## Light

Published in: Physics
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• ### Debarun S

• Kolkata
• 10 Years of Experience
• Qualification: B.Tech
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this presentation is about what light is , how human eye acts as a camera,concave and convex lenses, refraction of light etc. useful for class viii students

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Angle of incidence = Angle of reflection Reflecting surface Incident ray Angle of Angie of incidence I reflection Normal Reflected ray Figure 6.6 Law of reflection
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Refraction ' Refraction Fact #1: As light goes from one medium to another, the velocity CHANGES! ' Refraction Fact #2: As light goes from one medium to another, the path CHANGES!
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Refraction — Going from Air to Water ' If a light ray goes from a LOW refractive Index (FAST MEDIUM) to a HIGH refractive index (SLOW MEDIUM), it's soeed DECREASES and the angle BE Direction light would have gone if medium dd not onenge "dencrrg toward" the normal Normal uto surface 1 1 1 1 1 1 1 Actual direction Air (Fast medium) Water (Stow medium)
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Refraction — Going from Water into Air ' If you are going from a HIGH refractive index to a LOW refractive index, your speed INCREASES and the angle BENDS AWAY the normal Air (Fast medium) Water (Slow medium) Normal to surface Direction light would have gone if medium did not change Bending away" from the normal Actual direction
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There is one exception... That is when light hits 'head-on', perpendicular to the boundary. The light does not bend but its speed still changes.
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plastic sheath unner core outer hyer Total internal reflection total internal reflection ' happens when the angle between the light ray and the normal is greater than the critical angle. The critical angle is formed when the light ray travels along the boundary between the two substances. Optical fibres use multiple total internal reflections to transmit light.
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' Copy the diagrams into your workbook. Draw a normal wherever the light rays enter a new substance. ' Demonstrate what will happen to the rays as they enter and exit from the substances by continuing the ray through the shape and out the other side. b air 'rvzlcr
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Apparent Depth Prac Coin and Fish Physics Ghost fish Air Water tha coin in the cup cannot be seen from this positon coin seen the Cup IS filled witt water
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Why does it look bent away from normal and not towards?? Light from the lower part of the ruler is travelling into a region of lower optical density (air), and so has been bent away from the normal. The image in the water is actually an illusion—an image our brain constructs based on where light from the lower part of the ruler appears to come from. ' It assumes that the light traveled in a perfectly straight line, even though it didn't.
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, part B: Refraction in water - ANSWERS Refract; towards. b Emerge; away from. Incident; a glass surface; no. c 2 When light rays at an angle to the surface travel from a medium of low density to one of higher density, the light rays refract towards the normal. a a From the tip, to the eye. From the coin to the eye. Water Water
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Lenses There are two main types of lenses: convex lenses—these curve outwards and are fatter in the middle ' concave lenses—these curve inwards (a little like a cave) and are thinner in the middle. Convex lenses Concave Lenses
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Lenses — An application of refraction There are 2 basic types of lenses convex lens focal point 1 1 focal length A converging lens (Convex) takes light rays and bring them to a point. Focal Concave Lens int Focal length A diverging lens (concave) takes light rays and spreads them outward.
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Movement of Light through Lenses CONVEX LENS ' In a convex lens, an incoming ray parallel to the principal axis is refracted through the principal focus (F). plane of lens parallel rays oilight prineipal axis principal focus i focal length Convex lens
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Concave Lens ' In a concave lens, an incoming ray parallel to the principal axis is refracted so that it appears to come from the principal focus (F). principal focus principal axis parallel rays of light i plane oflens focal length Conveave lens
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Movement of Light through a Lens The distance from the centre line (plane) of the lens to the principal focus is called the focal length of the lens. plane of lens parallel •flight i principal axis principal focus focal A ray passing through the centre of either type of lens is unaffected. As with all images, rays of light that come from a part of the object come together again at that same part of the image.
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Focal Length The greater the curvature of a lens, the more it bends light and hence the shorter the focal length. lew lor,s lengh lew lerwh
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Image type and Location Convex lenses produce two different types of images, depending on where the object is located. ' If the object is at a distance greater than the focal length of the lens, a real image is formed. A real image can be projected onto a screen Object focus fmus real image
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Convex Lenses ' If the object is at a distance less than the focal length of the lens, a virtual image is formed. This image can't be projected onto a screen. Ray tracing virtual image
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Concave Lenses ' Concave lenses produce only virtual images. object focus virtual Image eye traces rays back to form a virtual image
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Finding the focal length ' Rays coming into a lens from a distant object are almost parallel and form an image very close to the focus. We can then measure the distance from lens to image to determine the focal length of the lens. distant obieet approximate focal length almost parallel rays convex lens real, inverted image
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Images in a Convex and Concave Lens Prac AIM: To investigate the image formed by different convex lens and concave lens Complete Convex prac — using 2 different convex lenses Diagram Convex lens focal length: Object Description of position Object more than 'two focal lengths from lens Object focal lengths from lens Object between one and two focal lengths from lens Object less than one focal length from lens (i.e. object inside the focal length) Object exactly at the focus (one focal length from lens) cm Distance from lens (cm) Distance from lens (cm) Image Description (e.g. larger/smaller, Inverted/uprlght)
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Questions CONVEX - 1) What happened as the object was brought closer to the lens? 2) When does; ' aa real image (on a screen) is obtained b a virtual image (one that cannot be 'caught' on a ' screen) is obtained ' c no image is obtained CONCAVE - 1) Assess whether it is possible to form a real image (one that may be 'caught' on a screen) using a concave lens. 2) Explain how the image changes as the object-to-lens distance is varied.
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Refractor Focus -Incoming light is bent into a bright point Pu il of the Eye Eyepiece -brings the bright image from Objective lens -gathers light and bends it into focus. Incoming light the focus and magnifies it to the size of your eye's pupil e 2000 How
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Optical instruments Telescopes ' Telescopes make small, far objects appear larger. Two lenses are used. The objective lens produces a real, inverted image just inside the focus of a second lens, called the eyepiece lens, The image produced by the first lens now acts as the object for the second lens. Because the first image is inside the focus of the second lens, the second 'l nonage (the one seen by the telescope user) is virtual and enlarged compared to the first one
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Sclera Retina Case film Iris Iris diaphragm Lens Lens E elid Shutter Eye (b) The eye compared with a camera Simple camera
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Eye ball prac Part of eye Part of camera Black lining Sclera Iris Lens Retina Aperture to admit light Hole in diaphragm

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