PHY205H1F Summer Physics of Everyday Life Class 10: Colour, Optics Color in Our World Mixing Colored Light Why the Sky Is Blue Why Sunsets Are Red Law of Reflection Virtual Image Formation Image Reversal Concave Mirrors Diffuse Reflection Refraction Total Internal Reflection Lenses Clicker Accuracy Leaderboard (just for fun) Number of correct UTORid clicker answers so far 1 (1) chenyuz2 112 2 (2) tombergs 106 3 ( 5) zhangx89 103 4 ( 10) halekath 101 5 ( 11) guoyepen 100 6 ( 13) kunghsia 99 6 ( 2) kuoyu 99 6 (6) wanggua8 99 Chs. 27 & 28 Pre-Class Quiz Notes 1. When red, green and blue light are combined, white light results. That is a consequence of the three types of light-sensitive cells in the human retina. It is a property of human biology, not the physical universe. 2. The sky is blue due to atmospheric scattering of sunlight. 3. The splitting up of white light into its component colours is called dispersion. 4. A converging lens can form a real image (such as the upside-down one of me currently on your retina!) Problem Set 4 is due right now. Last Chance for no late penalty! Any Problem Sets not turned in by now will receive the 10% per day (or portion thereof) late penalty. Recall from Chapters 25 and 26 A changing electric field creates a magnetic field, which then changes in just the right way to recreate the electric field, which then changes in just the right way to again recreate the magnetic field, and so on. This is an electromagnetic wave. E M CHECK YOUR NEIGHBOR What property of a light wave mainly determines its colour? A. Its frequency B. Its amplitude C. Its timbre D. A mix of all of the above Color Color we see depends on frequency of light. 0.4 μm 0.7 μm Selective Reflection Mixing Colored Light The spectrum of sunlight is a graph of brightness versus frequency. High Frequency, Short wavelength f λ Low Frequency, Long wavelength A red ball seen A red ball seen under white light. under red light. Only red is reflected, other colors are absorbed. A red ball seen under green light. There is no source of red light to reflect! 1
Slide from http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colcon.html CHECK YOUR NEIGHBOR Rods and cones contain stacks of membranes. Rhodopsin is a transmembrane protein complex. A beam of white light, when shone through a prism, will end up many different colours. What will happen if a green laser is shone through a prism? Opsin (protein component) Retinal (pigment) Response Curves for the three types of cones in the retina of the human eye. A. It will spread into a small range of colours. B. It will change to a single colour other than green. C. It will stay green. Cone Rod 0.5 µm Light Light The retinal molecule inside rhodopsin changes shape when retinal absorbs light. cis conformation (inactive) Opsin Light Rhodopsin trans conformation (activated) Opsin Slide courtesy of Ross Koning, Biology Department, Eastern Connecticut State University http://plantphys.info/sciencematters/vision.ppt Use both eyes, stare at area shown in blue for 15 seconds or so What do you see on this blank white slide? Blink if needed! Use Stare both at eyes, the bird s stare eye at the for 10 parrot s seconds eye without for 15 seconds tilting your or head. so This is called an after image Does it move around as you move your gaze? Bird Was Red Use Stare both at eyes, the bird s stare eye at the for 10 parrot s seconds eye without for 15 seconds tilting your or head. so Bird Was Green What colour is the afterimage bird? A. orange B. green C. blue D. yellow E. None of the above What colour is the afterimage bird? A. red B. orange C. blue D. violet E. None of the above 2
Use Stare both at eyes, the bird s stare eye at the for 10 parrot s seconds eye without for 20 seconds tilting your or head. so Bird Was Blue Additive Primary Colours (light bulbs) and Subtractive Primary Colours (ink) What colour is the afterimage bird? A. red B. orange C. yellow D. green E. None of the above Discussion Question Why is this square red? A. The light bulbs in the projector emit light with blue frequencies B. The light bulbs in the projector emit light with green frequencies C. The light bulbs in the projector emit light with red frequencies D. Both A and B Discussion Question Why is the red folder in Harlow s hand red? A. The pigments in the paper absorb light with blue frequencies B. The pigments in the paper absorb light with green frequencies C. The pigments in the paper absorb light with red frequencies D. Both A and B Discussion Question : Using the colour wheel.. When the colour yellow is seen on the screen of your computer, what are the lights being activated in that region of your screen? A. Mainly yellow B. Blue and red C. Green and yellow D. Red and green Discussion Question : Using the colour wheel.. If an object is blue, which of the coloured lights below would make the object appear most black? A. blue B. cyan C. yellow D. magenta Discussion Question : Using the colour wheel.. If an object is red, which of the coloured lights below would make the object appear not appear red? A. red B. orange C. white D. cyan 3
Why the Sky Is Blue For small scattering particles, like nitrogen or oxygen molecules, higher frequency blue light is scattered much more readily than lower frequency red light. Why the Sky Is Blue frequency If molecules in the sky scattered orange light instead of blue light, the sky would be A. orange. B. yellow. C. green. D. blue. CHECK YOUR NEIGHBOUR Doppler Shift for Light When a light source is moving away from you, the spectrum is shifted toward the red. When a light source is moving toward you, the spectrum is shifted toward the blue. Doppler Shift for Light The Doppler shift can be observed in the headlights of cars on the highway. The cars moving away from you appear more red, while the cars moving toward you appear more blue-ish or white. Doppler Shift for Light wthhh hhhhh hhhh?? The Doppler shift can be observed in the headlights of cars on the highway. The cars moving away from you appear more red, while the cars moving toward you appear more blue-ish or white. Doppler Shift for Light (yes, really!) The Doppler shift can be observed with carefully obtained spectra of very fast moving objects like stars There is a slight shift in absorption lines Light that is least scattered is light of low frequencies, which best travel straight through air. If molecules in the sky scattered orange light instead of blue light, sunsets would be A. orange. B. yellow. C. green. D. blue. CHECK YOUR NEIGHBOUR 4
