ESCI 215 Chapter 9

Theories of Light In ancient times it was believed that an object “sent” a small image of itself to a person’s eye so we could see it Similar to how we send an ! If this was true, we should still see in the dark 1600s scientists started to form theories about light In 1655 Robert Hooke and Christian Huygens supported the wave theory 1666 Isaac Newton supported the particle theory Light 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 Light 1800s scientific evidenced seems to support the wave theory Augustin 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 waves Interference Light Waves When 2 waves hit an object at the same time the effect is different than if the waves hit the object separately Constructive Interference – increases brightness Wave crests of both waves hit the object together and form a larger wave crest Destructive Interference – decreases brightness Wave 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 Light 1801 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 wavelength Visible light spectrum is 4000 – 7000 angstroms long 1 angstrom = millimeter 1865 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 particles Light is made up of bundles of energy called photons

Photons are released and absorbed as particles, but travel as waves Light was finally understood! Properties of Light: Made of energy particles called photons Travels as waves It can be reflected and refracted Has mass so is affected by gravity Exerts a force when it hits an object Ex: the light from the sun exerts a force of pressure of 1/6 kilogram per km 2 – can push a large tinfoil satellite out of orbit my 100s km/day Theories of Light

Light is Invisible Light 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 object Ex: on a clear night the sun’s light fills the sky, but we only see it when if reflects of something like the moon Note: Stars are there own source of light Event 9-B shows that light is invisible as it travels from the flashlight to your hand

Speed of Light 1675 Olaus Roemer discovered that light did not travel from the source to the object instantly Studying one of Jupiter’s visible moons (Io), he noticed depending on the time of year, it took longer for our sun’s light to reflect off Io and return to earth so he could see Io The distance the light had to travel changed depending on the position of the earth (due to the season) See Fig 9.5 on page 155 Roemer calculated that light traveled 300,000 km/second 299, km/s

Transmission of Light Materials can be classified by how well they permit light to pass through them: Transparent Light goes through without distortion (i.e. Clear window) Translucent Light goes through, but images are not clear (i.e. Stained glass, wax paper) Opaque Light does not go through (i.e. Wood, metal) Event 9-D shows the difference between transparent and translucent materials

Brightness of Light Depends on: Brightness of the light source Distance from the light source to the observer Photometer is a device used to measure brightness of light Events 9-E and 9-F use a photometer to exam the effect of the 2 factors above on the brightness of light Event 9-G shows that light’s brightness decreases by the square of the distance – inverse square law Ex: at a distance of 2cm, light’s intensity will be ¼ the intensity it would be at 1cm distance

Light Reflected Light is reflected in 2 ways: Even reflection On smooth surfaces, light rays bounce off at the same angle (ie. mirror, polished chrome) Shows image but no reflecting surface Uneven reflection On rough surfaces, light rays bounce off in all directions after hitting it (i.e. wall, desk) Shows the reflecting surface but no image

Angle of Reflection The angle of the incoming ray (the incidence) = the angle of the outgoing ray (the reflection) See Fig 9.15 on page 161 Shine a light on a mirror in a dark room The beam is invisible Blow chalk dust on it to see the incidence and reflection rays Events 9-H and 9-I shows the angles of incidence and reflection Reflected images are “folded over” from left to right, but not top to bottom Look 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 effect Kaleidoscopes and Periscopes can be used to investigate reflection

Light Refracted Refraction is the bending of light when: it goes from one medium to another AND at an angle This bending occurs because light travels at different speeds in different mediums Event 9-N shows how different mediums can change the direction of objects Event 9-P shows the refraction of light Event 9-O shows that refraction does not happen when light does not enter at an angle

Lenses Your eyeglass lenses work because of refraction Event 9-R shows how lenses can be used to bend light and a convex lens can project an image on a screen The 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 edges Light shines through the convex lens and converges Hold the lens close to an object an it appears larger Concave lenses are thinner in the middle than at the edges Light shines through and diverges Hold the lens close to an object an it appears smaller Event 9-S shows that images are only reversed when they are beyond the focal point

Lenses Cont’d How to make a lens: Place a drop of water on a pinhole in a piece of tinfoil Water becomes shaped like a lens and can be used as a magnifying lens Place an object (printed word) under the lens to view it Event 9-T uses eyeglass lenses to make objects bigger or smaller Nearsighted people have poor distance vision Concave lenses correct their vision by making the image clearer Farsighted people have trouble seeing close images Convex lenses correct their vision

Colour – Additive Colours White light contains all colours of light White 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 light Event 9-V shows that when all colours are blended together, the result is white When 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 white An object is the colour of the light that it reflects; if an object absorbs all light and does not reflect any, it will be black

Colours – Complementary Colours Any 2 colours of light that produce white when added to each other Blue and yellow light create white Magenta and green light create white Red and cyan light create white Event 9-X shows complementary colours Staring 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 Colours Primary colours of paint, dyes, inks and natural colorants are subtractive These colour pigments are used to absorb and reflect some wavelengths of light When mixing pigments, many colours are absorbed or subtracted Black often results from mixing pigments Event 9-AA demonstrates this

Optical Illusions Event 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 foreground Event 9-CC shows the persistence of vision because the 2 images of the fish appear to be one image This is similar to how we see motion on TV – the picture is changed 30 times per second Create a flip picture movie by drawing a stick figure on a notepad and making small changes on each page

Optical Illusions - Parallax When eyes are focused on a distant object, they cannot focus on a near object Each eye sees the object in the peripheral vision The 2 images overlap Floating finger – hold 2 fingers close together and look past them See figure 9.37 on page 175

Assessment and Instruction Task: Students create a concept map of the big ideas from the light unit that shows the relationship/connections between the ideas The students could create the concept list or the teacher could provide it Evaluation: Key terms and concepts are present and organized to show understanding Linking words show understanding of connections Hierarchy of concepts is evident and logical Examples are used to show understanding See Fig 9.39 on page 177

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