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Transcript
Light Light is a form of energy Crooke’s Radiometer proves light has energy Turns in sunlight as the light heats the black side Can you think of another example to demonstrate that light is a form of energy? Light travels in straight lines How are shadows formed? Reflection Reflection is the bouncing of light off an object. When light bounces off objects it scatters in all directions – diffuse reflection. Highly polished surfaces (mirror) behave in a more predictable way. Reflection Angle of incidence = Angle of reflection Normal Reflected ray Incident ray Angle of incidence Angle of reflection Mirror Laws of Reflection The angle of incidence ,i, is always equal to the angle of reflection, r. The incident ray, reflected ray and the normal all lie on the same plane. Reflection Laws of Reflection Animation 1 Laws of Reflection Animation 2 How is an image formed in a plane mirror? Virtual Image An image that is formed by the apparent intersection of light rays Can not appear on a screen d d Curved Mirrors Curved mirrors consist of a series of small mirrors combined together. Each individual mirror must obey the laws of reflection. Real Image An image that is formed by the actual intersection of light rays. Can be formed on a screen 2F F All ray diagrams in curved mirrors and lens are drawn using the same set of rays. Concave Mirror Object Principal Axis F Pole F You can draw any ray diagram by combining 2 of these rays The only difference is where the object is based. F Ray Diagrams- Object outside 2F 1/. Inverted 2/. Smaller 2F F 3/. Real The images can be formed on a screen so they are real. Object at 2F 1/. Inverted 2/. Same Size 3/. Real 2F F The image is at 2F Object between 2F and F 1/. Inverted 2/. Magnified 2F F The image is outside 2F 3/. Real Object at F 2F F The image is at infinity Object inside F F 1/. Upright 2/. Magnified 3/. Virtual The image is behind the mirror Convex Mirror The image is behind the mirror 1/. Upright 2/. Smaller 3/. Virtual F Convex Mirror – only one ray diagram F The image is behind the mirror Uses of curved mirrors Concave Mirrors Dentists Mirrors Make –up mirrors •Convex Mirror Security Mirrors Rear view mirrors Ray Diagram Example An object 4 cm high is placed at right angles to the axis of a concave mirror and at a distance of 30 cm from the mirror. If the focal length of the mirror is 10 cm find the position, size and nature of the image. This can be done using a diagram or by calculation. Calculations Use the formula 1 1 1 f u v u F v f=focal length u=object distance v=image distance Example An object is placed 20cm from a concave mirror of focal length 10cm find the position of the image formed. What is the nature of the image? Collect info f=10 and u=20 Using the formula 1 1 1 10 f 20 u v 1 1 1 f u v 1 1 1 v 10 20 1 1 V=20cm real v 20 Magnification What is the magnification in the last question? Well u=20 and v=20 As v 20 v m m u 20 u m 2v 2u • m=1 • Image is same size Example An object is placed 20cm from a concave mirror of focal length 30cm find the position of the image formed. What is the nature of the image? Collect info f=30 and u=20 Using the formula 1 1 1 f u v 1 1 1 1 1 1 1 30 20 v v 30 20 60 V=60cm Virtual Example An object is placed 30cm from a convex mirror of focal length 20cm find the position of the image formed. What is the nature of the image? Collect info f=-20 and u=30 Using the formula 1 1 1 20 30 v 1 1 1 f u v V=60/5cm =12cm The minus is Virtual Because the 1Mirror1is 1 5 convex v 30 20 60 MEASUREMENT OF THE FOCAL LENGTH OF A CONCAVE MIRROR Concave mirror Crosswire Lamp-box Screen u v Approximate focal length by focusing image of window onto sheet of paper. Place the lamp-box well outside the approximate focal length Move the screen until a clear inverted image of the crosswire is obtained. Measure the distance u from the crosswire to the mirror, using the metre stick. Measure the distance v from the screen to the mirror. Repeat this procedure for different values of u. Calculate f each time and then find an average value. Precautions The largest errors are in measuring with the meter rule and finding the exact position of the sharpest image. Refraction The fisherman sees the fish and tries to spear it Fisherman use a trident as light is bent at the surface Refraction into glass or water AIR WATER Light bends towards the normal due to entering a more dense medium Refraction out of glass or water Light bends away from the normal due to entering a less dense medium Refraction through a glass block Light bends towards the normal due to entering a more dense medium Light slows down but is not bent, due to entering along the normal Light bends away from the normal due to entering a less dense medium Laws of REFRACTION The incident ray, refracted ray and normal all lie on the same plane SNELLS LAW the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for 2 given media. sin i = n (Refractive Index) sin r Proving Snell’s Law i r Repeat for different angles of incidence Real and Apparent Depth A pool appears shallower Re al n Apparent MEASUREMENT OF THE REFRACTIVE INDEX OF A LIQUID Cork Pin Apparent depth Mirror Real depth Water Image Pin Finding No Parallax – Looking Down Pin at bottom Pin reflection