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Optics Notes Intro to Light - Light is the only form of energy that acts as a wave and can be absorbed by matter - Electromagnetic waves are waves of energy that have both electric and magnetic fields - They are invisible - Can travel through a vacuum or a medium - Travel through a vacuum at the speed of light → 3.00 * 10^8 m/s - Wavelength - Distance from one peak of a wave to the next peak - Measured in nanometres → 1 nm = 10^-9 m - Electromagnetic spectrum - Ordered from longest to shortest wavelength - Radio waves, microwaves, infrared light, visible light, UV light, X-rays, gamma rays - Newton discovered that white light was actually composed of all of the colors of the visible spectrum - He did this by shining a beam of white light from the sun through a triangular glass prism - ROYGBIV - Shorter wavelength → higher energy - Violet light has the highest energy of the entire visible spectrum - Visible spectrum is from 400 to 700 nm Sources of Light - We need light for - Seeing - Oxygen and food (photosynthesis) - Two main sources of light - Natural - Artificial - Light is usually associated with heat - Hot objects emit white light (mixture of all different colors) - Some sources do not release heat, these sources only emit one main color of light - Two ways we can see things - They are luminous: produce their own light - They are non-luminous: don’t produce their own light, but instead reflect it - Types of light production - Incandescence - Produce light when heated - Incandescent light bulbs only convert 5% of electrical

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energy into light, rest into heat → very inefficient Fluorescence - When something absorbs high-energy UV radiation and then immediately releases the energy as visible light - Fluorescent light bulbs AKA Compact Fluorescent Light bulbs - The glass tube is filled with a mixture of mercury and inert argon gas, the inside of this tube is coated with phosphor - The mercury-argon mixture produces UV light from the electric current, and the phosphor absorbs this UV and emits visible light - More efficient, last longer than incandescent bulbs - However, cost more, the mercury can cause environmental pollution if not disposed of properly - Bleach, money notes, the tongue (used for diagnosing harmful conditions by looking for dark spots) - Electric discharge is when passing electricity through a gas creates different colors of light - Neon signs - Electric discharge bulbs have an electrode at each end that emits electrons with a near-vacuum tube that has a sodiummercury vapor - The electrons exit these atoms and cause them to release light - Thus, fluorescence is a type of electric discharge Phosphorescence - Similar to fluorescence, but instead stores the energy from UV light and releases it gradually during and after exposure - Glow in the dark objects, watches Luminescence - Bioluminescence - Chemical reactions inside a living organism that produce light - Fireflies: have a chemical called luciferin that reacts with oxygen to produce oxyluciferin and release light - Angler fish use bulbs to lure prey, it hosts colonies of symbiotic, light-producing bacteria - Chemiluminescence - Chemical reactions produce light, no heat produced - When you crack a glow stick, the glass tube breaks and the two chemicals mix together, a dye in the chemicals produces the color - Triboluminescence

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Light formed from friction, pressure, or mechanical shock Breaks chemical bonds Crush a sugar cube with a flat-bottomed glass

Ray Diagrams - A ray is a single path followed by light - When light strikes matter, 3 things can happen - Transmitted: passes through the medium - e.g. windows - Reflected: bounces off and object - e.g. mirrors - Absorbed: disappears (all of it or just certain wavelengths) - 3 classifications of an object’s ability to transmit light - Transparent - Opaque - Translucent - Regular reflection is when light bounces off of smooth shiny surfaces

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Diffuse reflection is when light is scattered off in many directions because of an uneven surface In a ray diagram, an image is a representation of an object formed by the intersection of light rays SALT: size, attitude, location, type

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Characteristics of a plane mirror image - Size: same size - Attitude: upright and laterally inverted - Location: behind the mirror and same distance - Type: virtual

Images in Concave Mirrors - Concave: curved inwards, convex: curved outwards - Concave lens diverges light, convex lens converges - Centre of curvature is where all of the normals for a curved mirror intersect - Any ray sent from C will reflect back from C - The principal axis is the horizontal line that passes through C - Vertex (V) is where the principal axis meets the mirror - A light ray reflecting off of the vertex acts as if it is reflecting off of a plane mirror - Focal point (F) is the point through which reflected rays will pass if the incident rays are parallel to the principal axis

