Reflection and refraction Last Time… Total internal reflection Reflection and refraction

Light rays and images Each point on object reflects light Light propagates out, represented by rays perpendicular to wavefront. Lens in our eye does some ‘imaging’ so that we identify origin of light rays. Mon. Feb. 4, 2008 Physics 208, Lecture 4

Question Does the fish appear Closer than actual Farther than actual Same as actual n=1.00 n=1.33 Virtual image This is a kind of a lens. It creates an image by bending the rays. In this case the image is a virtual one. The rays diverge from a point, just like the original ones, but it is a different point. Mon. Feb. 4, 2008 Physics 208, Lecture 4

How do you ‘see’ this? Object Lens Image Lens bends (refracts) light rays— forms image on retina Sensors on retina report to brain. Color information, intensity information Here explain how eye lens makes a real image. Mon. Feb. 4, 2008 Physics 208, Lecture 4

Making a real image Lens: Refracts light so that rays originating from a point are focused to a point on the other side. Do demo of projecting light bulb filament on screen This is a real image Mon. Feb. 4, 2008 Physics 208, Lecture 4

How a lens works Position surfaces to bend light rays in just the right way Spherical surfaces are very close to the right ones. n1 n1 n2>n1 Optical Axis F Do light board demo with lenses. Mon. Feb. 4, 2008 Physics 208, Lecture 4

Thin-lens approximation: Ray tracing Optical Axis F Image Object F 1) Rays parallel to optical axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays through F emerge parallel to optical axis. Here image is real, inverted, enlarged Mon. Feb. 4, 2008 Physics 208, Lecture 4

Different object positions Image (real, inverted) Object Image (real, inverted) Image (virtual, upright) These rays seem to originate from tip of a ‘virtual’ arrow. Here we use ray tracing to find where image location. Mon. Feb. 4, 2008 Physics 208, Lecture 4

Quick Quiz I project a focused image onto a screen 2 meters away. I now want to make the image bigger without changing the lens. I should Move screen farther away only Move screen closer only Move screen closer and object toward lens Move screen farther and object toward lens Move screen farther and object away from lens Mon. Feb. 4, 2008 Physics 208, Lecture 4

Making an image s s’ Object Image How are all these related? Image distance Object distance f focal length How are all these related? Use lens board to demonstrate how s’ changes when s changes. Mon. Feb. 4, 2008 Physics 208, Lecture 4

Your eye can change focal length Lens Object Image What is range of focal lengths if it can focus from near point (25 cm) to inf. onto retina 1.7 cm away? Object at infinity: Object at near point: Very limited range Mon. Feb. 4, 2008 Physics 208, Lecture 4

Image size vs object size: Magnification Object height Image distance Object dist. Image height = M = Magnification Mon. Feb. 4, 2008 Physics 208, Lecture 4

Question At what object distance does image size equal object size (magnification=1)? Object distance = f Object distance = 2f Object distance = f/2 Object distance = infinity Object distance =0 Mon. Feb. 4, 2008 Physics 208, Lecture 4

Different object positions Image (real, inverted) Object Image (real, inverted) Image (virtual, upright) These rays seem to originate from tip of a ‘virtual’ arrow. Here we use ray tracing to find where image location. Mon. Feb. 4, 2008 Physics 208, Lecture 4

Virtual images Virtual image But rays can be focused by another lens Image (virtual, upright) These rays seem to originate from tip of a ‘virtual’ arrow. objects closer to a converging lens than the focal length form a virtual image Virtual image can’t be recorded on film, Can’t be seen on a screen. But rays can be focused by another lens e.g. lens in your eye (focus on retina) e.g. lens in a camera (focus on film plane) Mon. Feb. 4, 2008 Physics 208, Lecture 4

Do these rays come from real image, a virtual image, or an object? Can’t tell. Rays are exactly equivalent, and can be imaged by a lens in exactly the same way. Mon. Feb. 4, 2008 Physics 208, Lecture 4

Magnifying glass o Object at the near point - this is the biggest it will appear and be in focus Object closer than focal point Lens produces virtual image Light rays appear to originate from virtual image Virtual image is used as object for eye lens. Have moved object ‘closer’, while permitting eye to focus Image (virtual, upright) p q  Can work this out. Suppose object is at near point = 25 cm. Mon. Feb. 4, 2008 Physics 208, Lecture 4

Image (virtual, upright) Normal magnifier config, virtual image, upright Image (real, inverted) Image real, but eye cannot image it Blurry 3rd line: image is bigger than object, but not as big as just with simple magnifier. Ask how can we make it bigger? Answer: use a simple magnifier with the image as the object. Image (real, inverted) Eye can now form image of inverted lens image Mon. Feb. 4, 2008 Physics 208, Lecture 4

Lens combinations Image of one lens acts as object for next. Rays originate from image, whether real or virtual Can then directly apply Mon. Feb. 4, 2008 Physics 208, Lecture 4

Compound Microscope Real, inverted, image p q Virtual image ‘Object’ Eyepiece Object Objective Object outside focal point Forms a real image Real image used as object for eyepiece. Eyepiece forms virtual image for eye. Mon. Feb. 4, 2008 Physics 208, Lecture 4

Far away objects The moon is 3.8x108 m away, and 3.5x106 m diameter. I use a 1 m focal length lens to make an image of the moon. About what diameter is this image of the moon? 0.5 cm 1 cm 2 cm 10 cm 1 m Not a very big image. How can It be made ‘bigger’? Mon. Feb. 4, 2008 Physics 208, Lecture 4

Telescope: object far away Mon. Feb. 4, 2008 Physics 208, Lecture 4

Nearsightedness Object I can’t focus on this This, I can see Mon. Feb. 4, 2008 Physics 208, Lecture 4

Fixing nearsightedness Object Mon. Feb. 4, 2008 Physics 208, Lecture 4