Light Year 9 Science 1. Light energy Light is a form of energy that travels in straight lines and is...

Light

Year 9 Science

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Light energy

Light is a form of energy that travels in straight lines and is visible to the human eye.

Light travels from luminous and reflecting objects, and in a vacuum its speed is always constant (remains the same) at 300, 000 km/s.

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Light energy

Some objects give off light because they are very hot – these light sources are said to be incandescent.

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Light energy

Other sources of light can be fluorescent (like fluorescent tubes) …

… or bioluminescent as in the case of glow worms and some jellyfish.

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Light energy Year 9 Science

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Exercise 1 Give another example of an incandescent, fluorescent or bioluminescent light source.

Shadows

Light travels in straight lines and shadows are formed on surfaces where the light has been completely or partly blocked.Sharp shadows are formed when the light source is far away from the blocking object and screen. This shadow is called the umbra.As the light source gets closer, the edges of the shadow get ‘fuzzy’ and this part of the shadow is called the penumbra.The closer the light source, the larger the penumbra and the smaller the umbra.

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Shadows

Locate the umbra and penumbra on the screen.

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Reflected Images

When light reflects from a polished shiny surface like a mirror, water or polished metal an image is often formed.

What has happened in this reflection?

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Reflected Images

Reflected images are ‘back-to-front’ and are either vertically or laterally inverted.

Vertical inversion (Which is the real dog?)

Lateral inversion(Which is the real violinist?)

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Shadows and Light Rays Year 9 Science

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Exercise 2 Emergency vehicles often have signs that are inverted. Is this vertical or lateral inversion?

Exercise 3 In your book write your first name properly and then write it laterally inverted.

Light Rays

When investigating light we often imagine a very, very thin line of light. We represent this as a line drawn with an arrowhead to show direction.

A thin line of light is called a ray.

A stream of light rays is called a beam.

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Drawing a Light Ray

To experimentally draw a light ray the Hodson ray box is set up with a single slit that produces a beam across a piece of paper flat on the bench.

Using a pencil mark an ‘X’ in the centre of the beam near the ray box and …

another ‘X’ in the centre

of the beam far away from

the ray box.

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Drawing a Light Ray

The ray box is then removed, the X’s are joined using a ruler and an arrow head is added.

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X X

Drawing a Light Ray

The ray box is then removed, the X’s are joined using a ruler and an arrow head is added.

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X X

Light Shining on Objects

When light falls on an object it may be:1. Transmitted or partly transmitted

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2. Scattered

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3. Reflected (bounce off) or partly reflected

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4. Absorbed or partly absorbed

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5. Refracted (bent)

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Transparent materials (like glass) transmit almost all the light that shines on them.

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Translucent materials (like shower screens) transmit enough light to see objects on the other side but not clearly in detail.

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Opaque materials (most materials) transmit no light and they cast strong shadows.

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When drawing ray diagrams a reference line is often drawn to help in understanding what is happening.

A line drawn at right angles ( 90o ) to a surface is called a normal. Often normals are drawn as dashed lines.

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Reflection

When a ray of light falls on a mirror (or any other surface), the angle between the ray and the normal is called the angle of incidence (i).

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i

Reflection

The Law of Reflection states that the angle of incidence (i) is equal to the angle of reflection (r).

i = r

This is a law because it is true in all circumstances of reflection.Light rays always obey this law when they reflect.

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i r

Reflection

When light reflects from a curved surface the Law of Reflection is still obeyed.Draw normals at the points indicated on both the outside and the inside of the curved mirror.

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Reflection

Draw normals at the points indicated on both the outside and the inside of the curved mirror.

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Reflection

Draw parallel incoming rays as indicated.

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Reflection

Draw reflected rays taking into consideration the Law of Reflection.

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Reflection Year 9 Science

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Refraction

When light travels from one transparent material into another it changes speed. This change in speed cause it to change direction.The change in direction that a ray of light experiences when it moves from one material to another is called refraction.

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Refraction

Refraction causes objects in water to appear to look different in size or position.

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Refraction

When light moves into a more dense material (e.g. air into glass) the rays bend towards the normal.

In this situation the angle of refraction (R) is smaller than the angle of incidence (i).

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i

R

Refraction

When light moves into a less dense material (e.g. glass into air) the rays bend away from the normal.

In this situation the angle of refraction (R) is largerthan the angle of incidence (i).

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i

R

Refraction

When light moves through a rectangular prism, refraction occurs at both entry and exit boundaries. The ray emerges from the prism in the same direction as it entered.

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i1

R1i2

R2

Drawing a Light Ray Through a Prism

Set the ray box up as before with a single slit showing a beam of light. Direct the beam towards a prism.

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X X


Set the ray box up as before with a single slit showing a beam of light. Direct the beam towards and through a prism making sure the prism is in the beam path.

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Mark the rays as before but also draw around the prism in outline.

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X XX

X


Remove the prism and raybox.Draw the rays to and from the prism.

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X XX

X


Join the rays inside the prism.

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X XX

X


Remove the prism and raybox.

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X XX

X

Lenses

Lenses are designed to use refraction to form an image.

