Name:Dr. Julie J. Nazareth

Assignment Partner(s) (if any):Physics 122L/132L

Date assignment performed:Section:

Furlough Day Alternative Assignment - Thin Lenses

Be sure to read the entire Thin Lenses lab in your lab manual – all of the material is fair game for the lab final exam. I also recommend that you read the sections in your lecture textbook on light, index of refraction, and thin lenses. Students are encouraged to work together, but each student must turn in their own assignment written in their own words (make sure your answers do not sound like your partner’s or neither will receive credit). This assignment will have the equivalent points value of a regular lab report.

To complete the assignment, you must access the Geometric Optics interactive simulation at Click on the Play with sims …> button. Choose Physics from the left-hand column. Choose Light and Radiation from the left-hand column. Click on Geometric Optics. To run the simulation, click on Run Now! I encourage you to play around with the simulation, changing things up and seeing what happens to the image, image distance, magnification, etc. This is a great simulation to help you understand geometric optics and thin lenses.

Complete the Ray Optics PhET Lab below (provided by Chris Bires at Spring Valley H.S.; version 1/09). If you need more space to answer a question than is provided, please answer on an attached sheet of paper. Please show all work for calculations. Be sure to include units where appropriate, and be careful with unit conversions (m to cm and visa versa). In this lab, follow all sign conventions and record numbers with a positive (+) sign or negative (-) sign as appropriate. I will NOT assume that no sign means positive. Refer to the Thin Lenses Notes for sign conventions. In this class, we always work from left to right (light travels from a real object on the left through a lens on the right). In real life, of course light can go left to right or right to left thru a lens.

Ray Optics PhET Lab

Introduction:

When you hold your glasses far away from your face, what do you see? If you look through a magnifying glass and move it away from your face, something odd happens as you look through the moving lens. The light that reflects off images and passes through a lens before it arrives at your eye can be simulated as a series of rays. In this lab, you will investigate lens optics using the 3-ray system (parallel-focal, focal-parallel, central). Remember, the real focal point of a lens is behind the lens. The lens we will use in this simulation is a thin double-convex lens.

Important Formulas:

or or

Please note that in some texts, the object’s distance is given by “p” and the image’s by “q”

Procedure:PheT Simulations  Play With Sims Light and Radiation  Geometric Optics

  • Take some time and familiarize yourself with the simulation. You are able to move the object and the lens and change the characteristics of the lens. During this lab, be sure to always anchor your image on the principal axis. The pencil’s eraser works well for this.
  • Click to draw the rays using the 3-ray system you learned in class.
  • Move the object towards the lens. What happens to the image formed on the other side of the lens? ______
  • As you move the object inside the lens’ focal point something odd happens. Rays that don’t meet diverge. Does this mean no image will be formed? ______Where is the image? ______
  • Click on “Virtual Image.” How are a real image and a virtual image similar? ______How are they different? ______
  • Click on the ruler. You will need to make several measurements during the lab. You may, if you wish, leave your measurements in cm when using the formulas given above.
  • Set the lens’s refractive index (n) to 1.8 and the radius of curvature (R) to 0.7m. Use the appropriate equation above to solve for the focal distance (f). f = ______(Measure the focal distance to confirm your answer.)
  • Using the focal distance you just found, complete the table below and check your work in the simulation.

Table 1: Focal length, image distance, and magnification of for lens with R = 0.7 m and n = 1.8

focal distance,
f ( ) / object distance,
do (cm) / image distance,
di ( ) / magnification, m
120.0
90.0
60.0
30.0
15.0
  • Repeat the previous exercise, but with a very different lens with the following characteristics: R = 80cm, n = 1.25

WATCH YOUR SIGNS: Real images’ di should be + / - while virtual images should be + / -

(Note: in line above, circle + or – as appropriate)

Table 2: Focal length, image distance, and magnification of for lens with R = 0.8 m and n = 1.25

focal distance,
f ( ) / object distance,
do (cm) / image distance,
di ( ) / magnification, m
solve for this first / 120.0
90.0
60.0
30.0
15.0

Conclusion Questions and Calculations: In questions 1, 2, 3, 9, and 10, choose the appropriate italicized choice. Show all work in calculations. When more space is required than given, use an attached sheet of paper.

  1. Images found behind a lens are real / virtual images that will be upright / inverted.
  2. As the radius of curvature of the lens increases, the focal point of that lens becomes closer to / further away from that lens.
  3. As the refractive index of the lens increases, the focal point of that lens becomes closer to / further away from that lens.
  4. What advantage does a larger lens have over a smaller lens (all other characteristics being equal)? ______
  5. What was the focal distance (f) when the radius of curvature was .70 and index of refraction was 1.8? ______
  6. Calculate the radius of curvature of a lens with a focal distance of 40. cm and an index of 1.2. ______
  7. An object placed 35cm away from a lens projects a real image .55m behind the lens. What is this lens’ focal distance? ______
  8. What is the lens’ magnification? ______
  9. An object 20. cm to the left of a convex lens is 1.0 m in height. What is the height and location of its image if the lens has a magnification of -2.0? ______m and ______cm on the left / right side of the lens
  10. Imagine you are nearsighted (can only see close objects clearly and far objects are blurry). At your last eye doctor’s appointment, your optometrist tells you that she will need to increase your prescription because as it turns out light is focusing “too soon” or in front of your retina. You respond, “Obviously, I will need a converging / diverging lens with a higher / lower focal distance.”

Note: The above Ray Optics PhET Lab was created by Chris Bires at Spring Valley H.S. and downloaded from the PhET Interactive Simulations (University of Colorado at Boulder) website. The lab assignment has been slightly modified to add alternate text to pictures and formulas, to adjust the formatting of the data tables, remove points values, and to add a few specific directions to questions and calculations. – Dr. Julie J. Nazareth, Cal Poly Pomona

Furlough Day alternative assignment – Thin Lenses10/21/2009