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Optical Systems make use of Mirrors and Lenses. Unit 2: Chapter 5. The Ray Model of Light. light is represented as a straight line or ray that shows the direction the light wave is travelling. Remember that light is made up of waves and can be described using wavelength and frequency.
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Optical Systems make use of Mirrors and Lenses Unit 2: Chapter 5
The Ray Model of Light • light is represented as a straight line or ray that shows the direction the light wave is travelling. Remember that light is made up of waves and can be described using wavelength and frequency.
Transparent, Translucent, Opaque Transparent – a material that allows light to pass through freely. All or most of the light is transmitted. Translucent – a material, such as frosted glass, that lets most light rays through, but scatters them. Some light is transmitted. Opaque – a material that prevents any light from passing through. All or most of the light is absorbed or reflected.
HW: What examples did you find? • Transparent? • Translucent • Opaque
Ever wonder about shadows? • You can use the ray model to predict where shadows will form and how large they will be. • The closer the object is to the light source the larger the shadow. • The further the object is from the light source the smaller its shadow will be. • No shadow can be perfect*
Law of Reflection • If a wave strikes the sea wall on an angle how will it bounce off? • ** precisely the same angle • Incident ray= incoming ray • Reflected ray= ray that bounces off barrier • Normal= “dotted line” perpendicular to the boundary between two materials. Drawn at right angles to the solid barrier • Angle of incidence= formed by incident beam and normal • Angle of reflection= formed by the reflected beam and normal • Why is it called the Law of Reflection? • ** angle of reflection = angle of incidence
Law of Reflection Normal line
Law of Refraction When light moves into a medium of higher density it slows down (i.e. air to water, or air to glass) and moves towards the normal line.
Law of Refraction When light moves into a medium of lower density it speeds up (i.e. water to air or glass to air) and bends away from normal.
Let’s experiment Bowl of Water • Place a coin at the bottom of a bowl/cup. • Step back until the coin just stops being visible over the lip of the bowl/cup. Then have someone slowly pour water into the bowl. • What do you see? Why does this happen? Pencil • Insert a pencil at an angle into the cup. • Try looking at the pencil at different angles (above and below the water line). • What do you see? Does anything change, or stay the same? Why? Vegetable Oil and a Cup • Put the shot glass inside the larger glass. • Fill the larger glass with water. Just enough to cover the smaller glass by an inch • Look at the glass from different angles. • Do you see the drinking glass? Why? • Spill out the water and dry the shot and larger glass. • Put the shot glass back inside the larger glass • Fill the larger glass with cooking oil. • Look at the glass from different angles. • Do you see the drinking glass? Why? • Pour the vegetable oil back into the plastic cup. Dry and clean both glasses for the next group.
Hmm... Let’s Explain Bowl of Water • When the bowl is empty the edge of the bowl stops you seeing the coin. When the bowl is full of water the light bends over the edge so you can see the coin. • Have you ever noticed that things at the bottom of a pool or river always look closer to the surface than they really are? This is because of the way light is bent through water and is an effect of refracted light. Pencil • From above, the pencil seems to bend at the water line because refraction has occurred between the two mediums of water-air. The pencil also looks bigger underwater. This is because of the glass has curved surfaces and therefore the water in it acts as a convex lens. Vegetable Oil and a Cup • As light passes through one medium to another (water-air, glass-air), the light is bent (refracted) at the boundary between the two mediums. This happens because light travels at different speeds through different mediums. Light moves through petroleum products (including cooking oil) at about the same speed as it does through the glass. Therefore, as light passes between glass and oil it doesn’t bend at the boundaries, leaving the boundaries invisible.
Law of Refraction – Mirage Refraction of light can also occur when light travels through air at different temperatures. Warm air is less dense than cooler air. When light enters cooler air it will slow down and bend towards normal.
Law of Refraction – Mirage Mirage of water on pavement on a hot summer day. How does this happen?
