Post on 30-Dec-2015
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INTRODUCTION
• We step outside on a sunny day and our surroundings are awash with every colour of the spectrum. How does the energy known as light produce such spectacular effects?
• The answer is that white light, radiated from the sun, is perhaps not really white at all! White light is every colour radiating together, and it is only when this light is split that we see the dazzling array of colour that we know
HEADSTART BOOKLET
• Successful completion of this booklet will allow you to:– Explain the refraction of light in various situations– Compare the effects of concave and convex lenses
on light and explain how they form images– Describe how white light can be split into its
constituent colours– Explain the interaction of white light with coloured
objects and describe how coloured filters are used
HEADSTART BOOKLET
• Bring your booklet to every class• It will need to be submitted at the end of
Headstart for assessment• Must choose ONE homework activity from the
end to complete before the book is submitted for assessment
• Your booklet is NOT for taking notes – it will be used for an assessment at the end of the unit. Your exercise book must be used for notes
• Put a coin in a polystyrene cup and move back until the coin is just out of sight. Stand in this position while a friend pours water into the cup.
• What happens?• Draw a diagram to explain what has happened.
REFRACTION OF LIGHT• As light moves from one medium to another,
its speed changes
• This causes the light wave to change direction, or bend (except when light hits a surface at right angles)
Air
Water
Ray of light: the incident ray
Where light would go if travelling in a straight line
Where light really goes: the refracted ray
REFRACTION OF LIGHT• To investigate the bending of light as it moves from one
medium to another, we use a reference line called the normal
• The normal is an imaginary line that is at right angles to the boundary between the two materials
Air
WaterBoundary
Normal
ANGLE OF REFRACTION• Angle between the normal and the path of the
refracted light
Glass
Air Normal
Refracted ray
Incident ray
Angle of refraction
ANGLE OF INCIDENCE
• Angle between the normal and the ray of light approaching a surface
Glass
Air NormalIncident ray
Angle of incidence
CRITICAL ANGLE• The angle at which there is total reflection of light
as it hits a surface – called total internal reflection• i.e. There is NO refracted ray of light
Glass
Air
Normal
Incident ray Reflected ray
Critical angle
REFRACTION OF LIGHT• The direction that light bends as it moves from
one medium to another depends on the density of each material
• How did the light bend (in relation to the normal) as it entered the glass block?
• How did the light bend (in relation to the normal) as it left the glass block?
REFRACTION OF LIGHT
• When light moves from a less dense medium to a denser medium, the light slows down
• This causes the light to bend towards the normal
Glass
Air Normal
REFRACTION OF LIGHT
• When light moves from a denser medium to a less dense medium, the light gets faster
• This causes the light to bend away from the normal
Air
Glass Normal
REFRACTION AND FISHING
• Hunters who use spears to catch fish know about refraction. Light bends as it leaves the water so the fish always appears closer to the surface than it really is.
REFRACTION AND FISHING• Aim in front of the fish• Light from the fish comes out of the water and bends
away from the normal (speed increases)• Our brain knows that light travels in straight lines. For
this reason, we think that the fish is located in a straight line from our eyes to the water
TOTAL INTERNAL REFLECTION
• Light is refracted into the diamond, then totally internally reflected twice within the diamond, producing the sparkle
TOTAL INTERNAL REFLECTION• What does total internal reflection mean? Use
the following diagram to explain.
LENSES
• Look through a concave and then a convex lens at your classmates and at your book.
• What do your friends look like through each of the lenses?
• What does your book look like through each of the lenses?
• Suggest some uses for each type of lens.
CONVEX LENSES• Also called converging lenses because they
focus parallel light rays to a point• Your eyes contain these type of lenses
• Parallel rays come to a focus after passing through a convex lens
CONVEX LENSES• Were the images upright or inverted?
• If the object is not past the focal length, the image is upright
CONVEX LENSES• Were the images upright or inverted?
• If the object is past the focal length, the image is inverted
CONVEX LENSES• Was the image bigger or smaller?
• Images may be bigger or smaller
• However, images are mostly bigger than the real object
CONVEX LENSES• Was the image bigger or smaller?
Where would the object have to be for the image to be smaller than the object?
CONCAVE LENSES• A concave lens can be easily recognised by its thicker
rim• Also called diverging lenses because they spread out
parallel rays of light• Produce images that are always upright, smaller in
size and virtual in nature
REAL AND VIRTUAL IMAGES
• Virtual images are seen on a mirror• Real images can be projected on a screen
CONCAVE LENSES
• Use a concave lens to try to obtain an image of your laboratory windows.
• What do you find?
VISIBLE LIGHT & THE COLOUR SPECTRUM
• Visible light is only one section of a wide variety of waves known as the electromagnetic spectrum
• The electromagnetic spectrum also includes: UV rays, infrared rays, gamma rays
VISIBLE LIGHT & THE COLOUR SPECTRUM
• The electric field travels in waves – the energy alternates in positive and negative directions
• Positive direction
• Negative direction
VISIBLE LIGHT & THE COLOUR SPECTRUM
• All components of the electromagnetic spectrum consist of a changing electric field at right angles with a changing magnetic field.
A field is an area in space where an introduced electrically charged/magnetic field will experience a force.
