Physical Science 20 RCSUnit 4 Light4.1 What is light? (13.1 Pg 410-412)

Sometimes science asks questions that seem obvious, but that turn out to have much more complicated answers when examined.

What is light is one of these questions. This is because we have dealt with light in our lives for so long that we assume that our familiarity with it means that we understand it.

Let’s examine briefly the history of light.o Plato and Socrates in ancient Greece thought that the eye emitted invisible light rays that reflected back

to it allowing it to see objects.o Pythagoras thought that light particles bounced off an object and then into our eyes , allowing us to see

it. o Aristotle in 4 th century BC thought it travelled like a wave in water. o Leonardo da Vinci in the 1500 hundreds also thought it had wave like properties and was a wave, similar

to sound.o In the 17 th and 18 th century the debate between whether light was a wave or particle began to be able

to be tested. o Isaac Newton (1642-1727) developed a strong theory of light as a particle that explained the bending of

light (refraction), reflection, and colour.o Christian Huygens (1629-1695) showed mathematically how a wave could also demonstrate these

phenomena. Plus his theory also explained diffraction (interference of waves with themselves).o Thomas Young, in 1801, does the famous double slit experiment showing that light definitely has a

diffraction pattern.o Augustin Fresnel (1788-1827) develops a comprehensive mathematical wave theory of light.o James Clerk Maxwell (1831-1879) developed very robust mathematical formulas that explained how

light was formed due to electrical forces acting within the atom that produced a wave called electromagnetic radiation (er) that has frequencies from 10 1 to 10 25 Hz. (visible light is at about 10 15 Hz)

o Maxwell’s theory is very strong and has allowed us to develop things like radar, microwaves, radio, lasers, etc.

o So it is a wave right? Unfortunately light also behaves as only a particle does at times. This means that scientists have had to say we truly do not know the exact nature of light. We might call it a wavicle (fully particle and wave at the same time)

So while we do not fully understand what light is, we have a good idea and we can study and use the properties of light.

4.2 The Speed of Light (13.2 Pg 412) How fast is light? We know that it is faster than sound because lightning shows up before the thunder reaches us. Is it instantaneous? There were many reasons to think this for a long time because it is just so very fast. Also you

cannot see it move in our everyday life. Only when observing things really far away can you start to notice the lag that is produced by light. Olaus Roemer, a Danish astronomer, noticed this while observing eclipses of the moons of Jupiter. (P.413)At one

point of the year when Earth was on the opposite side of the sun from Jupiter it took longer for the light to reach Earth and therefore the eclipses appeared later than their calculated times. When Earth was on the same side of the sun as Jupiter the eclipses appeared earlier.

Speed of light = 3.00x10 8 m/s Complete Questions #1,2

4.3 The Pinhole camera (13.3 Pg 415) Light travels in straight lines (in the same medium). This is called linear propagation A beam of light is a stream of light rays. The stream of rays can be converging, parallel, or diverging (p. 415) The transmission of light allows for a very simple kind of camera to be constructed called the pinhole camera. Most of us when we think of a camera think of lens that focus light onto a film or digital detector. There is a simpler

definition though. A camera can be anything that allows the formation of an image on a film. It does not have to have a lens.

This may seem odd at first. Without a lens how is an image supposed to be made? The trick to forming an image is to make sure that the light only shows one

point of view. Most cameras do this by having a lens focus (direct) the light. A pinhole camera does this by only having a very tiny opening. This tiny opening is the only viewpoint possible then. Since light travels in straight lines making it so that there is only one path the light can take through the hole makes it so that only one image forms.

If an opening is too large then too many viewpoints come through and a proper image does not form.

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We can even control the size of the image by controlling how far away the back of the camera is and how far away the object is from the pinhole.

If the object is moved closer the image grows. If the screen is moved closer the object shrinks. The object size and image size are related by the MAGNIFICATION formula:

P.417

height of imageheight of object

=distance of image ¿hole ¿distance of object¿

hole ¿hiho

=d ido

- this equationuses properties of s❑' imilar triangles '

Read pages 410-417 You will also notice that the image is upside down. We call this inverted. Since the image can be formed on a screen we call it a real image. (Virtual images cannot be put onto screens

because they are “behind” a mirror) P. 418 #1-3

4.4 Laws of reflections (13.5 pg 418) Most of us have a fairly good idea how mirrors work. Here we will formalize those ideas and by doing that we will

be able to study them better. A mirror or other smooth surface will reflect light in a predictable way. This is called specular reflection or direct

reflection. A rough surface will scatter the light in a random way and this is called diffuse reflection. The terms used when studying reflections are (p. 418 diagram)

o Incident ray – this is the ray of light that hits the mirror.o Reflected ray – this is the ray of light that bounces off the mirror.o Point of incidence – this is the part of the mirror that the light bounces off of. o Normal – this is an imaginary line that is 90 degrees to the surface of the mirror from the point of

incidence. o Angle of incidence – this is the angle between the normal and the incident ray.o Angle of reflection – this is the angle between the normal and the reflected ray.

