snc2p - unit 4

SNC2P

Lesson 16

SNC2P – Science

Unit 4 – Lesson 16

Unit 4 – Physics: Light & Application of Optics Overall Expectations By the end of this course, students will: • Analyze how properties of light and colour are applied in technology and the impact of these technologies on society; • Investigate, through inquiry, properties of light, and predict its behaviour in mirrors and as it passes through different media; • Demonstrate an understanding of characteristics and properties of light, particularly with respect to reflection and refraction and the addition and subtraction of colour.

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SNC2P – Science

Unit 4 – Lesson 16

Lesson 16: Kinds of Light What is Light? Visible light is any electromagnetic wave that the human eye can detect and the visible spectrum is the continuous sequence of colours that make up white light. Think for a moment. Think of a campfire with all of your friends. You are by a lake, the moon and stars are out. There are fireflies in the distance. Some of your friends have brought flash lights, glow sticks and sparklers. Others are roasting marshmallows or drinking soda out of cans. How many sources of light can you name in this mental picture? From your list, how many are natural and how many are artificial? Check your answers; Natural Fireflies Stars

Artificial flashlight glow sticks Marshmallow glow fire

Reflection from the moon, ground, trees, water, soda cans, etc. are not considered the light source and therefore not light. In general – Natural Light is light that originates from the sun, animals or plants. Artificial Light is anything that is man-made that emits light. Sources of Light – there are many sources of light – they are classed as non-luminous (reflective light) or luminous (light producing and reflecting). Non-luminous Luminous

Type Not capable of producing light BUT can reflect light Fluorescence – emission of light by a substance that has absorbed light or other EM radiation Chemiluminescence – emission of light and heat as a result of a chemical reaction Bioluminescence – production and emission of light by a living organism Electric Discharge – flow of electric charge through solid, liquid or gas that can emit light Incandescence – emission of light from a hot body due to temperature Phosphorescence – absorbs radiation and re-emits it slow over a long period of time

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Example Moon Florescent bulbs Glow sticks Firefly Neon lights Light bulb Many glow in the dark items

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SNC2P – Science

Unit 4 – Lesson 16

Light travels at a very high speed (c = 3.0×108 m/s) – it circles earth’s equator about 7.5 times in just 1 second. Light travels in straight lines (referred to as Rectilinear Propagation) however, the path of the straight line can be manipulated or bent to change the direction of travel of the light. Light does not require a medium for transmission. Light is an Electromagnetic Wave. Energy is transferred through radiation. A wave is a disturbance that transfers energy from one point to another without transferring matter. Imagine a duck on the surface of the lake. The waves move up and down so then the duck moves up and down with the wave, meaning wave transfers energy to the duck. Parts of a Wave Crest - the highest point in a wave. Trough - The lowest point in a wave. Rest position - the level when there is no wave at all.

Crest

Rest Position Trough

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Page 4 of 34

SNC2P – Science

Unit 4 – Lesson 16

Wavelength: (lambda - λ) Distance from one place in a wave to the next similar place on the wave

λ

λ Amplitude: maximum height from the rest position, the highest or lowest spot in a wave.

Amplitude

Amplitude

The electromagnetic spectrum is a diagram that illustrates the range, or spectrum of electromagnetic waves, in order of wavelength or frequency. Notice the colours of light are just different wavelengths of light. The colour red has the longest wavelength and the violet colour the shortest.

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SNC2P – Science

Unit 4 – Lesson 16 Electromagnetic Spectrum

Visible Spectrum Colours – (from shortest to longest wavelength) violet, blue, green, yellow, orange, red.

Support Questions 1. State two properties of light. 2. What makes up white light? 3. Using the electromagnetic spectrum diagram in the lesson, sort the following wavelengths from shortest to longest - Ultraviolet, x-rays, Infrared, Gamma, AM Radio 4. Why is the moon considered “non-luminous” even though it lights up the night sky? 5. What is the difference between phosphorescence and fluorescence? 6. What is the symbol for wavelength? 7. Draw a wave diagram and label the following: amplitude, wavelength, trough, crest and rest position.

