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Light Year nines it is time to light up and understand some physics. Mr Posthumus 1 Terminology Explain the meaning of the following SE 9 page 170 • Luminous • Transparent • Translucent • Opaque 2 • Luminous- luminous objects produce light. Example, sun, lights and fires. • Transparent-materials like glass that allow light to pass through. • Translucent- a translucent material allows light to pass through, but scatters it so you can’t see through clearly. • Opaque -light can’t pass through. Light is absorbed or reflected 3 The sun ; Why is the sun Luminous • Hydrogen- Helium 4 Is the moon Luminous? • Why do we see the moon? 5 Answer • it is non-luminous and reflects light © 2003 Mike Maloney 6 Light • What is LIGHT? • How does light travel • WHERE DOES IT COME FROM? 7 What is Light • Light is a special type of wave called an electromagnetic wave • LIGHT is actually a type of something called ELECTROMAGNETIC RADIATION. • So, what is electromagnetic radiation and electromagnetic waves? 8 Electromagnetic Waves • When something creates energy it also emits radiation. Depending on the amount of energy, the object will emit different types of electromagnetic radiation. 9 Electromagnetic Waves • Electromagnetic waves are special in the fact that they do not need a medium to propagate through. • But what is creating the disturbance? What is emitting this energy? 10 Electromagnetic Waves • Electrons in materials are vibrated and emit energy in the form of photons, which propagate across the universe. • Photons have no mass, but are pure energy. • Electromagnetic Waves are waves that are made up of these “photons”. 11 Electromagnetic Waves • Electromagnetic waves are everywhere. • Light is only a small part of them – – – – Radios TVs Microwaves Light (Visible/UV/InfraRed) – Radiation – Lasers – X-Rays 12 Electromagnetic Spectrum © 2003 Mike Maloney 13 Visable spectrum • The light we see is know as visible or white light – although it is not that simple. • The light is not really white, the white we see is a combination of all the colors of the rainbow. • Remember R-O-Y G. B-I-V from art class. Or VIBGYOR • When all of these light waves are combined we see white light. 14 Back to Light • So, why can we only see a small portion of these E-M waves? 15 Visible Light • We now know what we see is part of the electromagnetic spectrum. We know that the light waves enter our eye, and stimulate the retina. Electrical impulses to be sent to the brain which creates this visual image. 16 Our eyes • • • • Lens : Focus of light (refraction) Cornea: Transparent allows light to enter the eye Iris: Controls amount of light entering the eye Retina : Converts light into nerve impulses 17 Why do we have two eyes? Purpose of binocular vision • the use of both eyes together to create a three dimensional image (Have depth of vision) 18 Eye defects and type of lens to correct the problem • Short sightedness: Cannot focus on distant objects Concave lens to correct • Far sighted: Cannot focus on near objects Convex lens to correct 19 Colour • Different objects may emit different wavelengths of E-M radiation, so we would see that light as different colors. • But why do we see colors in objects that reflect light? 20 Color Reflection • So if we see something as WHITE, that means … – It reflected back all the wavelengths of light to our eyes (White light is a mixture of different colours) • If we see something as RED or BLUE – It reflected only the RED or only the BLUE wavelengths – The others were absorbed. • And if we see something as black? – It did not reflect back any of the light. 21 Colour • Why is the sky blue? • Why are sunsets red? • Why is water greenish-blue? 22 Answer • The sky is blue because light is scattered by dust particles. Blue light is scattered more so it enters the eye from all parts of the sky. • Sunsets and sunrises are orange or red. Red is less scattered which means more of it left to enter the eye when looking at a sunset. • the answer to ‘why is the sunset red?’ is: At sunset, light must travel farther through the atmosphere before it gets to you, so more of it is reflected and scattered and the sun appears dimmer. The colour of the sun itself appears to change, first to orange and then to red because even more of the short wavelength blues and greens are now scattered and only the longer wavelengths(reds, oranges) are left to be seen. • Interesting fact • It is said that outback skies(in desert regions) are more vividly blue than European