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Concave Mirror Equations - R is the radius of convergence - F is the focal point, f is the focal length, f = 0.5R - Virtual image: produced when light rays appear to come from somewhere - Real image: produced when the light rays intersect and can be displayed on a surface - Concave mirrors can be used to produce magnified virtual images - Mirror equation - 1/f = 1/d_o + 1/d_i - Magnification equation - M = h_i/h_o = -d_i/d_o - When M>1, image larger than object, otherwise not - When M is +, image is upright, otherwise inverted

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Convex Mirrors - For convex mirrors, C and F are behind the mirror

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Size - smaller Attitude - upright Location - behind the mirror and before F Type - virtual Same mirror equation and magnification equation as concave

Applications of Mirrors - Applications of plane mirrors - Periscope - contains two plane mirrors - Submarine - In WW1 to look over the tops of trenches - At a hairdresser - Concave mirror applications - A radar antenna detects radio waves by focusing them at the focal point - Solar ovens - Flashlights produce a parallel beam by placing a light bulb at F - Vanity mirrors and dental mirrors: produce enlarged virtual image - Convex mirror applications - Since all images in convex mirrors are smaller, you can get a wider field of view - Security mirrors in convenience stores

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Mirrors in hospitals to peek around the corner when in a rush At truck inspection stations and border crossings, guards use them to see the underside of vehicles Used in rear view mirrors to give a wide field of view, that’s why “objects in mirror are closer than they appear”

Refraction - Fermat’s Principle - Light travels in a straight line and at a constant speed in a medium - It follows the path that will take the least amount of time in a medium - When travelling between mediums, the path that will take the least amount of light is not a straight line - Refraction is the bending of light when it travels from one medium to another - The particles in the medium slow down the light and cause it to change direction - It occurs at the boundary of the two media - The light’s speed depends on the density of the medium

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Angle of incidence - i Angle of reflection - r Angle of refraction - R If the light travels from a faster to slower medium, i > R Light that passes straight between the two media does not get refracted The amount by which a medium decreases the speed of light is indicated by the index of refraction (n) n = c/v c = 3.00 * 10^8 m/s Higher n → more the medium decreases the speed of light In a vacuum, n = 1, in air, n = 1.0003 Snell’s Law - Relates the indices of refraction of two media

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- n_1*sin(i) = n_2*sin(R) Dispersion: the process of separating colors in white light by refraction

Critical Angle and Total Internal Reflection - When light travels from a slow to fast medium, i < R - As i increases, the reflected ray becomes stronger and the refracted ray becomes weaker - Critical angle - The angle of incidence that produces an angle of refraction of 90 deg - Only happens when light travels from medium with high n to a medium with low n - To calculate the critical angle, you use Snell’s law, setting R to 90 deg - Total Internal Reflection - 2 conditions for TIR to occur - Angle of incidence is greater than the critical angle, so there is no more refraction, instead all the light is reflected within the first medium - Light has to be travelling from a medium with high n to a medium with low n - The critical angle for medium 1 is small if n is large - In a diamond, n is very large - 2.42 - so the critical angle is very small - 24 deg - The bouncing of light caused by TIR because of such a small critical angle is why diamonds sparkle - In fibre optic cables, light is transmitted because of TIR (the light cannot escape because of it) - Some periscopes also use triangular glass prisms to reflect light instead of mirrors as mirrors absorb some of the light and deteriorate more quickly

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Rainbows are also caused by TIR of the sunlight inside raindrops, which causes the white light to separate into the color spectrum

Optical Phenomena in Nature - Red light is faster than violet - For this reason, red light is bent less than violet during refraction - Rainbow - When white light from the sun enters a raindrop, it is dispersed, producing the 7 colors - Then when the light hits the back of the raindrop TIR happens - Finally when the light exits the raindrop, it disperses even more - Rainbows are usually seen when the sun is on one side of the observer and the rain shower is on the opposite side - The sun must also be at around 42 deg for a rainbow to be visible - Secondary rainbows occur when sunlight reflects twice inside rain droplets - The second rainbow is less bright and has its colors reversed - Sundogs AKA parhelia are bright spots on both sides of the sun - Are similar to rainbows, except occur when ice crystals in the atmosphere refract the light