The laws of reflection are (p. 419): o The angle of incidence is equal to the angle of reflection. o The incident ray, the reflected ray and the normal all lie within the same plane. (this means that they

can be drawn on a flat surface)

4.5 Images in a Plane Mirror (13.6 Pg 419) When you look into a mirror you see your image. There are some rules for how that image appears. We can

predict, based on where the object is, where the image will appear. A plane mirror is the simplest mirror to understand. It is a flat mirror and the rules for where the image appears are

the simplest of all the mirror types. First we must understand a little about how we see. An object will reflect light in all directions but we only see that

light that passes through the narrow window that is our eye. This light cone produces the image on the back of our retina.

When we draw a diagram of a mirror, an observer, and an image we call it a ray diagram . Based on how our eyes see, and how light travels we have some simple rules for drawing ray diagrams.

o The mirror is drawn as a solid line with dashes behind the side that the image appears to be on. o The object is drawn with a solid outline. o The observer is drawn as an eye.o The image is drawn the same size as the object and the same distance behind the mirror as the object is

from the front of the mirror. Images are often drawn with dotted lines. o Two rays are drawn from the observer to the image. One from the top of the eye one from the bottom.

The rays are solid on the real side of the mirror and are dotted on the virtual side of the mirror. o The rays are then drawn from the points of incidence on the mirror to the object. These rays are also

solid.

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Characteristics of images in a plane mirror are:- same size as the object- vertically upright (erect)- virtual

o Make your own notes on the applications of plane mirrors (13.7). Include diagrams and and explanations that will help you understand the applications (10 apps)

o Chapter 13 Assignment: Choose any 20 of the 32 review questions on page 432-435.

4.6 Curved Reflectors (14.1 Pg 438) If you pay attention you will start to notice that you see curved mirrors all of the time in everyday life. They can be

thought of as part of a glass sphere that has been cut away. If the part that is cut away curves towards us at the edges we call it concave. A concave mirror is also called a

converging mirror because it causes parallel light rays to converge on a single point called a focus. If the part that is cut away curves away from us at the edges we call it convex. A convex mirror is also called a diverging

mirror because it causes parallel light rays to diverge as if coming from a single point.

4.7 Reflection in a Converging Mirror (14.2 pg 439) There are 4 key words that we use to describe a converging mirror:

o Principal axis – The line that goes through the center of the mirror and the principal focus. o Center of curvature (C) – The point that would lie at the center of the sphere from which the curved

mirror would have been cut. The distance from C to the mirror is the radius of curvature.o Principal focus – The position where reflected parallel light rays

come together. It is half way between the surface of the mirror and C. It lies along the principal axis.

o Focal length(F) – the distance from the mirror to the principal focus to the middle of the mirror.

If the light rays do not come in parallel to the principal axis but they are still coming in parallel then they will focus at a different point. This point will still be somewhere along the focal plane. This plane if parallel to the center of the mirror surface and passes through the principal focus.

4.8 Images in a Converging Mirror (14.3 Pg 440) With a plane mirror the image is always behind the mirror the same distance away from it as the object is from the

front of it. Images are a little more complicated to predict with a concave mirror. The light rays still obey the same rules as with a plane mirror. That is angle of incidence equals angle of reflection.

The thing that changes is that every single point on the mirror has its own normal (90 o line) and therefore not every single ray is going to follow the same path.

Luckily for us if the mirror is a parabola that is part of a sphere and the arc length of the mirror is small compared to the radius of the mirror then we can follow three simple rules for drawing ray diagrams. (p. 441)

o A ray that is parallel to the principal axis is reflected through the principal focus. o A ray that passes through the principal focus is reflected parallel to the principal axis. o A ray that passes through the center of curvature is reflected back along the same path.

Let’s consider some possible situations.

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Physical Science 20 RCSo The object is beyond the center of

curvature (2 times the focal length)

o The object is at the center of curvature

o The object is in between the center of curvature and the focal point.

o The object is in between the focal point and the mirror.

