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LightLight
1) Properties of light2) Reflection3) Colours
Part 1 – Properties of Part 1 – Properties of LightLight
Light travels in straight lines:
Laser
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:
1) Thunder and lightning start at the same time, but we will see the lightning first.
2) When a starting pistol is fired we see the smoke first and then hear the bang.
We see things because they reflect light into our eyes:
Homework
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
-The Sun- Lamps- Lights- Lasers- Campfires
-The Moon- Mirrors- People- Objects
Shadows
Shadows are places where light is “blocked”:
Rays of light
Out of the DarknessOut of the Darkness-If the light source is small compared to the size of the object, then the shadow will be SHARP
-If the light source is of comparable size to the object, then the shadow will be “fuzzy”. The dark inner region is called the UMBRA; the grey region is called the PENUMBRA.
Properties of Light Properties of Light summarysummary
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
Part 2 - ReflectionPart 2 - ReflectionReflection 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 THE SAME ANGLE it hits it.
The same !
!!
Clear vs. Diffuse ReflectionClear 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
Using mirrorsUsing mirrorsTwo examples:
1) A periscope
2) A car headlight
Flat MirrorsFlat Mirrors
Image is VIRTUAL, UPRIGHT, UNMAGNIFIED
Spherical Mirrors - Spherical Mirrors - ConcaveConcave
Image is REAL, INVERTED, and DEMAGNIFIED !!!
C F
Spherical Mirrors – ConcaveSpherical Mirrors – ConcaveObject Inside the Focal Object Inside the Focal
PointPoint
Image is VIRTUAL, UPRIGHT, and MAGNIFIED
C F
Spherical Mirrors - Spherical Mirrors - ConvexConvex
Image is VIRTUAL, UPRIGHT, and DEMAGNIFIED
C F
ColourColour
White light is not a single colour; it is made up of a mixture of the seven colours 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.
The colours of the rainbow:The colours of the rainbow:
RedOrangeYellowGreenBlue
IndigoVioletRemember: Roy G. Biv !!!
Light from the SunLight from the Sun
The light from the sun contains a grand mixture of electromagnetic waves of varying frequency (i.e. wavelength)
The spectrum of light is characteristic of the TEMPERATURE of the sun!
Wavelengths of LightWavelengths of Light
Red Light – nm
Green Light - nm
Blue Light - nm
Adding coloursAdding coloursWhite light can be split up to make separate
colours. These colours can be added together again.
The primary colours of light are red, blue and green:Adding blue and
red makes magenta (purple)
Adding blue and green makes cyan
(light blue)
Adding all three makes white again
Adding red and green makes yellow
Seeing colourSeeing colourThe colour an object appears depends on the
colours 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 colours:
A pair of purple pants, in addition to being ugly, would reflect purple light
(or red and blue, as purple is made up of red and blue):
Purple light
White
light
Using coloured lightUsing coloured light
If we look at a coloured object in coloured light we see something different. For example, consider the outfit below – I mean, from a physics standpoint, not as a fashion choice:
White
light
Shorts look blue
Shirt looks red
In different colours of light this kit would look different:
Red
lightShirt looks red
Shorts look black
Blue
light
Shirt looks black
Shorts look blue
Using filtersUsing filtersFilters can be used to “block” out different colours of
light:
Red Filte
r
Magenta
Filter