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To see and be seen

Stephanie b.

on 24 May 2013

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Transcript of LIGHT

To see and be seen LIGHT light sources and light rays How do you see and how are you seen? Light, a colourful thing Why are there shadows? Seeing colours Shadows of celestial bodies Mirrors light is radiated by a light source. Our most important light source is the sun. The sun and the stars are natural light sources, because they give light without you having to do something. Artificial light sources like, light bulbs, candles, oil lamps and torches, are light sources made by mankind.
From the light source, light spreads out in a straight line: a light ray. You show the direction of a beam of light with arrows. By turning the head of the torch, the beam changes. You've got three types of beams:
- a parallel beam: it remains the same width.
- a convergent beam: it goes to a point
- a divergent beam: it appears to come from a point
Light takes time to move. The speed of propagation of light is almost 300.000 kilometres per second. Nothing can go faster than light.

There are natural and artificial sources of light.
light propagates from a light source in straight lines in all directions.
The speed of light is 300.000 kilometres per second.
A light beam can be parallel, convergent or divergent. You see light sources, because they emit light which is captured in your eye. You can also see objects which do not emit light themselves. They reflect back the light from a light source. This is called reflection. The moon reflects the light of the sun.
Bicycles, mopeds and cars need reflectors, so other can see them in the dark. Reflectors reflect a lot of light, so you can see it very well when light shines on it. Also wearing light-coloured clothing are easier to see in the dark, instead of dark clothes.

When you see, light from a light source or an illuminated object, enters your eye. You can see illuminated objects, because they reflect light. You can demonstrate the colours of a rainbow by using a prism, a triangular glass object. 1. put a prism into a light beam. 2. hold a piece of paper in the beam leaving the prism. 3. Rotate the prism until the colours appear.
White sunlight consists of different colours, which include the six principal colours: red, orange, yellow, green, blue and violet. These are the same colours you see in a rainbow. The colours of the rainbow and the experiment consist of the same array of colours emerging into one another. Such an array is called a spectrum.
Sunlight also contains radiation which you cannot see: infrared and ultraviolet radiation. The infrared lies on the left side of the red part and the ultraviolet lies on the right side of the violet part. Infra and ultra mean further on and beyond.
infrared radiation is heat radiation. It's used in remote controls for appliances, night-vision glasses, making photographs in space and treating muscular pain(infrared lamp). the radiation is converted to heat in the muscle.
Your skin turns brown in the sun due to the UV radiation. UvV radiation is therefor also used in sunbeds. Too much Uv radiation is harmful: it can cause skin cancer. UV radiation is stopped by window glass. Part of the UV radiation of the sun is stopped by the atmosphere. This occurs at great altitude, mostly due to ozone. if the ozone layer gets thinner, the number of people with skin cancer will increase.

You can split white light into a spectrum with a prism.
This spectrum consists of the principal colours: red, orange, yellow, green, blue and violet.
Sunlight also contains infrared radiation and ultraviolet radiation. Sunlight is white. However the world is colourful. How can this be? If you've got a green boot, which is illuminated by white light (which consists of the principal colours) the green boot absorps all the colours, except green. Therefore green light enters your eye.
When the green boot is illuminated by red light, the boot again absorps all the colours but green. The red light is therfor absorbed and the green boot looks... black. The absorption of certain colours can give rise to surprising effects.

White objects do not absorp any colours.
Black objects absorp all the colours.
Coloured objects absorp all the colour except their own colour. A light source spreads light out in all directions. However a non-transparent object stops the light.Light travels in straight lines. The place where the light cannot reach is called the shadow. If you move the light source or the object the shadow image changes.

A shadow is a place where light cannot reach.
To make a shadow, you need a light source and a non-transparent object.
To see a shadow a screen is also needed. Where the sun illuminates the earth it's day. On the shadow side of the earth is night. In position2 the moon is in the earth's shadow. this is called a lunar eclipse. The sun, earth and the moon are practically in a straight line. The sun is the light source and the earth the object. In clear weather you can see the moon slowly move into the shadow of the earth. in position 4 part of the earth is in the shadow of the moon. In this solar eclipse the moon completely covers the sun. In the middle of the day it's dark for several minutes. Animals become restless. If you look at the sun, you see a dark disc with a ring of light around it: the corona. A total eclipse of the sun lasts 7 minutes.