4.9 Images formed by diverging Mirrors (14.4 Pg 442) If a concave mirror is called a converging mirror then a convex mirror is a diverging mirror. When parallel light hits a diverging mirror it reflects it as if it came from the focal point of the mirror. The unusual

thing in this case is that the focal point is behind the mirror so no actual light did go through the focal point.

The rules for diverging mirrors is the same as the rules for the converging mirrors. o A ray that is parallel to the principal axis is reflected as if from the principal focus. o A ray that appears to pass through the principal focus is reflected parallel to the principal axis.o A ray that appears to pass through the center of curvature is reflected back along the same path.

4.10 Equations for Curved Mirrors (14.5 Pg 443) Ray diagrams are useful for figuring out where an image will appear but you do not always want to have to draw a

picture for every mirror that you are going to deal with. That is why the mirror equation is so useful. The mirror equation is an equation that works for both converging and diverging mirrors. (see p 445)

1do

+ 1d i

=1f

do=object distance ,d i=imagedistance , f= focal length In order for the equation to work properly we need to make sure that we are using the correct signs (+, -). The

rules for the use of signs is called sign conventions. o All distances are measured from the vertex of a curved mirror.o Distances of real objects and images are positive. (in front of the mirror) o Distances of virtual objects and images are negative. o Object heights and image heights are positive when measured upward and negative when measured

downward form the principal axis. RECALL: Magnification of the image is given by the equation:

M=hihohi=imageheight , ho=object height

Sample problem p447p.448 #1-3 questions

4.11 Spherical Aberration (14.6 pg 448) The rules that we have covered for diverging and converging mirrors do not work if the mirror is too large

compared to the radius of the mirror (center of curvature). To understand why this happens look at the image below.

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Physical Science 20 RCS As each ray strikes the mirror they all have their own angle of incidence and angle of reflection. You can see that as

you go further away from the principal axis the angles of reflection stop hitting the focal point.

Because this happens in spherical mirrors we call it spherical aberration. Aberration means a deviation from the expected.

To keep this from happening engineers and physicists must use parabolic mirrors instead of spherical mirrors when designing large projects that rely on mirrors.

Ch 14 Review asn’t: p. 455 #1-6,8-11,13,18

4.12 Refraction of light (15.1 Pg 464) Refraction , we learned in the sound unit, is the bending of a wave when it enters a new medium. Since light has

wave like properties that must mean that light also refracts when it enters a new medium. A good example of refraction with light that everyone is likely familiar with is when you look at a straw in a glass of

water. The straw appears bent or disjointed even though it is not. We can see that light bends when it enters a new material, such as a glass block, or water. When light travels from the air into the glass most of it continues into the glass but some reflects. This is called

partial reflection and partial refraction . The incoming ray of light is still called the angle of incidence. The point that the light hits the new material is the

point of incidence, the reflected ray is still called the reflected ray but now there is a third ray that travels into the material called the refracted ray.

The interesting thing is that if things were reversed and the refracted ray were to be the incidence ray then the light would follow its path back. This is called the principle of reversibility.

4.13 Index of Refraction (15.2 pg 465) An index is a list of information. In the back of a book the index gives page numbers where you can find particular

pieces of information. With light we have something called the index of refraction.

The index of refraction is a list of materials with a number that shows how the speed of light in that material compares to the speed of light in a vacuum (the fastest light can go) (p. 466)

So what does the speed of light have to do with refraction you might ask? (go on ask it.) Well it turns out that the reason that light or in fact any wave bends when it enters a new material is that the

velocity of the wave changes in that material. This causes the wave to bend. So what is the index of refraction then? It is a index of how the speed of light in a material compares to the speed

of light in a vacuum.

n= cv

The n = the index of refraction, the c = the constant speed of light in a vacuum 2.99 x 108 m/s, the v = the speed of light in the material that the light is traveling in.

4.14 Laws of Refraction (15.3 Pg 467) We can predict with accuracy how a light ray bounces of a mirror. This is thanks to the law of reflection; the angle

of incidence equals angle of reflection. It turns out that the amount that light bends when it hits the surface of a new medium can also be predicted

thanks to the careful work of Willebrod Snell (1591-1626). He discovered the mathematical relationship between the angle of incidence and the angle of refraction. Therefore it is called Snell’s Law:

sin isinR

=n i=angle of incidence R=angle of refraction

n=index of refraction

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Physical Science 20 RCS Written as word the laws of refraction are:

o The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant equal to the index of refraction.

o The incident ray and the refracted ray are on opposite sides of the normal at the point of incidence, and all three lie in the same plane.

