Light+and+Sight



= Light and Sight = = =

Definitions
Copy this text, fill in the blanks, save as notes and then submit for grading.


 * Light is produced by L _ _ _ _ _ _ _ _ objects, such as the sun and torches. These are known as sources of light. Light travels as a beam of photons, otherwise known as R _ _ _. We can see things when light bounces off an object and is detected by our eyes. When light bounces off an object it is known as R _ _ _ _ _ _ _ _ _. If something absorbs or reflects light it is said to be O_ _ _ _ _, if light can pass through it it is T _ _ _ _ _ _ _ _ _ _, if light passes through it, but is so scattered that we cannot see clearly through it, then it is said to be T _ _ _ _ _ _ _ _ _ _. ||

//For this unit, you will be using pages to take notes, and the templates have been prepared for you. You can download them below.// media type="youtube" key="T5oxC9l_u50" height="505" width="640"

Reflection of light
Reflection are not real. Not many animals can tell the difference. Watch how a small puppy behaves when it first sees its reflection in a mirror :-). Although the image may look the same, it is back the front.




 * ===Mirror writing===
 * try to sign your name in mirror writing. Practice until you are good at it. Take a picture using PHOTOBOOTH and save it with your notes on LIGHT.


 * [[file:LASReflection.pages]]
 * [[file:LASCurved Mirrors.pages]]

Refraction of light
Refraction refers to when light changes direction as it travels from one thing to another. For instance, have you ever looked down at your feet while in a swimming pool and they look closer than they should? This is due to the refraction of light. How does it work? Well, light has a fixed speed in a vacuum (when there is no particles in the air) and this is 300000000 m/s or 1080000000 km/h. However, when light has to move though things which have particles, like the water or glass, it slows down. As it slows down it changes direction slightly. Look at the picture below to help you understand what happens.

//What we can see is the light reflected from NEMO speed up as it leaves the dense water and bends away from the **normal** and hence the fisherman see the fish as being further away than it actually is. Our eyes believe that light travels in straight lines and hence they back trace the rays and see a virtual image of NEMO. If we were to put it in scientific terms we would write "light is reflected from the fish and as the reflected ray leaves the water, it speeds up and bends away from the normal, this means a virtual image of the fish is seen at a distance further that the object actually is."//

//Look at what happens to this light ray as it passes through glass. As it enters the glass, it slows down (more particles) and then upon leaving it speeds up!//

Refraction Refraction is the bending of light through a substance. Different substances bend light at different angles. White light is made up of a full spectrum of colors. Each color has a different wavelength, and bends at a different angle. This is the same effect that produces rainbows in the atmosphere. The most common illustration of this is a glass prism as seen here.



//The colors of the spectrum can be remembered easily with the acronym **ROYGBV** (pronounced: //roy-gee-biv//) ||
 * ~ ROYGBV ||
 * < **Red** ||< **Orange** ||< **Yellow** ||< **Green** ||< **Blue** ||< **Violet**

Download the following document and fill in the relevant sections.

Watch these videos to help you understand refraction and reflection. media type="youtube" key="kDB0hMyIRJw" height="344" width="425" media type="youtube" key="kc2o73FyN3I" height="344" width="425"

=How much have you understood?= media type="custom" key="6543855"

Crazy questions... and their scientific answers
The answers to some of theses questions are on p140-4 of your text. You may also find answers in the internet, or from the library.
 * 1) [|Why is the sky blue?]
 * 2) [|How are rainbows formed?]
 * 3) [|Why are sunsets red?]
 * 4) [|If you were traveling at the speed of light and you turned on your headlights, what would happen?]
 * 5) [|What are auroras and what do they look like?]
 * 6) [|Why does the moon appear larger on the horizon?]
 * 7) [|What is the difference between the speed of light and the speed of sound?]
 * 8) How fast does darkness travel?
 * 9) Where do the colours in white light come from?
 * 10) Can light bend?
 * 11) How does a laser work?
 * 12) How does light affect our health?
 * 13) How do eyes work?
 * 14) What is peripheral vision and how does it work?
 * Can you think of some more questions involving light?
 * In groups of two create a one minute video explaining the answers to one of these questions to grade 6 students.
 * Your video should be like a news report... with you as a speaker. It should include graphics and may include short animations. It can include background music where appropriate.
 * The best videos will be posted on this website.
 * Good luck.

Optic Fibres - using refraction to bring high speed internet to schools:
Theory p132 Model p132

Optic fibres use the principal of total internal refraction to transmit light along thin strands of glass or plastic. Total internal refraction occurs when light hits a surface at or less than the critical angle for that substance. The critical angle for a substance depends on its optical density - that is, how much light slows down when it passes through it.

> ===Demonstration: comparing internal refraction in water to internal refraction in an optic fibre.=== > We will use a laser to check the critical angle of water (the angle at which total internal refraction occurs), by shining a laser up through a beaker of water. > Then we will shine the laser through a stream of water and hope that it stays within the stream of water as it curves downwards into the sink.

