Light, Lenses and Colour

Light, Lenses and ColourESCI 215Chapter 9Theories of LightIn ancient times it was believed that an object sent a small image of itself to a persons eye so we could see itSimilar to how we send an email!If this was true, we should still see in the dark1600s scientists started to form theories about lightIn 1655 Robert Hooke and Christian Huygens supported the wave theory1666 Isaac Newton supported the particle theoryLight is made of particles that travel in straight lines, making shadows and images with sharp edges; if light traveled in wavy lines, the edges of images would be blurry

Theories of Light1800s scientific evidenced seems to support the wave theoryAugustin Fresnel used a strong magnifying glass to show that the edges of images were blurry (fringes)Event 9-A show the fringes that Fresnel examined; these are caused by the interference of light wavesInterference Light WavesWhen 2 waves hit an object at the same time the effect is different than if the waves hit the object separatelyConstructive Interference increases brightnessWave crests of both waves hit the object together and form a larger wave crestDestructive Interference decreases brightnessWave crests of one wave and the wave trough of the other wave hit the object (see diagram on page 152)Look at light through a pinhole and see a fuzzy spot Theories of Light1801 Thomas Young proved that light was the result of wave action (2 slit experiment)John Angstrom measure the length of waves Each colour has a different wavelengthVisible light spectrum is 4000 7000 angstroms long 1 angstrom = 0.0000001 millimeter1865 James Maxwell discovered that light is only the visible part of a longer spectrum the electromagnetic spectrum includes visible light, x-rays, radio waves, etc. (see Figure 9.2 on page 153)1900s Albert Einstein and Max Planck discovered that light acted as waves, but also as particlesLight is made up of bundles of energy called photons

Photons are released and absorbed as particles, but travel as wavesLight was finally understood!

Properties of Light:Made of energy particles called photonsTravels as wavesIt can be reflected and refractedHas mass so is affected by gravityExerts a force when it hits an objectEx: the light from the sun exerts a force of pressure of 1/6 kilogram per km2 can push a large tinfoil satellite out of orbit my 100s km/day

Theories of LightLight is InvisibleLight is visible only if:It is the source of light (luminous object i.e. light bulb)It is reflected off something (illuminated object)Light is invisible as it travels from the source to an objectEx: on a clear night the suns light fills the sky, but we only see it when if reflects of something like the moonNote: Stars are there own source of lightEvent 9-B shows that light is invisible as it travels from the flashlight to your handSpeed of Light1675 Olaus Roemer discovered that light did not travel from the source to the object instantlyStudying one of Jupiters visible moons (Io), he noticed depending on the time of year, it took longer for our suns light to reflect off Io and return to earth so he could see IoThe distance the light had to travel changed depending on the position of the earth (due to the season)See Fig 9.5 on page 155Roemer calculated that light traveled 300,000 km/second299,792.458 km/sTransmission of LightMaterials can be classified by how well they permit light to pass through them:TransparentLight goes through without distortion (i.e. Clear window)TranslucentLight goes through, but images are not clear (i.e. Stained glass, wax paper)OpaqueLight does not go through (i.e. Wood, metal)Event 9-D shows the difference between transparent and translucent materialsBrightness of LightDepends on:Brightness of the light sourceDistance from the light source to the observer