VISIBLE LIGHT & THE COLOUR SPECTRUM
• For example: a magnetic field is an area where if you introduce a magnet into it, the magnet will experience a force– A compass has a magnet in it that experiences a
force which pushes the needle to point North
COLOUR SPECTRUM• The light that you can see is the visible spectrum• This includes red, orange, yellow, green, blue, indigo
& violet• This can be abbreviated to ROY G BIV• Light wavelengths are extremely small, being less
than 1,000 of a millimeter
COLOUR SPECTRUM• Each colour of light has a different wavelength
• Special cells called cones in the retinas of your eyes respond to different wavelengths and determine the colour you see• Colour your graph to match that on the screen
COLOUR SPECTRUM• The white light that comes from the sun and
light bulbs is a mixture of all the colours of the visible spectrum
• It consists of mixed waves of different wavelengths and frequencies all travelling at the same speed
• The frequency of a light wave refers the number of waves that pass a specific point in one second
• Visible light is released from atoms when ‘excited’ electrons drop to a lower energy level
• List the colours of the visible spectrum• Apart from their colour, what is different
about the different colours in the visible spectrum?
• Which colour of visible light has the greatest wavelength?
• What colour of visible light has the highest frequency?
• Which colours is the human eye most sensitive to?
TRANSMISSION AND ABSORPTION OF LIGHT
• When light hits a surface it can be reflected, refracted, transmitted or absorbed
• If a material only absorbs light, no light will pass through it
• If a material transmits light, the light is able to pass through it
OPAQUE MATERIALS
• If a material reflects or absorbs light, it is opaque
• These materials do not transmit any light at all• This means that no light passes through the
object and we cannot see what is on the other side
• E.g. Wood, metal, mirror
TRANSPARENT MATERIALS
• Transparent materials transmit light• This means that they let light pass through,
allowing us to see what is on the other side• E.g. Glass
TRANSLUCENT MATERIALS
• Translucent materials scatters the light that passes through it
• The objects behind it are not clearly visible• May be fuzzy or unclear• E.g. Frosted glass, some plastics
COLOUR SPECTRUM• Pigments on the surface of an object give it
colour• An object is seen as red if it reflects red light
towards our eyes and absorbs orange, yellow, green, blue, indigo and violet light
• In reality, some of the other colours of light may also be reflected, but this will just affect the shade of red we see
COLOUR SPECTRUM• Why does a sheet of paper look white?
• White light is made up of all the colours of the visible spectrum, therefore white objects reflect all light colours
• Black objects absorb all colours, reflecting none
COLOUR SPECTRUM• White light can also be
made using just three colours of the spectrum:• Red, blue and green
• For this reason, these are called the primary colours
• Red + green = yellow• Red + blue = magenta• Green + blue = cyan
RETINAL FATIGUE• What happens when our eyes tire?• What is retinal fatigue & how does it work?
• Cone cells in the retina of our eye are responsible for detecting colour
• Video – Is that a colour photo?
RETINAL FATIGUECone cells in the retina of the eye
detect colour
Red light from an object hits
cone cells
Red-sensitive cone cells give off
a chemical
This chemical stimulates nerve receptors to send messages to the
brain, telling you that you’re seeing something red
After a few seconds the chemicals deplete, so that part of your retina becomes less sensitive to red light
If you look at an object of another colour, you will see it minus the red
(e.g. If you look at something white you will see cyan, which is all light minus red)
Called inverse colours
RAINBOWS
• A rainbow is a common and spectacular optical phenomenon, but how is it formed?
• It is a complex process involving refraction, total internal reflection and the dispersion of sunlight within water droplets
RAINBOWS • Prism animation
Light is refracted as it enters a
droplet of water
It gets dispersed in to the colours of the spectrum
Light is refracted again as it leaves the water
droplet, each colour at a different angle
Total internal reflection occurs
at the back surface of the
droplet
Red light is always refracted at 42O, meaning it will always be at the top of the rainbow, while violet is refracted at 400, so it will always be at the bottom
RAINBOWS
• We see a rainbow only occasionally because conditions need to be right for total internal reflection to occur
• This will be when the Sun is at our back, usually low in the sky
• On some occasions, light is reflected twice within the droplets of water to produce a second rainbow, like a mirror image above the first
RAINBOWS
• No two people ever see exactly the same rainbow, because the rainbow that you see is formed by a particular set of droplets that direct rays of the particular colour towards your eyes
• The droplets that form the orange part of your rainbow may produce the blue section of the rainbow seen by a friend standing next to you
RAINBOWS
• Why does the white light separate into the spectrum of colours when forming a rainbow?
• Why is red always at the top of the rainbow?• Explain why the rainbow you see is one of a kind.• Why does a rainbow only form when the sun is
at a particular angle? What would happen at the other times?
• Have you ever seen a double rainbow? How does it happen? • Video – Double Rainbow
RAINBOWS
• CHALLENGE QUESTION: Gina and Clare decide to wear their new clothes to the nightclub. Gina looks stunning in her red top, blue pants and black shoes and Clare is resplendent in her white top, yellow skirt and black shoes. The lighting inside the nightclub is nearly all red and blue. What would Gina and Clare’s clothes now look like?