[] [|Video: the fibre optic revolution] media type="custom" key="6576723" media type="custom" key="6576735"

Prisms - using refraction to separate light into the colours of the rainbow:
mind tease p145 Critical angle p132

Light enters through the **cornea** (a transparent layer) which is coveres by the protective **conjunctiva**. The light then passes through a small opening in the eye called the **pupil** (why does it look black?). This **pupil** can get bigger or smaller depending on how bring the day is. The **iris** is a muscle which controls the amount of light entering the eye. When its dark it relaxes and gets larger letting light in, when its light it contracts and gets smaller, letting less light in. The **lens** of the eye is behind the pupil. This is a **flexible** lens and can be made fatter or thinner by muscles in the eye.

Focus one finger then focus on something further away. What is the closest thing you can focus on, use a ruler the flexibility of the lens means its focal length can be changed.

Inside the eye there is a clear jelly this acts like a filler to keep they eye round, this is called the **vitreous humour**. The image formed by the lens is focussed onto the retina at the back of the eye. Should it be real or virtual, upright or inverted, diminished or enlarged?

The retina contains light sensors which send **electrical** signals to the brain via the optic nerve. The brain does many thing to the image including inverting it and bringing the two images together to form 3d (this is called binocular vision).

The spot where the retina enters the optic nerve has no sensory cells, this forms a blind spot.

Foveea (FOE vee a) is the centre of the retina and contains the most colour sensors.

Eyes and Vision
Colour blindness
 * what is colour blindness: wikipedia ,
 * colour blindness test: ishihara colour test, colourvisiontesting
 * Define colour blindness
 * Copy this table into your notes
 * [[image:colorblindtable.png width="476" height="356"]]

Healthy eyes

 * Find out what causes (click here)
 * spots before your eyes.
 * sparkles and flashes
 * eye strain
 * Write a sentence explaining each in your notes for this topic
 * How can we keep our eyes healthy?
 * Give 3 tips that could help you or your family have healthier eyes.
 * Answer: Does eating carrots really help you see in the dark?

How your eyes work
> > > > > >
 * KidsHealth: [[file:Eyes - how your eyes work.pdf]]
 * [[image:Snellen_chart.png align="center"]]

Naming parts of the eye
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 * Can you identify the different parts of the eye?

Copy and paste your certificate into your notes once you have passed. ====



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Mini-experiments: fun activities which help you understand your sight
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 * Measure your blind spo t. Copy your favorite test diagram and write the distance in cm that you were able to detect your blind spot for each eye. Explain what a blind spot is.
 * Binocular vision . Scroll down to "X-Ray Vision" This fun activity provides an insight into how the brain combines the images from each eye to provide an overall picture. If you do this correctly it will look as though you have a hole in your hand.
 * Judging distance: Our brain notes the angle each eye turns to focus on an object to calculate depth. It can do this because each eye can how big an object actually is and how big it appears to be. Try the distance perception activities here.
 * Judging distance: Our brain notes the angle each eye turns to focus on an object to calculate depth. It can do this because each eye can how big an object actually is and how big it appears to be. Try the distance perception activities here.

Did you know???

 * The resolution of the human eye is equivalent to a 81 MP (megapixel) camera and a hawk's is 8 times better than that!
 * The human eye actually sees everything upside-down and it's the brain that actually inverts the image right-way-up again
 * The visual pathway contributes up to 65% of all brain pathways and is responsible for up to 85% of our knowledge
 * Humans have a 200 degree horizontal binocular field of view
 * We blink on average over 10 thousand times a day
 * The eye is the second most complex organ in the body after the brain
 * The eye has over 2 million working parts and processes 36,000 bits of information every hour

How eyes work:
> > > > > > media type="custom" key="6603693" align="center"
 * When you want to look at something your eyes turn towards it. If it is too close you may go cross-eyed! They also use muscles on either side of the lens to alter its shape so that the object is in focus. When the object is close the muscles on either side of the lens relax, allowing it to thicken. When the object is far away, they pull the lens tight, causing it to become thinner. As a person ages, the flexibility of the lens decreases and it becomes more and more difficult to see things up close. This is why more and more people need reading glass as they age.
 * [[image:Focus_in_an_eye.png align="center" caption="Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens."]]
 * The iris is a circular muscle containing pigment. This pigment is the colour of your eyes. Its role is to protect the retina at the back of the eye from intense light, but also to allow enough light into the eye when it is dark. It does this by expanding or contracting. The dark spot in the middle is the pupil. When the iris relaxes the pupil looks large, letting as much light as possible. When light is bright the iris contracts and the pupil appears small. Next time you look into the light, or are in a dark room get someone to watch your iris. :-)

Eye problems

 * Some vision problems are inherited, others can be caused by injury, illness, or the environment we live in (lighting, habits... ). Some, like difficulty focussing on near objects lead to people getting reading glasses as their age increases.



**HYPEROPIA** Also known as long-sightedness, hyperopia occurs when the image is focussed behind the retina of a resting hyperopic (long-sighted) eye producing a blurred image. On average, the hyperopic eye is slightly smaller than a normal sized eye.

