# Where do we get light?.

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Where do we get light?

Luminous and Illuminated Objects
A luminous object is one that produces light. An illuminated object is one that reflects light. Luminous Objects Illuminated Objects

We see things because they reflect light into our eyes:
Homework

How does light behave?

Reflection of Light Reflection occurs when a wave bounces back after striking a barrier. Example: a reflection in a mirror.

Three things that affect how light is reflected:
Surface of the Object Color of the Object Shape of the Object

Clear vs. Diffuse Reflection
Surface of the Object Clear vs. Diffuse Reflection Smooth, shiny surfaces have a clear reflection: Rough, dull surfaces have a diffuse reflection. Diffuse reflection is when light is scattered in different directions

Color of the Object Ever notice that an asphalt driveway seems hotter on a summer day than a concrete sidewalk? This occurs because light that is not reflected from the surface of an object may be absorbed by the object and converted into thermal energy.

Shape of the Object Reflection from a mirror: Mirror Normal
Incident ray Reflected ray Angle of incidence Angle of reflection Mirror

Angle of incidence = Angle of reflection
The Law of Reflection Angle of incidence = Angle of reflection In other words, light gets reflected from a surface at ____ _____ angle it hits it. The same !!!

Plane (flat), Concave, & Convex
Types of Mirrors Plane (flat), Concave, & Convex

Plane Mirrors Flat Surface
Light is reflected straight back, resulting in an upright image that is the same size as the original object.

HINT: Think about angles….
Mirror Lab Read and follow directions for Flip-Flopped Messages to investigate plane mirrors. When Completed create a five word message for a partner using the mirrors. Have partner decodes. Using mirrors is fun. Justify the Law of Reflection Using the plane mirror. HINT: Think about angles….

Concave Mirrors Curves Inward (like the inside of a bowl)
If an object is very close to the mirror, light is reflected in a way that an enlarged, upright image is produced. If the object is very far away, the image is reduced in size and upside down.

Convex Mirrors Curves Outward
Results in an image your eyes detect as upright and reduced in size. The side mirrors on cars are convex mirrors. (Objects in mirror are closer than they appear.)

{Hint} With this understanding of convex and concave mirrors.
Spoons Steps: 1.  Look at your reflection on the inside of the spoon. Record what you see. The surface on the inside of a spoon bends in like a cave - it is concave. Dentist use concave mirrors to examine your teeth.  These mirrors make your teeth look bigger so the dentist can examine them more easily.  Likewise, shaving mirrors are concave because they make the face look bigger. 2.  Now turn your spoon out and look at your reflection on the outside of the spoon. Record what you see. The surface on the outside of a spoon bends out - it is convex. The next time you are at a funfair take a look at yourself in the crazy mirrors.  They bend in all sorts of ways so that you look bigger, smaller, fatter, thinner and even wavy. 3. Explain why you look different and how these things pertain to the standard: describe the behavior of light waves Be prepared to report out. 4. Give examples of what we use every day in our daily lives? {Hint} With this understanding of convex and concave mirrors.

Refraction of Light Refraction is the bending of a wave as it passes from one medium to another.

What causes the light to bend?
What’s different about air and water? DENSITY – the amount of mass in a certain volume of a substance (mass/volume) AIR WATER gas liquid

Example of Refraction When a straw is placed in water it looks like this: In this case the light rays are slowed down by the water and are bent, causing the straw to look broken. The two mediums in this example are air and water.

Another Example: When you use a magnifying glass, the object appears larger because of refraction. The light waves traveling in the air change direction when they enter the glass of the lens, and then again when they move from the glass back into the air again. Lens