Why Clouds Are White Clouds are clusters of various sizes of water droplets Why Clouds Are White Size of clusters determines scattered cloud color. Tiny clusters produce bluish clouds. Slightly large clusters produce greenish clouds. Larger clusters produce reddish clouds. Overall result is white clouds. Slightly larger clusters produce a deep gray. Still larger clusters produce raindrops. CHECK YOUR NEIGHBOR What does a white sky indicate? A. The atmosphere has mostly particles smaller than the wavelength of light B. The atmosphere has mostly particles larger than the wavelength of light C. The atmosphere has a mixture of particle sizes D. The atmosphere has a lot of water vapour E. The atmosphere is filled with pollutants. Law of Reflection The angle of reflection equals the angle of incidence. Mirror B A A dentist uses a mirror to look at the back of a second molar (A). Next, she wishes to look at the back of a lateral incisor (B), which is 90 away. By what angle should she rotate her mirror? Virtual Image Formation 2010 Pearson Education, Inc. A. 90 B. 45 C. 180 No light rays actually pass through or even near the image, so it is virtual. Two plane mirrors form a right angle. How many images of the ball can the observer see in the mirrors? A. 1 B. 2 C. 3 D. 4 Which picture is most likely a mirror image of Harlow? Virtual Image Formation Alice looks at Bob s image a mirror and sees he has a red shoe on the foot to Alice s left. Then she asks Bob to turn and face her, so she can compare the image to what Bob looks like in real life. Bob takes a couple of steps forward, turns around and faces Alice. Alice notes that the red shoe is now on the foot on the right. Alice concludes: Mirrors reverse left and right, not up and down. Is this true? Can you see any flaws in Alice s reasoning? mirror real 5
Virtual Image Formation Bob in Mirror Virtual Image Formation Bob in Mirror What really happens is the image is reversed front-to-back Bob turns to face Alice Bob chooses to rotate around a vertical axis, and therefore he looks flipped left-to-right. But if Bob wants to turn to face Alice, is there any other way to do it? Bob stands on his head to face Alice! If Bob had chosen to face Alice by standing on his head, he would have been flipped up-todown, and not left-to-right! Refraction When light bends in going obliquely from one medium to another, we call this process refraction. Cause of Refraction Bending of light when it passes from one medium to another Caused by change in speed of light 1 Dry Land A B A soldier wants to get from point 1 to Point 2 in the shortest amount of time. Marching through mud is much slower than marching on dry land. Which might be the best path? C Mud Dry Land 2 Refraction Light travels slower in glass than in air, so it minimizes the time it spends in the glass. air water Dispersion Process of separation of light into colors arranged by frequency A fish swims below the surface of the water. An observer sees the fish at: A. a greater depth than it really is. B. its true depth. C. a smaller depth than it really is. 2010 Pearson Education, Inc. Components of white light are dispersed in a prism (and in a diffraction grating). 6
Rainbows Rainbows are a result of dispersion by many drops. Dispersion of light by a single drop Rainbows Doublerainbow Antisun The second rainbow has blue on the top, and a radius of about 53 Radius of circle is about 41 Total Internal Reflection Total reflection of light traveling within a medium that strikes the boundary of another medium at an angle at, or greater than, the critical angle Discussion Question Light waves with speed v 1 are incident upon the flat surface of a material in which they have speed v 2. For what condition is total internal reflection possible? A. v 2 > v 1 B. v 2 < v 1 C. v 2 = v 1 D. All of the above An Optical Fibre Speed of light in cladding is higher than speed of light in core. Medical Fibrescopes Converging Lens Focal length, f Negative Focal length, -f Diverging Lens Videolaryngoscopy with a flexible fiberscope NOTE: Focal length is defined for initially parallel rays. Focal Point Virtual Focal Point Rays appear to emerge from Virtual Focal Point 7
Diverging rays through a Converging Lens Focal length, f f Real Image Formation Focal length, f image If an object emits rays at the focal point, they end up being parallel on the other side of the converging lens. What will happen to the rays emerging to the right of the lens if the face is moved a little further away from the lens? A. They will remain parallel. B. They will diverge (spread out). C.They will converge (toward a focus). Light rays actually pass through the image, and a screen can be placed there, so it is real. The Camera A camera takes a picture by using a lens to form a real, inverted image on a light-sensitive detector in a light-tight box. We can model a combination lens as a single lens with an effective focal length (usually called simply the focal length ) A zoom lens changes the effective focal length by varying the spacing between the converging lens and the diverging lens. This is the end!!!! The final exam, will be: Wed. June 26 at 1:00pm sharp! (not 1:10!) EX100 Test will begin promptly at 1:00 and will be 2 hours long. Please bring a calculator, and, if you wish, a lettersized aid sheet upon which you may write anything you wish This is the end!!!! The final exam will cover the entire course, including all of the assigned reading plus tutorial materials and what was discussed in class. Approximately even spread over the course material Final Exam: First Page Please Fill Out the Online Survey For This Course!!!!!! Your feedback is vital to me. I promise I will read every word you type in the online evaluations. The online system is open now (you should have received an email already) The deadline is Sunday June 23 please don t forget!! Keep in Touch!! My office hours are Mondays and Wednesdays: 10-11AM, Fridays: 9-10AM. I will have an extra office hour Tuesday June 25 from 2-3pm. I am in MP121-B and my land-line is 416-946-4071 (please don t send text messages to my land-line) You can always email me ( jharlow@physics.utoronto.ca ) with any questions, or if you d like to make an appointment outside my office hours. Good luck on the exam and have a great rest of the summer! 8