Photometer is a device used to measure brightness of lightEvents 9-E and 9-F use a photometer to exam the effect of the 2 factors above on the brightness of lightEvent 9-G shows that lights brightness decreases by the square of the distance inverse square lawEx: at a distance of 2cm, lights intensity will be the intensity it would be at 1cm distanceLight ReflectedLight is reflected in 2 ways:Even reflectionOn smooth surfaces, light rays bounce off at the same angle (ie. mirror, polished chrome)Shows image but no reflecting surfaceUneven reflectionOn rough surfaces, light rays bounce off in all directions after hitting it (i.e. wall, desk)Shows the reflecting surface but no imageAngle of ReflectionThe angle of the incoming ray (the incidence) = the angle of the outgoing ray (the reflection)See Fig 9.15 on page 161Shine a light on a mirror in a dark roomThe beam is invisible Blow chalk dust on it to see the incidence and reflection raysEvents 9-H and 9-I shows the angles of incidence and reflectionReflected images are folded over from left to right, but not top to bottomLook in a mirror and wink your right eye or hold up a word. What do you notice?Event 9-J uses 2 mirrors to counteract this effectKaleidoscopes and Periscopes can be used to investigate reflectionLight RefractedRefraction is the bending of light when: it goes from one medium to another ANDat an angleThis bending occurs because light travels at different speeds in different mediumsEvent 9-N shows how different mediums can change the direction of objectsEvent 9-P shows the refraction of lightEvent 9-O shows that refraction does not happen when light does not enter at an angleLensesYour eyeglass lenses work because of refractionEvent 9-R shows how lenses can be used to bend light and a convex lens can project an image on a screenThe image is inverted as the light rays are bent (see Fig 9.25)There are 2 types of lenses:Convex lenses are thicker in the middle than at the edgesLight shines through the convex lens and converges Hold the lens close to an object an it appears largerConcave lenses are thinner in the middle than at the edgesLight shines through and divergesHold the lens close to an object an it appears smallerEvent 9-S shows that images are only reversed when they are beyond the focal pointLenses ContdHow to make a lens:Place a drop of water on a pinhole in a piece of tinfoilWater becomes shaped like a lens and can be used as a magnifying lensPlace an object (printed word) under the lens to view itEvent 9-T uses eyeglass lenses to make objects bigger or smallerNearsighted people have poor distance visionConcave lenses correct their vision by making the image clearerFarsighted people have trouble seeing close imagesConvex lenses correct their vision Colour Additive Colours White light contains all colours of lightWhite light can be separated into its 6 separate light colours (ROYGBV)Event 9-U uses a prism to show how a beam of white light is divided into the different colours as it goes through the prism (because each colour was its own wavelength)The 6 light colours can be added to make white lightEvent 9-V shows that when all colours are blended together, the result is whiteWhen the disk is spun each colour stays briefly on the retina while the next colour is added (persistence of vision), all the colours combine on the retina and appear whiteAn object is the colour of the light that it reflects; if an object absorbs all light and does not reflect any, it will be blackColours Complementary ColoursAny 2 colours of light that produce white when added to each otherBlue and yellow light create whiteMagenta and green light create whiteRed and cyan light create whiteEvent 9-X shows complementary coloursStaring at a blue object for 30 seconds tires your eyes (cones), so when you look at a white screen (which reflects all colours) you will see the complementary colour, yellow

Colours Subtractive ColoursPrimary colours of paint, dyes, inks and natural colorants are subtractive These colour pigments are used to absorb and reflect some wavelengths of lightWhen mixing pigments, many colours are absorbed or subtracted

Black often results from mixing pigments

Event 9-AA demonstrates thisOptical IllusionsEvent 9-BB shows how our eyes can be deceived to think that the image of the man in the background is larger than the one in the foregroundEvent 9-CC shows the persistence of vision because the 2 images of the fish appear to be one imageThis is similar to how we see motion on TV the picture is changed 30 times per secondCreate a flip picture movie by drawing a stick figure on a notepad and making small changes on each pageOptical Illusions - ParallaxWhen eyes are focused on a distant object, they cannot focus on a near objectEach eye sees the object in the peripheral visionThe 2 images overlap Floating finger hold 2 fingers close together and look past themSee figure 9.37 on page 175

Assessment and InstructionTask:Students create a concept map of the big ideas from the light unit that shows the relationship/connections between the ideasThe students could create the concept list or the teacher could provide itEvaluation:Key terms and concepts are present and organized to show understandingLinking words show understanding of connectionsHierarchy of concepts is evident and logicalExamples are used to show understandingSee Fig 9.39 on page 177Websiteswww.eskimo.com/`billb/amasci.htmlwww.madsci.org/experimentswww.howstuffworks.com/category.htm?cat=Light

DiscussionWhere does this topic fit into the Science curriculum?Which grades and strands?Which curriculum objectives relate to the discrepant events?