4.15 Snell’s Law a general equation (15.4 pg 469) The original Snell’s law only dealt with the index of refraction of the material

that light was travelling into. That index of refraction is developed based on the speed of light in the material in comparison to a vacuum (air is fairly close). What if the light were not travelling from the air into a material but from something else like water, or crystal, etc.

In that case we would need to take into account the difference between the index of refraction of the material that you had been in and the compare it to the index of refraction of the material that you are travelling into.

When we do develop this formula what we end up with is what is often referred to as Snell’s law in general form:

n1 sin θ1=n2sin θ2 (In this equation the Ɵ is the angle measured from the normal.)

Textbook questions: Ch 15.2 – 15.4 p. 467 #1-4, p. 468 #1-3, p.470 #1-3

4.16 Total Internal Reflection and the Critical Angle (15.5 pg 470) When you are travelling from a fast material (air) to a slow material (glass) the angle of refraction is less than that

of the angle of incidence. The opposite is true when you go from a slow medium to a fast one. The angle of refraction is greater that than the angle of incidence.

So what this means is that it is possible for the light to be refracted so much that it never leaves the medium. When this happens we call it total internal reflection . (P. 470) You may have seen this in a pool. If you swim deep and look up at the surface at an angle you can only see what appears to be an upside down pool above you.

The angle of incidence that has a 90 o angle of refraction and therefore sends the light skimming along the surface is called the critical angle. Any angle of incidence greater than this will result in total internal reflection.

4.17 Lateral Displacement and Deviation of light rays (15.6 pg 473) When light hits a surface we can know calculate if the light will refract or reflect and to what degree. We can use

this to tell how light will travel through any flat faced object. In physics we typically send light through prisms. A prism is a solid object that has two identical ends and all flat

sides. We often use a square prism and a triangular prism. When light hits a rectangular prism it refracts the light twice. Once going in and then once going out. The light

leaves the material parallel to its original path but shifted over slightly. This is called lateral displacement. When light enters prisms of other shapes we need to calculate the refraction off of each surface. Diamonds are cut

so that most of the light that enters the top of the diamond reflects back out of the top of the diamond. This gives it its sparkle. P.478

15.7 Applications of Refraction- Apparent Bending of Straight Stick Water- Atmospheric refraction - Sparkling Diamonds

- Ch 15 Review Questions – page 482 #1,11,15-19,22,31,34,36

- Open notes quiz on applications of mirrors (sections 13.7 p. 422, 14.7 p. 449, 15.7 p. 474).

3.18 (16.1, 16.2 p. 491)- Lenses have been around since ancient times. They have many applications such as cameras, telescopes, microscopes, and projectors. - lenses are often made of glass but can also be made of plastic. - lenses refract light rays in a certain way as to create an image that is usually a different size and has different characteristics.- there are two types of lenses, converging and diverging.- converging lenses ‘bring the light rays closer together’ as they pass through the lens. - Diverging lenses ‘spread light rays out’ as they pass through. See diagrams on page 491.4.19 (16.3 p 492)- There are a few terms that need to be understood

- optical center: geometric center of the lens at the point the principal axis passes through.

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Physical Science 20 RCS- principal axis:- principal focus:- focal length: - secondary principal focus: F’ is the point that exists in front of the lens, the same distance from O as F.- focal plane: this is the vertical line (imaginary, perpendicular to the P.A.) where images are focused. All light rays

passing through lens (at nearly any angle) will focus somewhere on the focal plane.

Rules for converging lenses - Rules for lenses are very similar to rules for

mirrors, except that light rays always pass through, not reflect back.1. A ray that is parallel to the PA is refracted through the focus (F)2. A ray that passes through the optical center goes straight through un-refracted.3. A ray that passes through the secondary principal focus (F’), will refract parallel to the principal axis

***Please study the diagrams on page 494-5.

4.20 (16.4 p.495)

16.7 p. 501 Human Eye

Accommodation – process of changing the shape of the lens to focus nearby and far away objects.

16.8 Defects in visionMake your own notes on the following terms. Farsightedness (hypermetropia) – the inability to see ________________ objects clearly.

- Corrected by ___________ lenses.- Natural loss in __________________ is called ____________. Bifocals are used for people that become

farsighted.Nearsightedness (myopia) – the inability to see ________________ objects clearly.

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Physical Science 20 RCS- Corrected by ___________ lenses.

Astigmatism – inability to focus images in all planes due to the cornea or lens not being perfectly spherical

Diopters –

Blind spot – the portion of the image seen by the eye that falls on the optic nerve attachment site of the retina. This causes that portion of the image to not register to the brain.

