# Where do we get light?.

## Presentation on theme: "Where do we get light?."— Presentation transcript:

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.

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.)

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

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.

An electromagnetic wave is produced. The wave travels from the stations transmitter out in all directions at the speed of light. Even though you cannot hear radio waves when your radio’s turned off, the waves are still being transmitted in the room.

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.