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SNC2P – Science

Unit 4 – Lesson 16

Key Question #16 (29 marks) 1. Below are 3 diagrams of various wavelengths. All of the wave diagrams represent a type of wave from the electromagnetic spectrum, either visible light, infrared waves or x-rays. Compare the wave diagrams and decide which diagram would represent which wavelength and explain your decision. (3 marks each = 9 marks)

A

B

C

2. Fluorescent materials are used for many applications in our environment. Research one of the following applications and prepare a one page summary on how it is used and the benefits of its use. (20 marks) • • • • • •

Fluorescence in medicine Fluorescence in dentistry Fluorescent paints and dyes for application in theatre or clothing Fluorescence uses for counterfeit detection in banking, legal documents, etc. Fluorescence in lighting Fluorescent in mining, geology, gemology

Begin your search by using a good search engine and search “fluorescence applications in industry”.

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Page 7 of 34

SNC2P

Lesson 17

SNC2P – Science

Unit 4 – Lesson 17

Lesson 17: Additive and Subtractive Colour Theory Important Terms: Visible Light Spectrum – the light that humans can see. It typically contains the colours red, orange, yellow, green, blue, indigo, violet. Reflection – when light “bounces off” a surface and changes direction. Absorption – when light energy is absorbed by an object and is converted into heat. Think of wearing a black t-shirt on a hot day! You can feel the absorption! Transmission – when light penetrates an object and keeps traveling allowing you to see objects on the other side. Clear glass and plastic allow transmission of light where wood does not. Transparent – an object that allows light to penetrate it, making it possible to see objects on the other side. E.g. Clear glass & plastic Translucent – an object that allows light to pass through but scatters it in different directions. E.g. Tinted windows. Paper, bubbles Opaque – an object that will not allow any light to penetrate it. Ex. Wood, books & walls The Colour of an Opaque Object Opaque objects either absorb light or reflect light. Imagine a basket of fruit full of different colours. White light is striking all of the fruit in the basket but certain fruit absorb colours and then reflect others. Grapes would absorb all of the colours but reflect only purple. An orange would absorb all the colours in the visible spectrum and reflect orange! What happens though if you shine a blue light though, on a red apple? The red apple absorbs all of the blue light, but has no red light to reflect and will appear as dark blue or nearly black.

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Page 9 of 34

SNC2P – Science

Unit 4 – Lesson 17

The Colour of a Translucent or Transparent Object For a transparent or translucent object to have colour it must absorb all other colours of light AND it must also transmit and reflect the colour that it is! For example – a blue bubble would absorb all the colours in the visible light spectrum as well as transmit and reflect the colour blue. Additive Colour Theory of Light White light is composed of different colours of light. White light can be produced by combining red, blue and green light. These are the primary colours. Yellow, cyan & magenta are secondary colours of light. Google “Primary Colours of Light” images to see a full colour image. It should look similar to what you see below. When you “overlap” the three primary colours they “add up” to form white light. When you add any 2 or 3 of the primary colours they generate the secondary colours Subtractive Colour Theory of Light When a light strikes an object, some light gets reflected. Coloured matter selectively absorbs different colours or wavelengths of light. The colours absorbed are subtracted from the reflected light that is seen by the eye. Black absorbs all colours and white reflects all colours. Google “Subtractive Colour Theory” images

Support Questions 1. Think of the fruit basket example from the lesson. What colours of light would a banana absorb? Reflect? What about a whole pineapple? 2. What would happen if you shine a red light on a green apple? 3. What would happen if you shone a red + green + blue light together? 4. What would happen if you shone a red + green light together?

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Page 10 of 34

SNC2P – Science

Unit 4 – Lesson 17

Key Questions #17 (10 marks) 1. You are shopping at the mall and stop to look at a rack of t-shirts. You notice that the red t-shirt is really vibrant and bright and you purchase it. When you get it home though it appears more faded and less “red” then you thought. What “trick” did the store use with lights to fool you? What would have happened if they did the same “trick” on blue t-shirts? (5 marks) 2. Explain using a Venn Diagram “What are the similarities of the additive and subtractive colour theories?” (5 marks) Note: Not sure how to do a Venn diagram? Check out the following website: http://www.purplemath.com/modules/venndiag.htm or use a reliable search engine to fine out “how to do a Venn diagram”.