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Lenses

Lenses that cause rays to diverge (spread out) are called biconcave lenses.

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Lenses

Lenses that cause rays to converge (close in towards a focal point) are called biconvex lenses.

e.g. magnifying glass

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Lenses

The distance between the centre of the lens and the focal point is called the focal length.

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Lenses

When you look at an object through a lens what you see is an image of the object.

Images can be larger or smaller (magnified or diminished) than the object …

Images can be upright or inverted (upside down).

Images can be real images (the rays actually pass through) or virtual images (the rays do not pass through).

For example: An image in a mirror is a virtual image because the rays do not actually come from behind mirror, they just seem to.

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Lenses

Biconvex lenses produce three types of image:1. Inverted and smaller than the object, when the

object is distant (e.g. a camera).2. Inverted and larger than the object, when the

object is close to the lens but not closer than the focal point (e.g. a film projector)

3. Upright and larger than the object, when the object is closer to the lens than the focal point (e.g. a magnifying glass).

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The Human Eye

We ‘see’ objects when light reflects from them into our eyes.(Just remember teachers and parents have eyes in the back of their heads.)

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The Human Eye

Light enters the eye through the cornea, (a transparent front cover) and then passes through the pupil and the eye’s lens.The pupil is a small hole at the centre of the front of the eye – it looks black because most of the light entering the eye is absorbed inside the eye.

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The Human Eye

Paste the diagram of the eye into your notebook and label the:• cornea• pupil• lens

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The Human Eye

A sharp image is formed on a screen at the back of the eye called the retina.The retina’s surface has light sensitive cells on it and they transmit the image as a message to the brain via the optic nerve.Both the cornea (2/3)and then the lens (1/3) refractthe incoming light to form the image.

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The Human Eye

On your diagram of the eye label the:• retina• optic nerve

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cornea

pupil

lens

The Human Eye

The eye has an adjustable hole called the iris that controls the amount of light entering the eye.In a dark room the iris makes the hole large so that a lot of light can get in.In bright sunlight the iris makes the hole small so that the eye is not flooded by light.When you walk from a dark room into bright sunlight you may have to squint to give the iris time to adjust to the new light conditions.

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The Human Eye

Label the:• iris

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cornea

pupil

lens

retina

optic nerve

The Human Eye Year 9 Science

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cornea

pupil

lens

retina

optic nerveiris

iris

The Human Eye

Thicker lenses refract light more than thinner lenses.The lens in the human eye is adjustable so that it can focus on near or far objects forming a sharp image on the retina.Muscles are attached to the lens to make it thicker or thinner depending on whether it is focussing on near or far objects.

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Improving Sight

As we get older the lens tissue becomes less flexible and does not change its shape so easily.Reading can become a problem because the image is blurred. This problem is called presbyopia.The eye does not bulge as much as it should preventing light from focussing on the retina. Instead the focal length is greater and the converges to a focal point behind the retina.The reader has a desire to hold the book further away to bring the focal length back to the retina.Reading glasses work with the eye’s lens to help this.

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Improving Sight

Some people are naturally short-sighted; able to see close images clearly but unable to focus clearly on distant objects.This condition is known as myopia.In this situation the light focuses before the retina and diverging lenses are needed to refocus the light properly.

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Improving Sight

Some people are naturally long-sighted; able to focus clearly on distant objects but unable to see close images clearly.This condition is known as hyperopia.In this situation the light converges beyond the retina and converging lenses are needed to refocus the light properly.

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Improving Sight - Astigmatism

Astigmatism is a common eye problem usually caused by a cornea that does not have a uniform shape.Light is bent differently from different parts of the cornea and this means that a sharp image does not form on the retina.Hard contact lenses or special glasses are needed to correct this problem.

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Improving Sight - Cataracts

Cataracts are a problem usually associated with old age. Cataracts are parts of the lens that have clouded over causing blurred vision.If the problem is serious the lens may be surgically removed and replaced with a plastic lens.

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Improving Sight - Glaucoma

Glaucoma is one of the leading causes of blindness.It is a disease where the pressure inside the eye builds up and harms the optic nerve.If left untreated it will cause blindness and any damage is irreversible.A person could have this disease and not know it so it is important to have regular eye examinations.

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The Endoscope

An optical fibre is a thin length of glass strand that has the property of not allowing light to refract out and escape.It effectively becomes a ‘light tube’.

This happens because the light beam repeatedly reflects from the internal surface of the optical fibre without refracting out.

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The Endoscope

An endoscope (commonly used in medicine) is a flexible tube that contains two bundles of optical fibres. One bundle takes a beam of bright light and shines it on an object. The other bundle takes a reflected image of the object as a beam and allows a person to view that image.

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The Endoscope

Many things can catch in the esophagus, usually above a stricture or narrowing. Image 1 shows a chunk of chicken sitting above a stricture. In Image 2, a dried out piece of pork is seen. Image 3 shows a small jingle bell stuck in the lining of the esophagus. All of these were removed safely from the esophagus by endoscopy.

