1. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments 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. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Activity 1: Focal Length of a Lens Flashlight or table lamp at the end of classroom (long distance compared to focal length).  Light rays enter lens approximately parallel. Screen or sheet of paper to see image. Lens f Method 1: Move sheet until image is in focus. Then measure f.

3. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Move sheet until image is in focus. Then measure so and s i and calculate f with: Activity 1: Focal Length of a Lens Object (illuminated cross on the light source) Screen or sheet of paper to see image. sisi Method 2: soso

4. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments The Imaging Equation for Lenses s o : object distance s i : image distance f : focal length

5. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Sign Rules For Lenses Convex lenses: f is positive Concave lenses: f is negative Real objects: s o is positive Virtual objects: s o is negative Real images: s i is positive Virtual images: s i is negative Most objects are real. Virtual images cannot be picked up with a screen.

6. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments 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. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Activity 2: Magnification soso sisi h’ h   An inverted image means that h’and h have opposite sign.  M < 0

8. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments 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: eye f virtual image

9. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Verifying this magnification of a magnifying glass 1)Tape linear graph paper on viewing screen. 2)Place lens about 25cm away from screen. 3)Hold a second piece of graph paper close to lens. 4)Move your eye close to the lens. 5)Move the second piece of graph paper so it is in focus. 6)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. your eye (close to lens)  25 cm Optical Bench viewing screen linear graph paper lens f = +100mm hand held linear graph paper (close to lens)

10. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments What you should see …. lens viewing screen graph paper on viewing screen hand held graph paper 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) …then check whether this agrees with

11. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments 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 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. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Activity 3: Microscope eye sisi soso objective f o =200mm eyepiece f e =100mm need s o > f o  real image between lenses virtual image

13. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Microscope: Building Instructions Step 1: Install light source and objective lens. optical bench light source illuminated arrow on this side 200mm lens (objective) s o  30cm measure s i handheld piece of paper: move so that the image of the arrow is in focus. record s o and s i

14. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Microscope: Building Instructions Step 2: Install eyepiece lens. optical bench 200mm lens (objective) s o  30cm S i (as previously determined) 100mm lens (eyepiece) some small distance further

15. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Microscope: Building Instructions Step 3: Replace light source with white viewing screen optical bench 200mm lens (objective) s o  30cm S i 100mm lens (eyepiece) white viewing screen Viewing screen must be placed where the arrow used to be. Cover the viewing screen with linear graph paper. linear graph paper

16. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Microscope: Building Instructions Step 3: Look through eyepiece and adjust it’s position. optical bench 200mm lens (objective) s o  30cm S i 100mm lens (eyepiece) white viewing screen linear graph paper eye Adjust eyepiece position so that the image of the graph paper is in focus.

17. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Microscope: Measuring the magnification optical bench 200mm lens (objective) 100mm lens (eyepiece) linear graph paper eye 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. 25cm hand held linear graph paper

18. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Step 2: What you should see ….and measure viewing screen graph paper on viewing screen hand held graph paper (25cm from eye) image of 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. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Activity 3: Telescope eye s i  f o s o   (looking at far away objects) objective f o =350mm eyepiece f e =100mm virtual image  f e  f o + f e

20. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Telescope: Building Instructions Install objective and eyepiece optical bench 350mm lens (objective) 100mm lens (eyepiece) Separate objective and eyepiece by f o +f e (=450mm)

21. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Telescope: Measuring the Magnification 350mm lens (objective) 100mm lens (eyepiece) 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. lamp eye

22. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments What you should see … white board Compare scale seen through telescope with scale seen directly to determine M. Here: Magnification looks like M  - 2.3 (negative because inverted) telescope eyepiece

23. Physics 1809 Optics 2: Spherical Lenses and Optical Instruments Using the Desk Lamp Dimmer Lamp Plug (black) must be plugged into dimmer plug. Dimmer plug (white) must be plugged into power outlet. On/Off switch of lamp