Purpose of this Minilab

1. Purpose of this Minilab • Use lens formula to determine focal length of a lens. • Learn about image magnification in magnifying glasses, microscopes, and telescopes.

2. Activity 1: Focal Length of a Lens Method 1: f • Flashlight or table lamp at the • end of classroom (long distance • compared to focal length). • Light rays enter lens approximately parallel. Lens Screen or sheet of paper to see image. Move sheet until image is in focus. Then measure f.

3. Activity 1: Focal Length of a Lens Method 2: si so Object (illuminated cross on the light source) Screen or sheet of paper to see image. Move sheet until image is in focus. Then measure so and si and calculate f with:

4. The Imaging Equation for Lenses so: object distance si: image distance f : focal length

5. Sign Rules For Lenses Convex lenses: f is positive Concave lenses: f is negative Most objects are real. Real objects: sois positive Virtual objects: so is negative Real images: si is positive Virtual images: si is negative Virtual images cannot be picked up with a screen.

6. Virtual or Real Image? • In Activity 1.2 (using a converging lens) place the object at a distance larger than f away from the lens to get a real image. Hint: To answer Q1, do a similar analysis for the concave lens (f < 0).

7. Activity 2: Magnification An inverted image means that h’and h have opposite sign.  M < 0 h’ a so si a h

8. 2.2 Virtual image magnification (magnifying glass) Without the magnifying glass: eye 25 cm (typical nearest distance a human can focus on) With the magnifying glass: virtual image eye f

9. Verifying this magnification of a magnifying glass viewing screen your eye (close to lens) linear graph paper Optical Bench  25 cm lens f = +100mm • Tape linear graph paper on viewing screen. • Place lens about 25cm away from screen. • Hold a second piece of graph paper close to lens. • Move your eye close to the lens. • Move the second piece of graph paper so it is in focus. • Compare the size of graph paper seen through the lens • with the size of the graph paper on the screen (seen not • through the lens). See next page for illustration. hand held linear graph paper (close to lens)

10. What you should see …. viewing screen lens hand held graph paper seen through lens • Compare: • 3.5 divisions • on the graph paper • taped to the screen • = 1 division on the • hand held graph • paper seen • through the lens. • M=3.5 (in this example) hand held graph paper graph paper on viewing screen …then check whether this agrees with

11. Remarks to formula for magnifying glass…. • The actual magnification depends on exactly where the object is placed: • If the object you magnify is placed exactly at the focal point of the • magnifying glass, then you view with a relaxed eye and • If you move the object even closer to the lens, the magnification can get • as high as • You could get a theoretical value anywhere between those two magnifications, depending on where exactly you hold the paper.

12. eye so si objective fo=200mm eyepiece fe=100mm Activity 3: Microscope virtual image • need so > fo • real image between lenses

13. Microscope: Building Instructions Step 1: Install light source and objective lens. handheld piece of paper: move so that the image of the arrow is in focus. illuminated arrow on this side 200mm lens (objective) light source optical bench so  30cm measure si record so and si

14. 200mm lens (objective) Microscope: Building Instructions Step 2: Install eyepiece lens. 100mm lens (eyepiece) optical bench so  30cm Si (as previously determined) some small distance further

15. 200mm lens (objective) Microscope: Building Instructions Step 3: Replace light source with white viewing screen linear graph paper white viewing screen 100mm lens (eyepiece) optical bench so  30cm Si Viewing screen must be placed where the arrow used to be. Cover the viewing screen with linear graph paper.

16. 200mm lens (objective) Microscope: Building Instructions Step 3: Look through eyepiece and adjust it’s position. linear graph paper white viewing screen 100mm lens (eyepiece) eye optical bench so  30cm Si Adjust eyepiece position so that the image of the graph paper is in focus.

17. Microscope: Measuring the magnification linear graph paper hand held linear graph paper 100mm lens (eyepiece) 200mm lens (objective) eye optical bench 25cm Step 1: Hold a second piece of graph paper approximately 25cm from your eye. That extra graph paper should be a bit to the side so you can still see the image of the graph paper that is on the viewing screen.

18. Step 2: What you should see ….and measure viewing screen hand held graph paper (25cm from eye) image of graph paper on viewing screen graph paper on viewing screen • Compare: • 2.8 divisions • on the hand held graph paper • = 1 division on the image of • Graph paper taped to the screen. • M=2.8 (in this example) …then check whether this agrees with

19. Activity 3: Telescope  fo + fe virtual image eye si  fo  fe so   (looking at far away objects) eyepiece fe=100mm objective fo=350mm

20. Telescope: Building Instructions Install objective and eyepiece 100mm lens (eyepiece) 350mm lens (objective) optical bench Separate objective and eyepiece by fo+fe (=450mm)

21. Telescope: Measuring the Magnification 100mm lens (eyepiece) 350mm lens (objective) lamp eye optical bench View white board through telescope from the back of the room. White board in the front of the room. Draw a thick scale on the white board. Illuminate the scale with a lamp.

22. What you should see … white board telescope eyepiece Compare scale seen through telescope with scale seen directly to determine M. Here: Magnification looks like M  - 2.3 (negative because inverted)

23. Using the Desk Lamp Lamp Plug (black) must be plugged into dimmer plug. Dimmer plug (white) must be plugged into power outlet. Dimmer On/Off switch of lamp