Night and day, and solar and lunar eclipses are examples of the formation of shadows. In a mirror you see a mirror image or reflection. You see the image at the same distance behind the mirror as you are in front of it(?). Objects with a smooth and shiny surface give a mirror effect. Most mirrors consist of three part: a glass plate, a thin layer of shiny metal and a layer on the metal to protect it from scratches.
in the drawing you can see the mirror sp, a point A and its mirror image A'. A and A' are the same distance from the mirror. The line between A and A' is perpendicular to the mirror. This connecting lne is called the 'normal'. Object and image are thus the same distance from the mirror.
How to draw a mirror image of point A without using a mirror:
- Draw a line Through A which is perpendicular to the mirror: the normal.
- Maesure how far A is from the mirror.
- The mirror image A' is on the normal at the same distance from the mirror as A.

You can see a reflection in objects with a smooth and shiny surface.
An object and its mirror image are at an equal distance from the mirror.
The imaginary line connecting a point and its mirror image is perpendicular to the mirror. This line is called the normal. 2 1 3 4 sp A' A Field of vision and traffic In a mirror you can only see a part of the area around you. This part is called the field of view or the range of vision. Your range of vision depends upon your location with respect to the mirrors. this is very important in traffic. An ordinary wing mirror of a car can have a blind-spot mirror (a stuck-on mirror) which increases the field of view.

Your field of view is the area which you can observe with a mirror.
Your field of view depends upon your position with respect to the mirror. Many mirrors are used in road traffic situations.
A blind-spot mirror increases your range of vision. Refraction of light A teaspoon in a glass of water looks as if it is broken. You see a fish in water above the place where it actually is. Both examples show you that light is refracted.
The ligt rays from the head and from the tail refract at the interface between water and air. If you look at the fish from the place indicated, it appears as if the rays are not broken. you look along the dotted lines. Therefore you see the fish at B, above the place where it actually is (A).
Refraction of light also plays a role in the formation of a spectrum by a prism. Some colours refract more than others.

At the boundary between two materials light rays can change direction: the light rays are refracted. Lenses With a magnifying glass you can combine rays from the sun into one point. The pint where the rays come together is called the focus. A piece of paper positioned at the focus can even burn. The magnifying glass is a lens. The action of a lens is thus based on the refraction of light.
You can find lenses in all kind of appliences: glasses, camera's, slide projectors, binoculars and microscopes. Your eye also has an eye lens.
There are two types of lenses: the convex lens an the concave lens. A convex lens is thicker in the middle than at the edges and is called a converging lens. A concave lens is tinner in the middle than at the edges and is called a diverging lens.
A convex lens refracts light rays towards each other. A convex lens has therefore a converging effect. The imaginary line which goed through the middle of a lens and is perpendicular to a lens is called the principal axis. All light rays which are parallel to the principal axis meet at a single point after refraction. This is the pricipal focus of the lens (F).
After refraction of a concave lens the ligh rays spread out. A concave lens thus has a diverging effect. Light rays which are parallel to the principal xis appear to come from from a single point after refraction. This is the principal focus of the concave lens.
With lenses you talk about their power. The more curved a lens is the shorter the distance from the lense to the principal focus (the focal length). The smaller the focal length, the more powerful is the lens.
The power S is the inverse of the focal length f.
As a formula: S = 1 / f
Here f is in metres, so the unit of power is 1/m. This unit is called the dioptre. A 1 dioptre lens thus has a focal length of 1 m. A lens with a focal length of 40 cm has a power of 1/0,40 m=2,5 dioptres. You usually say: "I have a +2 1/2 lens".