** How does hyperopia affect vision? **
A little amount of hyperopia is usually not a problem as the lens can compensate for it quite easily. However, if there is a significant amount of hyperopia, the amount of effort required to focus (known as accommodation) can cause symptoms of stress, tiredness, blurry vision and also headaches, especially when doing alot of near work.

How can I tell if my child is long-sighted?
A complete eye test is the only sure way of determining if your child’s vision is normal or not. A child with hyperopia may complain of tired and sore eyes when reading and may not enjoy reading. An unequal or high amount of hyperopia in the eyes can also lead to turned eyes (strabismus) and lazy eyes (amblyopia).



**What is myopia?**
Most people call myopia shortsightedness. Shortsighted people do not see distant objects clearly. The eye's lens and cornea normally focus light into an image on the retina. In a myopic eye the light is focussed in front of the retina and so the image is blurred.

**Does a shortsighted person see close objects more clearly than a person with normal vision?**
No. Shortsighted people see close objects equally as well or often slightly worse.

How can I tell if I am shortsighted?
Shortsighted people have difficulty in seeing distant objects clearly. They find it hard to read road signs and scoreboards and to play ball games. Recognising people in the distance may be a problem for many shortsighted people. Often a person will not realise that they cannot see clearly but an eye examination by an optometrist will reveal the problem.

Virtual dissection



 * cow's eye dissection (Exploratorium: the museum of Science, Art and Human Perception)
 * watch online
 * learn more about eyes
 * do your own dissection

=Prac: eye dissection=

You will be shown then correct use of a scalpel.
 * 1) Take a picture of your eye using your macbook before you do anything.
 * 2) Make an incision and cut completely around the middle of the eye (through the sclera) parallel to the Lens (So you wont be cutting the cornea in half).
 * 3) A clear liquid will come our of the eye when you cut it. This is called the vitreous humour.
 * 4) Lay the two halves of the eye on the dissecting board and take another picture.
 * 5) Pick up the half of the eye which has the cornea. See if you can remove the lens (this might have already fallen out).
 * 6) Wipe the lens clean and then hold it up to your macbook camera (about 5cm away) take a picture looking through the lens.
 * 7) Now put the lens on a piece of news paper and take a picture looking through the lens.
 * 8) Now take a look at the rest of the eye. If the vitreous humor is still in the eyeball, empty it out. On the inside of the back half of the eyeball, you can see some blood vessels that are part of a thin fleshy film.That film is the retina. Before you cut the eye open, the vitreous humor pushed against the retina so that it lay flat on the back of the eye. It may be all pushed together in a wad now. Take another picture of this.
 * 9) Use your finger to push the retina around. The retina is attached to the back of the eye at just one spot. Can you find that spot? That’s the place where nerves from all the cells in the retina come together. All these nerves go out the back of the eye, forming the optic nerve, the bundle of nerves that carries messages from the eye to the brain.
 * 10) Under the retina, the back of the eye is covered with shiny, blue-green stuff. This is the tapetum. It reflects light from the back of the eye. Humans don't have this. Cows, foxes and other mammals have this layer which helps them see at night. Take a picture of this.
 * 11) Once you have finished doing all of these activities and have taken alot of pictures, you may pack up. Make sure to wash everything thoroughly in soapy water.



Glossary of the eye
aqueous humor = blind spot = cones = cornea = iris = lens = myelin = optic nerve = pupil = retina = rods = sclera = tapetum = vitreous humor =
 * A clear fluid that helps the cornea keep its rounded shape.
 * The place where all nerves from the retina join to form the optic nerve. Each eye has a blind spot where there are no light-sensitive.
 * One kind of light-sensitive cell in the retina. Cones give you color vision in bright light.
 * A tough, clear covering over the iris and the pupil that helps protect the eye. Light bends as it passes through the cornea. The cornea begins bending light to make an image; the lens finishes the job.
 * A muscle that controls how much light enters the eye. It is suspended between the cornea and the lens. A cow’s iris is brown. Human irises come in many colors, including brown, blue, green, and gray.
 * A clear, flexible structure that makes an image on the eye’s retina. The lens is flexible so that it can change shape, focusing on objects that are close up and objects that are far away.
 * The fatty layer that surrounds each nerve fiber.
 * The bundle of nerve fibers that carry information from the retina to the brain.
 * The pupil is the dark circle in the center of your iris.It’s a hole that lets light into the inner eye.Your pupil is round. A cow’s pupil is oval.
 * The layer of light-sensitive cells at the back of the eye. The retina detects images focused by the cornea and the lens. The retina is connected to the brain by the optic nerve.
 * One kind of light-sensitive cell in the retina. Rods respond in dim light.
 * The thick, tough, white outer covering of the eyeball.
 * The colorful, shiny material located behind the retina. Found in animals with good night vision, the tapetum reflects light back through the retina.
 * The thick, clear jelly that helps give the eyeball its shape.

Colours.
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Revision: Gap-fill exercise 

Revision: Multiple Choice Questions
Lights, Mirrors, Lenses

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