures that demonstrate the brightness of the image seen through a telescope. The reality is that it depends on both the light gathering power and what we call the image brightness for the eyepiece. (Hint: See the lesson "Choosing Wisely".)
A) What part of the telescope determines the light gathering power?
B) What is the relationship between the image brightness and the focal length of the eyepiece?
C) Based on what you wrote in the previous part of this question, of the three eyepieces you used in question 11, which of them should produce the brightest image? Revisit the pictures taken through those eyepieces. Did the pictures capture what you expected to see?
Analysis 4 (3 Points): Every measurement will have at least some amount of error to it, the amount being heavily dependent on the equipment and the technique used to make the measurement. In Part B, distance values were called out at 0.5 cm increments. So, it is reasonable to expect that your focal length values are subject to an error of 0.5 cm. Calculate the percent error for each lens using the following equation:
Percent Error = [(0.5 cm) / focal length] x 100
A) What are the percent errors for lens 1 and lens 2? B) Why are your percent errors different even though the error of 0.5 cm applied to both lenses?
Analysis 5 (2 Points): In Parts B and C, you made observations with combinations of lenses and a mirror for which you measured or were given the focal lengths. Use those values to calculate the magnifications for the following scenarios listed below. For the lenses in Part B, the focal length values are both in centimeters, so you do not need to make any unit conversions when calculating the magnification. However, for the reflecting telescope from Part C, the focal length of the objective is in centimeters and the focal lengths for the eyepieces are in millimeters. As in the "Choosing Wisely" lesson, you will need to convert between centimeter and millimeters before you can calculate the magnifications. A) lens combination from Part B (longer focal length divided by shorter focal length): B) reflector (objective focal length from question 9) with 25 mm eyepiece: C) reflector (same) with 10 mm eyepiece: D) reflector (same) with 6 mm eyepiece:
Analysis 6 (2 Points): In this lab, you weren't able to study the effect of the telescope design on the resolution of the images. However, getting high resolution images is critically important for our understanding of objects in space. Sadly, most telescopes are limited in resolution by the blurring of the atmosphere. For a ground-based telescope, describe two methods that Astronomers can employ to enhance the resolution.