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Page 11 of 34

SNC2P

Lesson 18

SNC2P – Science

Unit 4 – Lesson 18

Lesson 18: Reflection Important Terms: Incident Ray – the ray of light traveling towards the surface.

reflecting surface Reflected Ray – the ray of light that has “bounced” off a reflecting surface.

reflecting surface Normal – line perpendicular to a surface (like a mirror).

reflecting surface Angle of Incidence (i) – the angle between the incident ray and the normal

i reflecting surface

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Page 13 of 34

SNC2P – Science

Unit 4 – Lesson 18

Angle of Reflection (r) – the angle between the reflected ray and the normal.

i

r

reflecting surface Plane Mirror – a flat reflecting surface, such as a mirror or a still body of water like a lake. The Law of Reflection The angle of reflection (r) is equal to the angle of incidence (i). The reflected ray and the incident ray are on opposite sides of the normal. These laws apply to every reflecting surface.

So how does our eye see objects? Imagine our eye is looking at a basket of flowers and that the sun is shining on the basket of flowers. The surfaces of the flowers are not “mirror-smooth”. This means that the light rays reflect in all different directions, some of which reach your eye. This is happening all over the pot of flowers, allowing us to see the entire pot of flowers!

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SNC2P – Science

Unit 4 – Lesson 18

How does our eye see reflected objects? All of the light rays from the pot of flowers that strike the mirror reflect from it. This is what the Law of Reflection says! The rays that your eye sees appear that they are coming from behind the mirror.

Terms to Know for Ray Diagrams Object – the item in front of the mirror Image – the reflection of the object in the mirror Object distance – the distance from the mirror to the object Image distance – the distance from the mirror to the image back of mirror

object

image

object distance

image distance

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SNC2P – Science

Unit 4 – Lesson 18

Things to Remember 9 9 9 9

The size of the image is the same size as the object. The image distance is always equal to the object distance The image is always orientated in the same direction as the object. The image seen in the plane mirror is always a “virtual images”, located behind the mirror.

Support Questions 1. Explain in your own words The Law of Reflection. 2. Matching – match the terms in column A and column B together that correspond! Column A 1. Incidence ray 2. Reflected ray 3. Normal 4. Angle of incidence 5. Angle of reflection 6. Plane mirror 7. Object 8. Image 9. Object distance 10. Image distance

Column B a. the distance from the mirror to the object b. the angle between the reflected ray and the normal. c. the ray of light that has “bounced” off a reflecting surface. d. the ray of light traveling towards the surface. e. the reflection of the object in the mirror f. g. h. i.

the angle between the incident ray and the normal the distance from the mirror to the image line perpendicular to a surface (like a mirror). the item in front of the mirror

j. a flat reflecting surface

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Page 16 of 34

SNC2P – Science

Unit 4 – Lesson 18

How to Draw Ray Diagrams in a Plane Mirror Step 1 9 Draw a double line to represent a plane mirror 9 Draw a simple object 9 Label one point of the object “a” and one point “b”

A

B

Step 2 9 Draw an incident ray from point A directly to the mirror at a 90o angle 9 Draw the reflected ray (from the mirror) backwards along the same line as the incident ray.

A

B

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Page 17 of 34

SNC2P – Science

Unit 4 – Lesson 18

Step 3 9 Draw another incident ray from point A at an angle to the mirror 9 At the point where the incident ray hits the mirror, draw a normal. 9 Measure the angle of incidence with a protractor 9 Using the law of reflection, draw the reflected ray o angle of incidence = angle of reflection

A

i B

r

Step 4 9 use the dashed line extend both reflected rays behind the mirror until they meet 9 where they meet mark it as “Ai”. This shows that it is the image of point A.

A

Ai

i B

r

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Page 18 of 34

SNC2P – Science

Unit 4 – Lesson 18

Step 5 9 repeat steps 2 – 4 for point B 9 draw the image now between points Ai and Bi.

r

A

Ai

i

B

Bi

Support Questions 3. In your notebook, use a ruler and a protractor to draw object-image lines, lines of equal length that are perpendicular to the mirror, incident rays, and reflected rays for the object. Locate the apparent source behind the mirror. a.

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b.