A convex (converging) lens has a converging effect.
A parallel beam along the principal axis is brought together at the principal focus.
A concave (diverging) lens has a diverging effect.
A parallel beam along the principal axis appears to come from the principal focus.
The focal length of a lens is the distance from the principal fcus to the centre of the lens.
The power S of a lens is the inverse of the focal length f.
As a formula: S = 1 / f
The unit of the power of a lens is the dioptre (= 1/metre) If the focal length and the distance are known, you can also draw the image accurately. A beam of light rays travels from the top of the object through the lens. After refraction these rays of light come together at a point. This point is the top of the image.
Two lighht rays from the beam are used in drawing the image:
- The light ray parallel to the principal axis goes to the principal focus F after refraction.
- the light ray through the centre of the lens is not refracted.
With these two rays you can determine the top of the image. This image is an up-side down arrow. If you put a screen in the place of the image, you get a sharp picture.
The object and the image are not usually the same size. Just imagine if your eye saw everything as life size, or if you were just as tall on a photograph as in real life! In image formation you therefore often talk about magnification. This is the size of the image divided by the size of the object.

You can make an image on a screen with a convex lens.
This image is always inverted.
You can draw the image accurately with the aid of two light rays:
- the light ray parallel to the principal axis. This ray passes through the principal focus.
- The light ray through the centre of the lens. This ray is not refracted.
magnification= size of image/size of object Image forming with a convex lens Diagnostic TEST 1. Name three types of beams and explain them
- parallel beam. straight light rays
- convergent beams. go to one point
- divergent beams. appear to come from a point.
2. What's the speed of light?
300.000 km per second
3. There are natural and artificial light sources. What's the difference?
artificial light sources are made by mankind and natural light sources aren't made by mankind, but already 'work'by theirselves.
4a. How can you see a light source?
a light source emits light which your eye captures
b. How can you see objects that aren't light sources?
these are illuminated by a light source. so they are reflecting the light which our eye captures.
5. How can you split light into a spectrum?
by using a prism.
6. UV / IR ?
treating muscle pain -> IR
skin cancer -> UV
remote controller -> IR
sunbed -> UV
tan -> UV
night-vision -> IR
7. What does the rainbow and spectrum consist of?
the six prncipal colours: red, orange, yellow, green, blue and violet
8a. A black bag is illuminated with white light. Which colours would it absorp?
all the colours
b. A pink pen is illuminated by white light. How is it possible that you see that the pen is pink?
because coloured objects absorp all the colours except their own. So the pink pen absorps all the colours, but pink. so pink is what you see.
c. Which colours does a white shoe absorp?
no colours
9a.What is a shadow?
the shadow is the place that light cannot reach, because something is blocking it
b. What do you need to make a shadow?
an object and a light source. (and a screen if you want to see the shadow.
10. What do night, day, solar- and lunar eclipses have to do with shadow?
night is when a part of the earth isnt reached by the sun. day is the part of the earth that isnt a shadow, cause the light can reach it, lunar eclipse is when the moon is behind the earth so the sunlight doesnt reach it and there is nothing to illuminate the sun, so the sun isnt seen. a solar eclipse is when the moon is right in between the sun and the earth so the moon stops the sunlight from reaching the earth. now the whole earth is dark.
11. Why do animals get restless at a solar eclipse?
cause its dark at a solar eclipse. 12. how long does a total solar eclipse last?
7 min.
13. T/F? You can see a reflection in objects which are rough and shiny.
F. smooth and shiny
14. What is the imaginary connecting line of a point and its mirror image?
the normal
15. How can you sketch where the mirror image is using only point a and NO mirror?
1. draw a line from point A perpendicular to the mirror, than measure the distance and draw exactly the same line on the other side. this line is the normal.
16a. What is the area which you can observe with a mirror called?
field of view/range of vision
b. On what does this depend?
your position with respect to the mirror.
17. What is a blind-spot mirror?
a mirror stuck on wing mirror of a car to increase the range of vision.
18. You see a fish in water above the place where it actually is. How is this possible.
light rays that go trough two different materials refract.
19a. What are the two type of lenses?
convex (converging) lens and the concave (diverging) lens
b. what is their effect?
a converging an diverging effect
20. What is the distance between the principal focus and the centre of the lens called?
the focal length (F)
21. What is the formula to calculate the power of a lens?
S = 1/F
22. With which lens can you make an image?
the convex lens
23. With the aid of which two light rays of an object can you draw its image accurately?
a parallel light ray from the top pof the object to the lens where it refracts and all goes to one point: the principal focus.
a line from the top of the object to the centre of lens. this light ray doesn't refract.
24. How can you calculate the magnification?
size of image / size of object
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