What is Light?What is Light? Light is a small part of the Electromagnetic Spectrum or a form of ElectromagneticRadiation. Other parts of the Electromagnetic Spectrum include: Radio Waves, Microwaves (mostly used for radar) Infrared Radiation (we can feel some of this as heat) Ultra-Violet radiation, X-rays, Gamma Rays & Cosmic Rays
Properties of LightProperties of Light Light travels in straight lines Light travels VERY FAST – around 300,000kilometres per second. At this speed it can go around the world 8times in one second.Laser
Luminous and non-luminous objectsLuminous and non-luminous objects Luminous objects ReflectorsA luminous object is one that produces light.A non-luminous object is one that reflects light.
ReflectionReflection Reflection is the bouncing of light falling on a surface Reflection from a mirror:Incident rayNormalReflected rayAngle of incidence Angle ofreflection
The Law of ReflectionThe Law of ReflectionAngle of incidence = Angle of reflectionAngle of incidence = Angle of reflectionIn other words, light gets reflected from a surface at ____ _____angle it hits it.The same !!!
Reflection off Curved surfacesReflection off Curved surfacesConcave(curved in) Focuses parallel beamsConvex(curved out) Disperses parallel beams.
Using mirrorsUsing mirrors Two examples:1) A periscope2) A car headlight
RefractionRefraction Refraction: is the bending of light as it moves fromone medium (material) to another.
Refraction occurs because thespeed of light changes as thedensity of the material changes. Light speeds up in less densematerial (bending away from thenormal) and slows down in moredense materials (bending towardthe normal). Density is measured by refractiveindex
If light strikes the interfacebetween the materials alongthe normal (at right angles),then it does not changedirection.AirGlass
Refraction:Refraction:From Less Dense to More DenseFrom Less Dense to More Dense Light bends toward theNormal Example of this includeLight passing from : Air to Water Air to GlassNormalAirGlassAngle of Incidence (Ai)Angle of Refraction (Ar)
Refraction:Refraction:From Dense to Less DenseFrom Dense to Less Dense Light bends away from the Normal Example of this include Lightpassing from : Water to Air Glass into WaterAirGlassNormalAngle of Incidence (Ai)Angle of Refraction (Ar)
Passing Through a Block of GlassPassing Through a Block of GlassAirGlassNote: The path of the beam of light isparallel entering and leaving the block
Copy the diagram intoyou notebook and Mark on it: The Normal Angle Of Incidence Angle of Refraction State which materialhas the greater densityMaterial BMaterial A
SolutionsSolutions Copy the diagram into you notebookand Mark on it: The Normal Angle Of Incidence Angle of Refraction State which material has the greaterdensity Material B, as the light bends towardthe normal passing from A into BMaterial BMaterial A
LensesLensesConverging orConvex LensDiverging orConcave Lens•A lens is a piece of glass that bends 2 rays so that they crossand form an image• There are two different types of lenses, a converging lens anda diverging lens
Converging orConverging or ConvexConvex LensLens A converging lens is thicker in thecenter Parallel rays go through it andcome out so they all converge atone point (the focal point) Convex lens are found inMagnifying glasses, Microscopes(at least 2), Telescopes (at least 2),Cameras and glasses used tocorrect Long-sightedness.
Diverging orDiverging or ConcaveConcave LensLens A diverging lens is thinner in the center Parallel rays go through the lens anddiverged (spread). The beam after passingthrough the lens appears to be emanatingfrom a point in front of the lens; thedistance from this point to the lens is alsoknown as the focal length Concave lens are used in a peep hole, suchas youd find in the door of a hotel room andto correct Short-sightedness.
Bending Light Through a PrismBending Light Through a Prism Each wavelength (colour) of lighttravels through a material at its ownparticular speed . For example, when white lightpasses through a prism somewavelengths of light bend morebecause their refractive index ishigher, i.e. they travel slower This is what gives us the "Spectrum". The "red‘’ light travels slowest, and"violet" travels fastest, so are bentdifferently. All the other colours liein between. This most easily seem with a prismwere the light is bent twice in thesame direction.
RainbowsRainbows Rainbows occur because therefraction of light in waterdroplets.Refraction and reflection in a raindrop,producing a rainbow.White light separates into different colours onentering the raindrop and then furtherseparated on leaving.
SightSight In the back of the eye we have specialised receptorcells that react to light Photoreceptors
The Cornea, Pupil, Lens are all working to focuslight (and hence the image) on to the retina Retina Rods: black & white, night vision Cones: color, day vision Fovea: sharpest vision (concentration of cones)
Rods Do Not See Red!Rods Do Not See Red!The light response of the rods peaks sharply in the blue;they respond very little to red light.The ship captain has red instrument lights. Since the rodsdo not respond to red, the captain can gain full dark-adapted vision with the rods with which to watch foricebergs and other obstacles outside. It would beundesirable to examine anything with white light evenfor a moment, because the attainment of optimumnight-vision may take up to a half-hour. Red lights donot spoil it.
Light & ColourLight & Colour White light can be split up to make separate colours. Thesecolours can be added together again. The primary colours of light are red, blue and green:Adding blue and redmakes magenta(purple)Adding blue and greenmakes cyan (light blue)Adding all threemakes whiteagainAdding red andgreen makesyellow
Seeing ColourSeeing Colour The Primary colours correspond to the three kindsof color-sensitive cones in our retinas. We see objects because of the light they reflect. The colour an object appears depends on thecolours of light it reflects.For example, a red book only reflects red lightWhitelight Only red light isreflected
Seeing Black & WhiteSeeing Black & WhiteSeeing White White objects reflectsall colors.Seeing Black Black Objects absorball light (& Reflectnone)
Seeing Primary ColoursSeeing Primary Colours When we see Red, Blue or Green objects we areseeing just a single wave length of light reflected.•The object appears BLUE because RED and GREEN are absorbed, whileonly the BLUE light is reflected.•We see the color BLUE.•The object appears RED because GREEN and BLUE are absorbed, while onlythe RED light is reflected.We see the color RED.•The object appears GREEN because RED and BLUE are absorbed, while onlythe GREEN light is reflected.•We see the color GREEN.
Seeing secondary ColoursSeeing secondary Colours We see secondary colours when two wave lengthsof light are reflected while one is absorbed.
Using coloured lightUsing coloured light If we look at a coloured object in coloured light we see somethingdifferent. For example, consider the clothing below:WhitelightShorts look blueShirt looks red
In different colours of light this kit would look different:RedlightShirt looks redShorts look blackBluelightShirt looks blackShorts look blue
Some further examples:Object Colour of lightColour object seemsto beRed socksRed RedBlue BlackGreen BlackBlue teddyRed BlackBlueGreenGreen camelRedBlueGreenMagenta bookRedBlueGreen
Using filtersUsing filters Filters can be used to “block” out different colours of light:Red FilterMagentaFilter
Investigating filtersInvestigating filtersColour of filter Colours that could be “seen”RedGreenBlueCyanMagentaYellow