in mirror Parallax No Parallax Refractive Index(n) in terms of relative speeds Refractive Index Ratio of speeds cair 300000000m / s n 1.5 cwater 200000000m / s Refraction out of glass or water Light stays in denser medium Reflected like a mirror Angle i = angle r Finding the Critical Angle… 1) Ray gets refracted 3) Ray still gets refracted (just!) THE CRITICAL ANGLE 2) Ray still gets refracted 4) Total Internal Reflection Critical Angle Varies according to refractive index 1 sin C n 1 sin 45 n 1 0.7071 n 1 n 0.7071 n 1.141 Refractive Index and Critical Angle Refractive Index is defined in relation to light going from air into that medium (i.e. air to glass or air to water) Ex 1: The critical angle for a certain medium is 500 . Find its refractive index Ex 2: The refractive index of glass is 1.5. What is the critical angle for glass? Uses of Total Internal Reflection Optical fibres: An optical fibre is a long, thin, transparent rod made of glass or plastic. Light is internally reflected from one end to the other, making it possible to send large chunks of information Optical fibres can be used for communications by sending e-m signals through the cable. The main advantage of this is a reduced signal loss. Also no magnetic interference. Practical Fibre Optics It is important to coat the strand in a material of low n. This increases Total Internal Reflection The light can not leak into the next strand. 1) Endoscopes (a medical device used to see inside the body): 2) Binoculars and periscopes (using “reflecting prisms”) Mirages Lenses Two types of lenses Focal Point Focal Point Converging Lens Diverging Lens Ray Diagrams Optical Centre F F 2F F F 2F Converging LensObject outside 2F Image is 1/. Real 2/. Inverted 3/. Smaller 2F F F 2F Object at 2F Image is 1/. Real 2/. Inverted 3/. Same size 2F F F 2F Object between 2F and F Image is 1/. Real 2/. Inverted 3/. Magnified 2F F F 2F Object at F Image is at infinity F F Object inside F Image is 1/. Virtual 2/. Erect 3/. Magnified F F Calculations Use the formula 1 1 1 f u v f=focal length u=object distance v=image distance u 2 F F F v 2F Example An object is placed 30cm from a converging lens of focal length 40cm find the position of the image formed. What is the nature of the image? Collect info f=40 and u=30 Using the formula 1 1 1 40 f 30 u v 1 1 1 f u v 1 11 11 1 - = vu -120 v vf fu40 30 V=120cm virtual Magnification What is the magnification in the last question? Well u=30 and v=120 As 120 v v mm 30 u u m 4v 1u • Image is larger MEASUREMENT OF THE FOCAL LENGTH OF A CONVERGING LENS Show on OPTICAL BENCH Lamp-box with crosswire Screen Lens u v 1. Place the lamp-box well outside the approximate focal length 2. Move the screen until a clear inverted image of the crosswire is obtained. 3. Measure the distance u from the crosswire to the lens, using the metre stick. 4. Measure the distance v from the screen to the lens. 5. Calculate the focal length of the lens using 1 1 1 f u v 6. Repeat this procedure for different values of u. 7. Calculate f each time and then find the average value. The Eye Power of Accommodation - ability to focus a real image of an object on the retina The width of the lens is controlled by the ciliary muscles. For distant objects the lens is stretched. For close up objects the muscles relax. Why is not a good idea to water plants on a sunny day? The water forms droplets on the leaves. These droplets act as converging lenses and focus the sun onto the leaves, burning them. As a result the leaves will have brown spots. Why can’t we focus clearly under water yet swimming goggles will restore clear focus? Hint: your cornea and water have a similar refractive index Light refracts when travelling from air through the cornea of your eye, but water and the cornea have the same refractive index , so light does not refract. By wearing goggles however light which hits your eye is coming from air, so the usual focusing applies and objects appear normal. Diverging Lens Image is 1/. Virtual 2/. Upright 3/. Smaller F F Example An object is placed 30cm from a diverging lens of focal length 20cm find the position of the image formed. What is the nature of the image? Collect info f=-20 and u=30 Using the formula 1 1 1 20 30 v 1 1 1 f u v V=60/5cm =12cm The minus is Virtual Because the 1 1 1 5 Diverging lens v 30 20 60 Example An object is placed 30cm from a diverging lens of focal length 60cm find the position of the image formed. What is the nature of the image? (Remember f must be negative) Collect info f=-60 and u=30 Using the formula 1 1 1 -60 30 f u v 1 1 1 f u v 1 11 11 1 - = -60 uf 30vu -20v vf V=20cm virtual Magnification What is the magnification in the last question? Well u=30 and v=20 As v 20 v m m u 30 u m 2v 3u • Image is smaller Myopia (Short Sighted) Image is formed in front of the retina. Correct with diverging lens. Hyper-Myopia (Long-Sighted) Image is formed behind the retina. Correct with a converging lens Power of Lens Opticians use power to describe lenses. 1 P= f So a focal length of 10cm= 0.1m is written as P=10m-1 A diverging lens with a negative focal length f=-40cm=-0.4m Has a power of P = -2.5m-1 Lens in Contact Most camera lens are made up of two lens joined to prevent dispersion of the light. The power of the total lens is Ptotal=P1+ P2