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What is light?
• Light is a form of radiant energy that you can detect with your eyes. Light energy comes from chemical energy, electrical energy and nuclear energy.
• Light can pass through anything that is transparent, sort of passes through translucent objects (frosted window) but doesn't make it through opaque objects such as a brick wall.
Light travels VERY FAST – around 300,000 kilometres per second.
At this speed it can go around the world 8 times in one second.
• Light travels much faster than sound. For example:
Thunder and lightning start at the same time, but we will see the lightning first.
Luminous and non-luminous objects
A luminous object is one that produces light.
A non-luminous object is one that reflects light.
Luminous objects
Reflectors
•Sun
•Light bulbs
•Fire
•Water
•Mirrors
•Smooth shiny surfaces
Properties of Light summary
1) Light travels in straight lines2) Light travels much faster than
sound3) We see things because they
reflect light into our eyes4) Shadows are formed when light is
blocked by an object
Reflection• Reflection from a mirror:
Incident ray
Normal
Reflected ray
Angle of incidence
Angle of reflection
Mirror
•The Law of ReflectionThe Law of Reflection
Angle of incidence = Angle of Angle of incidence = Angle of reflectionreflection
In other words, light gets reflected from a surface at ____ _____ angle it hits it.
The same !
!!
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• White light is not a single color;
it is made up of a mixture of the seven colors of the rainbow.
We can demonstrate this by splitting white light with a prism:
This is how rainbows are formed: sunlight is “split up” by raindrops.
• A rainbow would form a complete circle, not just an arc, if the ground didn't get in the way
• Rainbows always face the observer. As the observer moves, the rainbow moves.
• And sorry, no one can ever get to the "pot of gold" at the end of the rainbow
Seeing color• The color an object appears depends on
the colors of light it reflects.
For example, a red book only reflects red light:
White
light
Only red light is
reflected
A white hat would reflect all seven colors:
A pair of purple shorts would reflect purple light (and red and blue, as purple is made up of red and
blue):
Purple light
White
light
RefractionRefraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through. When a ruler is placed in water it looks like this:
In this case the light rays are slowed down by the water and are _____, causing the ruler to look odd. The two mediums in this example are ______ and _______.
Words – speed up, direction, water, air, bent
Key Vocabulary
Reflection – The light energy that bounces off objects.
Refraction – The bending of light rays when they pass through a substance.
Convex and Concave Mirrors
Images in convex mirrors Images in concave mirrors are always smaller. are always bigger.
Convex and Concave Lenses
• Convex Lenses are used as a Magnifier
• Concave Lenses are used as a De-magnifier
Convex Lenses• People who are farsighted
have trouble seeing things that are close to them (reading) Glasses or contacts with convex lenses magnify the print
• Movie projectors use convex lenses. As light from the bulb shines through the film, light rays spread apart. As these rays pass through the projector lens, they bend toward each other again.
Concave Lenses• Concave lenses help
people who are nearsighted. These people have trouble seeing distant objects. Concave lenses bend light rays outward just enough to make distant objects seem closer.
• Some cameras have a concave lens in their viewfinder, you see a small version of what the final photograph wil look like.
Key Vocabulary
lens – a piece of clear material that bends, or refracts, light rays passing through it.
Convex lens – is thicker in the middle than at the edges.
Concave lens – is thicker at the edges than in the middle.
In Galileo's version, light
entering the far end (1) passed through a convex lens (2), which bent the light rays until they came into focus at the focal point (f). The eyepiece (3) then spread out (magnified) the light so that it covered a large portion the viewer's retina and thus made the image appear larger.
Refracting Telescope
Reflecting Telescope
In Newton's version, light streaming in one end (1) reflected off a concave mirror fixed inside the other end (2), then off a flat mirror set an angle (3). The light came into focus at the focal point (f) and then was magnified for the viewer by the eyepiece (4).