Cataracts – *corrective eye surgery - research

- Ch 16 Review questions p. 512 (Due Monday -day of exam) #3,8,14,15,16,19,27,33

Make up asn’ts - Ch- Recall that when any asn’t questions have been discussed in class or asn’ts returned, only reduced credit is

available for the same asn’t. This means that in order for you to get the full marks for the asn’t, you must add to the asn’t. Below are the original and additional questions that will allow full marks once reduced credit kicks in for original asn’ts

- Chapter 13 Assignment: Choose any 20 of the 32 review questions on page 432-435. o Make up asn’t: Choose 5 more questions to add to the 20.

- Ch 14 Review asn’t: p. 455 #1-6,8-11,13,18o Make up asn’t for full marks if late: add #12, 15, 19, 21

- Ch 15 Review Questions – page 482 #1,11,15-19,22,31,34,36o Make up asn’t: add #3,6,9,21,27

- Ch 16 Review questions p. 512 (Due Monday -day of exam) #3,8,14,15,16,19,27,33o Make up asn’t: add #1,2

If you are all caught up on Ch 13-16, please begin reading Ch 8 in your physics text. You are encouraged to take notes on all bold faced terms in the text (from 8.1-8.6)

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Optional Material

Dispersion and Recomposition (17.1 Pg 518) You have likely seen a rainbow before. It has been known since ancient times that white light passing through a

shard of glass will show bits of colour. It wasn’t until 1666 that Isaac Newton systematically showed that white light is composed of the colour spectrum. When we break white light into the spectrum we call that decomposition. When we but the spectrum back

together to form white light again we call that recomposition. In the same way that a prism will break apart white light it can combine coloured light. A simpler way of demonstrating this is using a spinner. Put the different colours of light onto a disk and spin it. The

disk will appear blurred and the colours will be averaged. If you do this with the spectrum you will get white light.

Colour by subtraction (17.2 pg 519) When you see an object it appears the colour it does because of the light it reflects. For example

white light falling onto a leaf will largely be absorbed by the leaf. The colour of light that is not absorbed is green and this is reflected.

A transparent filter appears a certain colour because it absorbs the other colours and allows that colour through.

Sometimes it allows adjacent colours through. We call this a compound colour then. You can see that both reflective light and filters work the same way. They absorb some light and

allow other light through. We use this when painting or using pigments to make the colours that we want.

When white light strikes an object all of the colours are there in the beam. If we use compound colours in our pigments then those colours will gradually filter out the extra colours until just the colour we want is left.

o Compound Cyan – It should absorb red and reflect or transmit blue and green.o Compound Yellow – It should absorb blue and reflect or transmit green and red.o Compound Magenta – It should absorb green and reflect or transmit blue and red.

So if you put two compound filters in front of each other only the colour that they both transmit or reflect should show through.

o Cyan and Yellow only have green in common so when combined they appear green. Cyan, Magenta, and Yellow are the true subtractive primary colours although most people still learn blue instead of

cyan, and red instead of magenta. This is because of historical reasons. Also most paint pigments are not truly pure colours so when mixing them we can get away with being slightly off.

Printers use 4 colours of ink Cyan, Magenta, Yellow, and to sharpen the image black. This is called the CMYK model or subtractive colour model.

Colour by addition (17.3 pg 524) So if we see the world around us by light reflecting off of objects why is it that our tvs, computer monitors, phones,

etc do not use the CYMK model but instead use the Red, Green, Blue model? Well we have to think about the difference between how we see most things and how we see screens. Most

objects are reflecting light, which means that they can absorb or filter out the initial white light. A screen like on your phone does not reflect light but produces light. Since there is no filtering it has to make sure that the light that reaches your eye is the same that would have reached it had it passed through a filter.

Basically the colour additive theory is the exact opposite of the colour subtractive theory. When the colours in the colour additive theory are added they produce the compound colours (composite) colours of cyan, yellow, and magenta. And if you put them all together they appear white.

o Red + Blue = Magenta (purplish)o Blue + Green = Cyano Green + Red = Yellow

The reason the colour by addition works is because of how our eye works. The way we see colour is with 3 colour sensors called cones on our retina. These cones respond to different wavelengths of coloured light. They respond best to Red, Green, and Blue.

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So the reason that Red and Green light make

yellow is because they both activate in our eye and our eye knows that normally yellow light is what activates both the red and green cone. You can also see that it means that we have the weakest sensitivity to cyan.

Other animals that have other cones would have the ability to see colours beyond the ends of the spectrum that we can see. Some may be able to see infra-red others ultra-violet.

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