Optics 2 Spherical Lenses and Optical Instruments Purpose

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.

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

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

Optics 2: Spherical Lenses and Optical Instruments The Imaging Equation for Lenses so: object distance si: image distance f : focal length

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

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 Q 1, do a similar analysis for the concave lens (f < 0).

Optics 2: Spherical Lenses and Optical Instruments Activity 2: Magnification An inverted image means that h’and h have opposite sign. M < 0 h’ h so a a si

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

Optics 2: Spherical Lenses and Optical Instruments Verifying this magnification of a magnifying glass viewing screen your eye (close to lens) Optical Bench linear graph paper 25 cm lens f = +100 mm hand held linear graph paper (close to lens) 1) 2) 3) 4) 5) 6) Tape linear graph paper on viewing screen. Place lens about 25 cm 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.

Optics 2: Spherical Lenses and Optical Instruments What you should see …. viewing screen 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 hand held graph paper seen through lens hand held graph paper on viewing screen

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 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.

Optics 2: Spherical Lenses and Optical Instruments Activity 3: Microscope virtual image eye si eyepiece fe=100 mm so objective fo=200 mm need so > fo real image between lenses

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

Optics 2: Spherical Lenses and Optical Instruments Microscope: Building Instructions Step 2: Install eyepiece lens. 100 mm lens (eyepiece) 200 mm lens (objective) optical bench so 30 cm Si (as previously determined) some small distance further

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

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

Optics 2: Spherical Lenses and Optical Instruments Microscope: Measuring the magnification linear graph paper 200 mm lens (objective) hand held linear graph paper 100 mm lens (eyepiece) eye optical bench 25 cm Step 1: Hold a second piece of graph paper approximately 25 cm 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.

Optics 2: Spherical Lenses and Optical Instruments Step 2: What you should see …. and measure viewing screen hand held graph paper (25 cm 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) graph paper on viewing screen …then check whether this agrees with

Optics 2: Spherical Lenses and Optical Instruments Activity 3: Telescope fo + f e virtual image eye si fo eyepiece fe=100 mm fe so (looking at far away objects) objective fo=350 mm

Optics 2: Spherical Lenses and Optical Instruments Telescope: Building Instructions Install objective and eyepiece 350 mm lens (objective) 100 mm lens (eyepiece) optical bench Separate objective and eyepiece by fo+fe (=450 mm)

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

Optics 2: Spherical Lenses and Optical Instruments 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)

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