Introduction of Magnifying Glass

With water in the glass, things start the same
With water in the glass, things start the same. Light still reflects from your finger and spreads outwards. This time as the light moves from the water to the glass and then to the air, something happens. Its speed changes. Wait a minute! The speed of light is a constant, right? 300,000 k/m per second. That speed is for light in a vacuum. It travels through other substances (air, water, glass, oil) at different speeds. As it changes speed, if it is traveling perpendicular to the surface (straight through), nothing much happens. If the boundary is at an angle to the direction the light is traveling, the light is bent from its path. The shape of the surface at the speed change also has an impact on what you see. If the boundary between two different substances is flat, then you don't notice much of a difference. The image may be shifted to the side as the light is bent, but everything looks the right size. If the boundary is curved, then the image is distorted. Depending on the shape of the boundary and the speed of light in each of the substances, the light waves can be spread apart or bent together. If they are spread outwards, the image looks bigger. If they are bent inwards, the image looks smaller. This has other implications besides making fingers look larger. Would a lens shaped to focus light on Earth (in air) work the same in space? If you wear eye glasses, do you think they would work well if you were underwater? For that matter, do your eyes work as well underwater as in air? What would you see if you were in a room filled with water and you stuck your finger into a glass of air?

Diffraction of Light Diffraction is the bending, spreading, and interference of waves when they go through a narrow opening.

Diffraction Patterns Radio waves can diffract around hills, mountains or even the whole planet. Light waves can diffract through tiny slits. X-rays can diffract around atoms.

Electromagnetic waves have a huge range of wavelengths.
If the wavelength is of a similar size to a gap in a harbor wall, then the wave will diffract as shown below.

If the wavelength does not match the size of the gap, then only a little diffraction will occur at the edge of the wave.

Transmission of Light Transmission is the passing of waves through a medium.

Safety Rules for lights and Prism
Lights out of eyes Handle objects with care

Prism Lab Separating light with a prism experiment
Try It Out Inside the box, place the prism on a sheet of dark paper. Shine the flashlight beam through the hole and adjust the prism as shown in the diagram. Use coloured pencils or pens to trace the individual bands of light that you see. In what order are the colours? Can you come up with a mnemonic to help you memorise the sequence of colours in prismatic light?        Make It In order to create a clearly visible spectrum of light with a prism, it's best to work in a darkened room with a single source of light. A cardboard box (such as a file box) makes a great prism box. Simply cut a small rectangular opening on the side of the box near the bottom edge, about 5 mm in width.  Experiment Finally, cut a second hole in the box and arrange two prisms so that their spectra cross paths. What happens to the various colours of light where they cross?    You will need: · A prism (or two) · A cardboard box (such as a file box) · Coloured pencils or pens · A sheet of plain white paper · A flashlight or reading lamp

Questions for Written Report
Discuss visible light spectrum: Explain why does the prism make the spectrum How do you make the spectrum narrow or wide? When adding double plane mirrors with a prism, describe what happens with the visible light spectrum? Synthesis the two types of light concepts used when there are prisms and mirrors in the experiment. Consider a (What if question) using the prisms and mirrors then analyze your response.

Be prepared to Report out your Lab findings and how they relate to the standard.
S8P4a,bCharaacteristics of the E/M Spectrum; describe the behavior of light waves

What happens when light hits these objects?
Glass of water School bus window Notebook paper Waxed paper Plastic wrap Tissue paper Cardboard Textbook Hand lens…

Many materials are classified by how well they transmit light.
Three Types of Materials: Transparent Translucent Opaque

Transparent A material that permits light to pass through

Transparent objects: ALL of these are transparent.
The windows on a school bus, A clear empty glass, A clear window pane, The lenses of some eyeglasses, Clear plastic wrap, The glass on a clock, A hand lens, Colored glass… ALL of these are transparent. Yes, we can see through them because light passes through each of them.

Translucent A material that transmits some light but also scatter light in all directions

Translucent objects: Thin tissue paper, Waxed paper, Tinted car windows, Frosted glass, Clouds, All of these materials are translucent and allow some light to pass but the light cannot be clearly seen through.

Opaque A material that allows no light to pass through

Opaque objects: Heavy weight paper, Cardboard Aluminum foil, Mirror, bricks, buildings, Your eyelids and hands, Solid wood door, All of these objects are opaque because light cannot pass through them at all. They cast a dark shadow.

Shadows Shadows are places where light is “blocked”: Rays of light

Compare and Contrast Transparent objects Translucent objects
Opaque objects Explain why each group is the type of material and how they are used in our daily lives. Be prepared to Report out your answers and how they relate to the standard. . S8P4a,b describe the behavior of light waves

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