Introduction
In this lab activity we will be conducting a series of explorations to better understand how lenses and mirrors work. In particular, we will be building simple microscopes, telescopes, projectors and cameras.
Students should read the related articles on the CD-ROM "The way things work" or on the web light and optics submodule lenses or light and optics submodule telescopes and microscopes
related class notes - light and its properties
General Instructions
As you proceed through the labs read
these notes before beginning each activity.
Use the questions to write a discussion for each section. Use a separate piece
of paper.
If you have time and interest go ahead and do the challenge questions.
Write answers to the conclusion questions after you have finished all the labs.
Activity 1. Magnifiers
This activity is an important prerequisite for the rest of the unit. Take your time and think carefully about the magnifiers that you are examining. The main objective of this section is for students to understand how the curvature of the lens determines both its magnifying power and its field of view.
Discussion Questions:
1.You have been given two lenses, A and B. How are they alike and different?
2. Which of the two lenses A or B magnifies more? Why do you think this is so?
3. Look at the jug filled with water on display on the front table. How does it make objects that are placed behind it look? Why do you think it acts like a lens? How does it act differently than a lens, why is this so?
4. Relate the curvature of the lens to its magnifying power. Are they directly or inversely proportional? How does curvature effect magnifying power?
5. The area seen through a lens is called the field of view. Which lens has the larger field of view? Is it the same as the lens that had the greatest magnifying power? Why or why not?
6. How does something Magnify an object? Describe this in your own words.
Going Further.....
Take time to explore the flip point of the lens. (The point at which the object observed through a lens flip upside down. ) Is the flip point for A or B closer to your eyes? The flip point is actually related to the focal point in the next activity, cameras.
Activity 2. Cameras
In this activity students project images of a light bulb onto a piece of paper, just like a camera projects images onto film. They also investigate the concept of image distance.
some things to think about... What are the differences between cameras?
Compare a telephoto lens with a wide angle lens? Which one magnifies more?
Which lens has a greater field of view? Which one has a greater curvature?
The challenge....
1. Measure the image distance of each of the lenses, that's the distance that it takes to make an image appear on a piece of paper. Follow the steps outlined on the data sheet, cameras. Record your measurements for both lenses, A and B.
Discussion Questions.
2. Which of the lenses is the short focus lens? Which the long focus? What does this mean?
3. Which of the lenses projected the largest image? Was it the long or short focus lens?
4. Explain in your own words why you think the image is upside down on the paper.
Activity 3. Telescopes.
By experimenting with a long and short focus lens, students will discover how to build the most powerful telescope. Students should also discover the relationship between magnification and field of view in this lab.
Procedure.
Project the image of the light bulb onto the paper. Observe the image by looking through a lens. try different combinations of the lenses to discover which on gives you the greatest magnification and field of view. Record this on your data sheet, Telescopes.
the lamp will need to be a the same height as the lenses. You may need to use books to raise the lenses or lower the lamp onto a stool.
When you have completed this part of the lab try to make a telescope using the two paper tubes and lenses that have been given to you.
Discussion questions.
1. Draw the set up that you used. Label the objective lens and the eyepiece.
2. Why was the image inverted by the objective lens? Why was it not reinverted by the eyepiece?
3. Why do you think commercial telescopes have tubes?
Activity 4. Projectors.
Using a slide, a flashlight and a lens in this activity students are challenged to make a projector. Through the investigation they should also discover which type of lens projects a larger image.
Procedure notes.
Use the projector data sheets.
After taping the slide to the end of the flashlight ( put the slide upside down on the flashlight), use the third stryo foam cup in your bin to hold the flashlight at the right height. Go ahead and tape it on the cup with masking tape.
Review what you have learned in the last labs. What is a more powerful magnifier? Lens A or Lens B?
What is an eyepiece, objective ? If you want a projector that magnifies as much as possible, which lens is the best one to use as the objective lens? Which one is the best one to use for the eyepiece ?
Discussion Questions
1. Which of the lenses projected a larger image?
2. How is the projector like a backwards camera?
3. How does the lens enlarge the image?
4. Why do you have to put a slide into a projector upside down?
Challenge Questions...
Make a projector involving two lenses. How is it different than the projector that you already built?
Explain how these optical devices are like the human eye.
Optional, dissect a cows eye. Sketch the pieces.
Conclusion Questions.
Can the same lenses be used to make magnifiers, cameras, projectors and telescopes?
What are the important differences between lenses?
How does a telescope combine the different functions of the lenses?
How are projectors alike and different from cameras?
If you have time and equipment investigate lenses in real devices like slide and overhead projectors. How are they alike and different from the lenses we have used in the lab?
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