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Occur when the sun is low - 22 deg and on cold, clear, sunny mornings and evenings Mirage - An image of a distant object produced as light refracts through air of different n’s - Common on hot sunny days on highways - On hot days, the air above the highways becomes very hot and therefore has a lower n than the colder air above it - As light rays pass from the colder to the warmer air, the light rays refract away from the normal - Eventually TIR happens and the light rays pass from the warmer to the colder air, as the rays refract towards the normal - The light passes in a concave upward shape - This makes it look as if the light is coming from the ground - So we see a puddle of water on the ground but it is really an image of the sky

Apparent depth: when the image of an object appears closer than the actual object due to refraction - Occurs when light rays from the water bend away from the normal as they exit (high n to low n) but we see them as if they have travelled in a straight line

Images in Lenses - Lenses change the appearance of objects, such as their size, upright direction, or

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can even cause them to appear misshapen Lenses can focus light because of refraction Converging lenses (convex) make parallel light rays come together and focus at the focal point on the opposite side of the lens

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Diverging lenses (concave) make parallel light rays move apart - The focal point on the side of the object (on the left in the following example) is called the virtual focal point

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f is positive (to the right) for converging lens, negative (to the left) for diverging lens For lenses, image distances to the right of the mirror are positive

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Lens Applications - Lenses make use of refraction - A camera uses a converging lens to produce an inverted, real, smaller image on either film or the sensor in a digital camera - In order for this to happen, the image must be beyond the centre of curvature of the lens - Modern cameras are digital and use a charge-coupled device instead of film

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A movie projector is the opposite of a camera - Takes a small object (film) and projects a large, inverted image on a screen - The film must be between F and C, and the film must be loaded into the projector upside down

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A magnifying glass AKA simple microscope uses a converging lens to produce an enlarged, virtual image of an object that is located between F and the mirror

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A compound microscope uses two converging lenses - Produces two enlarged, upside down images, one real and one virtual - The virtual one is bigger and it is the one you see

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A refracting telescope works the same way as a compound microscope except that the object is so far that the incident rays are basically parallel

The Human Eye

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The cornea and lens in the eye acts as a converging lens that produces a smaller, real, inverted image on the retina that is flipped left to right Electric impulses from the retina travel through the optic nerve to the brain, which interprets the signal so that we see the image correctly

Accommodation - The process of the eye using the ciliary muscles to change the shape of the lens to focus an image on the retina - The lens is fatter when the eye is focused on closer objects Myopia - Near-sightedness - Distant objects out of focus - The light rays from distant objects intersect in front of the retina because the lens is too fat (converges the light too much) - Diverging lenses can correct it by spreading out light rays before they enter the eye so that they focus on the retina Hyperopia - Far-sightedness

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Close objects out of focus The light rays from close objects intersect behind the retina because the lens is too thick (diverges the light too much) - Converging lenses can correct it by converging the light rays before they enter the eye so that they focus on the retina Contact lenses can be used instead of glasses - Are placed directly on the cornea of the eye - Can be used for changing the color of the eye Blindness - Any type of vision problem that prevents someone from being able to do important activities - Some might not be able to see in the middle of their visual field - Others might only be able to see in the middle! Photoreceptors are cells in the retina that are sensitive to light - Rods are able to detect low levels of light - Night vision - Cones are less sensitive to light than rods but allow color vision - Three types of cones - R, G, B - Eyes are most sensitive to yellow A pigment is a chemical that absorbs some colors of light and reflects others Colors of objects - In white light (RGB), a yellow flower appears yellow because red and green are reflected, blue is absorbed - In magenta light (R + B), the same flower appears red because that is reflected, blue is absorbed Color blindness is very rare, occurring in 1 person out of 40,000 - Can only see shades of grey Color vision deficiency is more common - Inability to distinguish some colors Blind spot - The spot on the retina with no photoreceptors - Where the optic nerve attaches to the retina...