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					            08/04/2010




KS3 Light
                                   08/04/2010



   Properties of Light

Light travels in straight lines:



Laser
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  Light travels VERY FAST – around
    300,000 kilometres per second.


At this speed it can
go around the world 8
times in one second.
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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.
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We see things because they
 reflect light into our eyes:




            Homework
                                             08/04/2010

 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
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                  Shadows
Shadows are places where light is “blocked”:


                        Rays of light
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Properties of Light summary

1) Light travels in straight lines
2) Light travels much faster than sound
3) We see things because they reflect light
   into our eyes
4) Shadows are formed when light is blocked
   by an object
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                 Reflection
  Reflection from a mirror:

                       Normal

Incident ray                               Reflected ray

                Angle of       Angle of
               incidence      reflection




                       Mirror
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       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 !!!
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     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
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           Using mirrors
Two examples:




                       2) A car headlight

      1) A periscope
    Refraction in a
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rectangular block
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Newton’s disc animation
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Splitting white light animation
                       Colour
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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.
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             Splitting white light with a prism
1. Shine a ray of bright white light at a prism and move
   the prism until colours appear.




2. Draw a diagram to show what you observed.
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The colours of the rainbow:



                        Red
                       Orange
                       Yellow
                       Green
                        Blue
                       Indigo
                       Violet
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             Splitting white light into colours
A prism splits a ray of white light into a spectrum of colours.
This is known as dispersion.




When white light is split, the colours always follow the
same order.
Use this phrase to remember the order of colours:

 Richard Of York Gave Battle In Vain
                                 08/04/2010




         Red



   Magenta      Yellow

         White

Blue                     Green
         Cyan
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               Adding colours
  White 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                       Adding blue and
makes magenta                            green makes cyan
(purple)                                       (light blue)


Adding red                                     Adding all
and green                                    three makes
makes yellow                                  white again
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                  Seeing colour
The colour an object appears depends on the colours
  of light it reflects.


For example, a red book only reflects red light:




         White                           Only red light
          light                           is reflected
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  A pair of purple trousers would reflect purple light
(and red and blue, as purple is made up of red and blue):



                                     Purple light




     A white hat would reflect all seven colours:


                                        White
                                        light
                                                08/04/2010



       Using coloured light
If we look at a coloured object in coloured
  light we see something different. For
  example, consider a football kit:


                             Shirt looks red

   White
    light

                                   Shorts look blue
                                                            08/04/2010

In different colours of light this kit would look different:



         Red
                                          Shirt looks red
         light

                                       Shorts look black




                                        Shirt looks black
         Blue
         light


                                       Shorts look blue
                                            08/04/2010
        Some further examples:
                                 Colour object
  Object       Colour of light
                                 seems to be
                    Red              Red
 Red socks          Blue            Black
                   Green            Black
                    Red             Black
 Blue teddy         Blue
                   Green
                    Red
Green camel         Blue
                   Green
                    Red
Magenta book        Blue
                   Green
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                  Using filters
Filters can be used to “block” out different colours of light:

                           Red
                          Filter




                          Magenta
                           Filter
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    Investigating filters
Colour of filter   Colours that could be “seen”
      Red
     Green
     Blue
     Cyan
   Magenta
    Yellow
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 Red     Blue    Green    White




Yellow   Cyan   Magenta
     Refraction through a glass block:
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                         Wave slows down and bends
                          towards the normal due to
                        entering a more dense medium




                                               Wave slows down but is
                                              not bent, due to entering
                                                  along the normal



 Wave speeds up and bends
away from the normal due to
entering a less dense medium
08/04/2010
                                                    08/04/2010


                  Refraction
Refraction is when waves ____ __ or slow down due to
travelling in a different _________. A medium is
something that waves will travel through. When a pen is
placed in water it looks like this:




In this case the light rays are slowed down by the water
and are _____, causing the pen to look odd. The two
mediums in this example are ______ and _______.

      Words – speed up, water, air, bent, medium

				
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