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Light By Neil Bronks
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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.
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Virtual Image An image that is formed by the eye
Can not appear on a screen d d
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Real Image Rays really meet Can be formed on a screen F 2F
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Ray Diagrams- Object outside 2F
1/. Inverted 2/. Smaller 3/. Real 2F F The images can be formed on a screen so they are real.
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Object at F F 2F The image is at infinity
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Object inside F 1/. Upright 2/. Magnified 3/. Virtual F
The image is behind the mirror
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Convex Mirror 1/. Upright 2/. Smaller 3/. Virtual
The image is behind the mirror 1/. Upright 2/. Smaller 3/. Virtual F
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Uses of curved mirrors Concave Mirrors Convex Mirror Security Mirrors
Dentists Mirrors Make –up mirrors Convex Mirror Security Mirrors Rear view mirrors
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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 V=60cm Virtual
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Magnification What is the magnification in the last question?
Well u=20 and v=60 As 20 m=3 Image is magnified 6 2
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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 The minus is Because the Mirror is convex Using the formula V=60/5cm =12cm Virtual
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MEASUREMENT OF THE FOCAL LENGTH OF A CONCAVE MIRROR
Crosswire u Lamp-box Screen v
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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 =constant =n (Refractive Index) sin r
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Proving Snell’s Law Sin i i r Sin r
Laser Sin i i Protractor Glass Block r Sin r A straight line though the origin proves Snell’s law. The slope is the refractive index.
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Refractive Index Ratio of speeds
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Real and Apparent Depth
A pool appears shallower
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REFRACTIVE INDEX OF A LIQUID
MEASUREMENT OF THE REFRACTIVE INDEX OF A LIQUID Cork Pin Apparent depth Mirror Real depth Water Image Pin
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Finding the Critical Angle…
1) Ray gets refracted 2) Ray still gets refracted 4) Ray gets internally reflected 3) Ray still gets refracted (just!) THE CRITICAL ANGLE
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Critical Angle Varies according to refractive index
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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.
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Lenses Two types of lenses Focal Point Converging Lens Diverging Lens
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Converging Lens- Object outside 2F
Image is 1/. Real 2/. Inverted 3/. Smaller 2F F
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Object inside F Image is 1/. Virtual 2/. Erect 3/. Magnified F
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Magnification What is the magnification in the last question?
Well u=30 and v=12 As 12 30 2 5 Image is smaller
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MEASUREMENT OF THE FOCAL LENGTH
OF A CONVERGING LENS Lamp-box with crosswire Screen Lens v u
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Diverging Lens Image is 1/. Virtual 2/. Upright 3/. Smaller F F
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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 The minus is Because the Diverging lens Using the formula V=60/5cm =12cm Virtual
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Magnification What is the magnification in the last question?
Well u=30 and v=20 As 20 30 2 3 Image is smaller
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Sign Convention f Positive V either f negative V f negative V f
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Myopia (Short Sighted)
Image is formed in front of the retina. Correct with diverging lens.
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Hyperopia (Long-Sighted)
Image is formed behind the retina. Correct with a converging lens
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Power of Lens Opticians use power to describe lenses. P=
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
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The power of the total lens is
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
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