Law of Refraction – Mirage Answer: Air close to the pavement becomes superheated. As light travels from the road to your eye, it slows down when it hits the cooler air above the highway. It will slow down and bends towards normal. The effect causes a water mirage.
5.2 http://hubblesite.org/gallery/spacecraft/22/ http://solarcooking.org/ http://www.physicsclassroom.com/Class/refln/u13l3d.cfm
VIRTUAL IMAGES VS. REAL IMAGES • More or less than meets the eye. • A real image is one in which light rays actually come from the image. • In a virtual image, they appear to come from the reflected image - but do not. In a flat mirror, for example, the virtual image of an object is behind, or "inside" the mirror, but light rays do not originate from there. Real images form outside the system, where emerging light rays cross and are "caught"...inside camera...on a screen...or in a “Mirage.” • All concave mirrors can produce real images under certain circumstances.
Mirror Mirror on the wall... Who’s reflection is the best of all?
Ok, but seriously ...... Let’s talk mirrors! Plane Mirrors • All mirrors reflect light according to the law of reflection. Plane mirrors form an image that is upright and appears to be as far behind the mirror as the object in front of it is.
Plane Mirrors Continued • Plane mirrors produce an image with the same orientation as well. Take a look at this video ... • http://www.youtube.com/watch?v=H_Dtsx-VGG0
Concave and Convex Mirrors • Concave mirrors curve inward. They can form an image that is inverted or right side up, and can be larger or smaller than the object. • Parallel light rays bounce off the curved surface and meet at a single point called the focal point. We describe the lights rays joining together as converging. • Object’s distance to focal point of mirror: far= small and upside down image. Close= remains inverted, but larger. If the object is between the focal point and the mirror=object is larger and upright.
Concave Examples • Spotlights • Flashlights • Lighthouses • Car headlights (bright light is placed at focal point)
Convex Mirrors • Convex mirrors bend outwards. Remember the question about why “objects in mirror are closer than they appear?” • The image is smaller than the object. • The image distance is smaller than the object distance. • The image is upright. • More can be seen in convex mirror opposed to a plane mirror. • They reflect parallel light rays as if they came from the focal point behind the mirror. The light rays do not meet. Instead, they diverge (spread a part after reflecting).
Exit Slip • Which mirror is best suited for security purposes in retail stores and why? (Plane, concave or convex?)
Lab 5-8 p. 190 Observing Light Rays • Materials: Ray box, concave/convex lenses, and a printed page • 1) Shine the ray box at a concave lens. Observe how the rays are affected. Draw your observations • 2) Look through the concave lens at some printed text. Observe the appearance of the print. Draw your observations • 3) Shine the ray box at the convex lens. Observe how the rays are affected. Draw your observations. • 4) Look through the convex lens at some printed text. Observe the appearance of the print. Draw your observations. • Compare what you observed about the appearance of the text with each of the two lenses. • Which type of lens would be best used as a magnifying glass? • What might the other kind of lens be used for?
Concave Mirror vs Concave Lens • Concave lenses are thinner in the middle than at the edge. Light rays that pass through a concave lens diverge The rays never meet at focal point and always bend outwards.
Concave Cont. And Examples • Concave lenses work to make something look smaller, so they're not quite as common as convex (magnifying) lenses. • Telescopes • Binoculars • Eyeglasses
Convex Lenses • Convex lenses are thicker in the middle of the lens and they thin out towards the edges. • When light rays pass through they converge and meet at a focal point. • When light rays passes through the center it continues in a linear path. • When light rays passes through the more curved sides, they refract, i.e. Bend • Images seen through convex lenses are larger than concave images.
Convex Examples • Eyeglasses • Magnifying glass
Mirrors versus Lenses • Convex lenses and concave mirrors both reflect light rays that converge at the focal point • Like concave mirrors, convex lenses also produce reflective images that are either smaller/ larger, or upright/inverted to the object itself. The object’s position to the focal length and lens will determine the image produced.
Exit Slip • Explain why a drop of water placed on the page of a book magnifies printing beneath it.