winter skies because the outback has more dust= more scattering 23 In the beginning……there were 3 colours • Most people will be familiar with the ideas of basic colour mixing ie • Yellow + Blue makes Green • Red + Yellow makes Orange • Blue + Red makes Purple (some texts refer to the mix as being “Violet”…..but you get the general idea) • So….we understand that we can mix colours and arrive at another colour. BUT there are some colours that cannot be mixed and they are called “Primary Colours”….they are Red, Yellow & Blue. We cannot create these colours from other colours but if you start with these 3 colours you can mix an almost infinite number of other colours eg Green, Orange & Purple. • The 3 colours that were the result of mixing two Primary colours are called Secondary colours. 24 Reflection • SE 9 pp.170-172 • Images are formed by mirrors because they reflect light. 25 Reflection. Plane mirror When light is reflected the angle of incidence is equal to the angle of reflection. A= incident ray B= angle of incidence C=Normal D=Angle of reflection E= Reflected ray 26 Image reflected by a plane mirror • The image reflected is the same size and the right way up © 2003 Mike Maloney 27 Curved Mirrors • Concave mirror: concave means to bend inwards(caved in) • Concave mirrors can give you an enlarged image if you stand close 28 Convex mirror • Convex mirrors curve outwards. Some examples of convex mirrors are a car's side mirrors, spoons and security mirrors. • When beams of light come into contact with convex mirrors the light rays diverge (spread out) • Convex mirrors improve the field of view (make it possible to see more the area behind 29 Optical fibres • Fibre optic technology has virtually replaced copper wire in longdistance telephone lines, and it is used to link computers within local area networks. Fibre optics is also the basis of the fibrescopes used in examining internal parts of the body (endoscopy) or inspecting the interiors of manufactured structural products. • Through a process known as total internal reflection, light rays beamed into the fibre can propagate within the core for great distances with remarkably little attenuation 30 Practical Time: Reflection • Inquiry 1,2 and 3 (can be done with Hodgson light boxes to show the ray paths and then the image formation can be done with concave and convex lenses) p. 170-171 31 Speed of light • It has been found that the speed of light is --– 3 x 108 or 300,000,000 m/s – 300 000 km/s. Speed of light is constant throughout the universe, as long as light is in a vacuum. – The density of the medium through which light passes effects the speed of light 32 Refraction – bending of light • When light changes direction as it enters a new material this is known as refraction. It is due to a change in the speed of the light. • Convex lenses are thicker in the middle .They cause parallel rays of light to converge. Convex lenses are used in magnifying glasses (make objects bigger) • Optical devices like a projector use a convex lens to create an enlarged image on the screen 33 Concave lenses Concave lenses are thinner at the middle. Rays of light that pass through the lens are spread out (they diverge). A concave lens is a diverging lens. They make objects smaller © 34 Mirages : Refraction of light • Mirages commonly form when the air near the ground is much warmer than that above it. Warm air has a different density than cold air. When light waves travel through one medium and into another, they are bent or refracted from their straight-line path. • What causes the mirage illusion is that our mind initially interprets the light rays reaching our eyes as having come along a straight path, which in this example must have originated on the ground. Thus, we see that patch of sky and cloud "on the ground" and interpret the image as a surface "pool of water. 35 Refraction and Images Clickview; Refraction and Images. 36 • Refraction – bending of light, total internal reflection. Inquiry 5 p. 174 SE9 Investigation 1 p. 175 SE9 (Skill p. 176 top class only and if there is time) Eyes and Seeing Inquiry 7 p. 177 • Investigation 2 p.178 (Eye dissection) (Omit refractive index p. 173) 37 38 How far is the sun? • I takes about 8 minutes for light to reach the earth from the sun. So how far away is the sun? • Remember speed of light = 300 000 Km per second. • If you calculate the number of seconds in 8 minutes you can work out the distance of the sun https://www.childcarejobs.com.au/ • 8 minutes = 480 seconds • 480 times 300 000= 144000000km 39