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SNC2P – Science

Unit 4 – Lesson 18

Other kinds of mirrors… Concave Mirrors This is a mirror that has a “caved-in” reflective surface, for example, the bowl part of a shiny spoon. The image distance is greater than the object distance. The image is larger than the object and the image is virtual. The image can be upright or flipped upside down depending on where the object is located. Convex Mirrors This is a mirror that bulges out in the centre, for example, the back of a shiny spoon, convenience store mirrors, or the passenger side mirror in a car. It is sometimes called a “fisheye mirror”. The image is closer to the mirror than then object is, the image is smaller than the size of the object, it is virtual and upright.

Support Questions

4. You will observe your image in a curved mirror and compared it with your image in a plane mirror. What You Need Plane (flat) Mirror

Large kitchen spoon

What to Do Hold the plane mirror about 25 cm from your face. On your answer sheet record the following information: a. Estimate the size of the image

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Page 20 of 34

SNC2P – Science

Unit 4 – Lesson 18

b. Estimate distance from the mirror to your image. Hold the spoon the same distance from your face as the plane mirror, looking at the inside or “caved-in” side of the spoon. Once again, with the spoon about 25 cm from your face, try to estimate the size of the image and its distance from the mirror relative to your face. On your answer sheet record the following information: c. size of image d. distance of image from your face Still looking at the “caved-in” side of the spoon, move the spoon as close to your face as you can and still see an image. Then move it as far away as possible and observe any changes in your image. On your answer sheet record the following information: e. How does the size of the image changes as you bring your spoon closer to your face. f. Record any other things you observed Turn the spoon over and look at your reflection on the back of the spoon. Hold it fairly close to your face and slowly move it away. On your answer sheet record the following information: g. How does your image change? Analysis In which “mirror” was your image larger? In which “mirror” did the image appear to be farther behind the mirror? What is the biggest difference that you notice between your images on the two sides of the spoon?

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Page 21 of 34

SNC2P – Science

Unit 4 – Lesson 18

Key Question #18 (20 marks) 1. Use a ruler and a protractor to draw object-image lines, lines of equal length that are perpendicular to the mirror, incident rays, and reflected rays for the object. Locate the apparent source behind the mirror. (Recreate these diagrams in your notebook prior to completing). (10 marks)

A.

B.

2. Is a dental mirror, shown here, a concave, convex or plane mirror? Think about the purpose of a dental mirror and then explain and giving proof to back up your choice. (5 marks) 3. Explain, giving proof, how a spoon can be both a concave and a convex mirror. (5 marks)

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Page 22 of 34

SNC2P

Lesson 19

SNC2P – Science

Unit 4 – Lesson 19

Lesson 19: Refraction What is Refraction? Refraction is the bending effect of light when it crosses a boundary between two different media. A ray of light can bend when it travels from one substance across the boundary to another substance Medium: is any substance light rays travel through (plural is media) Refracted ray: is the ray after crossing the boundary between 2 media (E.g. Air and water) Refracted angle: is the angle between the refracted ray and the normal. The path of light bends as it meets the boundary between one medium (air) and another medium (water)

Incident ray

Normal Angle of  incidence,   i

Angle   of      refraction,   R 

Refracted ray

What Causes Refraction? Light refracts because light travels at different speed in different media. Imagine a car travelling along smooth pavement and entering a sandy area. Imagine a car driving off an asphalt surface onto a sandy pit at an angle. As shown in the figure above, the right wheel gets onto the sand first, and slows down a little bit. Meanwhile, the left wheel is still moving at the original speed. This causes the car to turn slightly towards the right. Copyright © 2011, Durham Continuing Education

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SNC2P – Science

Unit 4 – Lesson 19

The light behaves the same way! The speed of light changes when light goes through different mediums! The speed of light in a vacuum (no air) = 3.0 x 108 m/s. All other mediums that light travels through will be less than this number. Rules of Refraction 1. The incident ray, refracted ray and the normal all lie in the same plane. The incident ray and refracted ray are on opposite sides of the line that separates the two media. 2. The refracted ray bends toward the normal when travelling from less optically dense to more optically dense – the speed is reduced.

Normal

θi > θR 

Angle of  incidence,   θi

 

  Angle  of        refraction,   θR 

 

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SNC2P – Science

Unit 4 – Lesson 19

3. The refracted ray bends away from the normal when travelling from more optically dense to less optically dense – the speed increases.

Normal Angle of  incidence,   θi

θi