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White Light Year 9 Science

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Light coming from an incandescent source (e.g. the sun) consists of many colours – it is called white light because it looks white when reflected from a white object.White light can be sent through a prism in such a way that it splits into the colours of the rainbow – this is called dispersion.

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Dispersion of white light produces the visible spectrum.

The colours of the visible spectrum are:red – orange – yellow – green – blue - indigo –

violet

In dispersion red refracts the least and violet refracts the most but the colours blend into one another.

Blue sky? Year 9 Science

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When light passes through water or the atmosphere it is scattered from molecules – blue is scattered to your eye most so the sky looks blue in the daytime.

White sky? Year 9 Science

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Bigger particles like dust scatter lower frequencies better so on dusty days the scattered light is a mixture of most frequencies – the sky looks whiter.

White clouds? Year 9 Science

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Water droplets or ice crystals are big enough to scatter all colours so no particular colour is missing and the clouds appear white.

Black clouds? Year 9 Science

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Clouds that are thick and heavy with water droplets do not allow much light through and so look black.

Red Sky? Year 9 Science

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When the sun is low in the sky the light has to travel through a thicker amount of the air and so more blue light scatters.

The red end of the spectrum does not scatter as much so often sunsets appear red/orange.

Black and White Year 9 Science

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Black is the absence of any reflected light.All the colours of the spectrum reflect from a white object as white.A black object reflects no visible light.

Coloured Objects Year 9 Science

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Coloured objects appear coloured because of the colours they reflect and absorb and how these colours interact with our eyes and brain.


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A red apple reflects red and absorbs all the other colours.A green leaf reflects green and absorbs all the other colours.A white piece of paper reflects all colours and absorbs none.A black piece of paper absorbs all colours and reflects none.


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Exercise 2 a) What colours does a yellow object absorb?b) What colours does a blue object reflect?

Exercise 3 Why does a white object appear white?

Exercise 4 Why is there no such thing as ‘black light’?


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One complication is that there are no perfect reflectors or absorbers.In real situations a black object will always reflect some light and a white object will always absorb some light.The result is what we see.


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Exercise 5 What colour does a white object appear in green light?Explain why.

Exercise 5 What colour does a red object appear in green light?Explain why.

Primary Colours Year 9 Science

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Red, green and blue light can combine to produce white light.Different combinations of these three colours can produce all known colours so they are called primary colours.

Colour Addition Year 9 Science

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Red and blue light add to produce magenta. Green and blue light add to produce cyan.Green and red light add to produce yellow.This is different to colour subtraction in paints and dyes.


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Magenta, cyan, and yellow are examples of secondary colours.Secondary colours are made by combining primary colours.


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The coloured print in your textbook is made of thousands of magenta, cyan, yellow and black dots combined in such a way as to produce coloured images.A similar process is used to produce coloured television images.Colour addition refers to light shone from something or light reflected from an object.

Colour Subtraction Year 9 Science

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Using coloured paint and thinking in terms of the visible spectrum:A coloured paint absorbs some colours and reflects others.The absorbing process means that some colours are subtracted from the light.


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GREEN PAINT

ABSORBS REFLECTS

red

orange

yellow

green

blueindigo violet


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RED PAINT

ABSORBS REFLECTS

red

orange

yellow

green

blueindigo violet


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RED PAINT

ABSORBS REFLECTS

red

orange

yellow

green

blueindigo violet


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BLUE PAINT

ABSORBS REFLECTS

red

orange

yellow

green

blueindigo violet


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Remember it is very difficult to get a pure red reflector or filter.The shade of red we see depends on other coloured impurities that are present.

Versions of red

‘Less than Pure’ Colours Year 9 Science

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Colour spectrum of a yellow lemon

‘Less than Pure’ Colours - Subtraction Year 9 Science

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Reflected spectrum from a green leaf


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Action of a green filter


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Action of a red and blue filter


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Yellow paint absorbs blue, indigo and violet and reflects some red, orange, yellow and green.


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White paint reflects all colours.


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Red paint absorbs green and blue.


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Green paint absorbs red and blue.


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If we mix green paint (absorbs red and blue) and red paint (absorbs green and blue) we get no light reflected.

It looks black (or brown/grey).


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We also get black (or brown/grey) if we mix red with blue or …

green with blue.


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Cyan is a mixture of green and blue so cyan paint absorbs red and reflects green and blue.

Yellow is a mixture of green and red so yellow paint absorbs blue and reflects green and red.


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Magenta is a mixture of blue and red so magenta paint absorbs green and reflects red and blue.


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If we mix cyan and yellow, the cyan absorbs red and the yellow absorbs blue so the colour that is not absorbed is green which is reflected.


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If we mix cyan and magenta, the cyan absorbs red and the magenta absorbs green so the colour that is not absorbed is blue which is reflected.


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The retina in our eyes have two main types of receptor cells: rods and cones.The rods detect how bright the light is and cannot detect colour.There are three types of cone cells. Each is sensitive to a different range of colours corresponding to each of the primary colours, red, green and blue.This enables the eyes to be sensitive to a huge range of colours by colour addition.

The Human Eye - Colour Year 9 Science

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Light Year 9 Science 1. Light energy Light is a form of energy that travels in straight lines and is...

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