1

PHY205H1F Summer

Physics of Everyday Life

Class 10: Colour, Optics

• Color in Our World

• Mixing Colored

Light

• Why the Sky Is

Blue

• Why Sunsets Are

Red

• Law of Reflection

• Virtual Image Formation

• Image Reversal

• Concave Mirrors

• Diffuse Reflection

• Refraction

• Total Internal Reflection

• Lenses

Clicker Accuracy Leaderboard

(just for fun)

UTORid Number of correct

clicker answers so far 1 (1) chenyuz2 112

2 (2) tombergs 106 3 (↑5) zhangx89 103 4 (↑10) halekath 101 5 (↑11) guoyepen 100 6 (↑13) kunghsia 99 6 (↓2) kuoyu 99 6 (6) wanggua8 99

Chs. 27 & 28 Pre-Class Quiz Notes

1. When red, green and blue light are combined,

white light results. That is a consequence of the

three types of light-sensitive cells in the human

retina. It is a property of human biology, not the

physical universe.

2. The sky is blue due to atmospheric scattering of

sunlight.

3. The splitting up of white light into its component

colours is called dispersion.

4. A converging lens can form a real image (such

as the upside-down one of me currently on your

retina!)

Problem Set 4 is due right now.

Last Chance for no late penalty!

• Any Problem Sets not turned in by now will receive the

10% per day (or portion thereof) late penalty.

Recall from Chapters 25 and 26…

A changing electric field creates a magnetic field, which then changes in just the right way to recreate the electric field, which then changes in just the right way to again recreate the magnetic field, and so on.

This is an electromagnetic wave.

E

M

What property of a light wave mainly

determines its colour?

A. Its frequency

B. Its amplitude

C. Its timbre

D. A mix of all of the above

Why Sunsets Are Red

CHECK YOUR NEIGHBOR

Color we see depends on frequency of light.

High Frequency,

Short wavelength

Low Frequency,

Long wavelength

𝑓

𝜆

Color

0.4 𝜇m 0.7 𝜇m

Selective Reflection

A red ball seen

under white light.

Only red is reflected,

other colors are

absorbed.

A red ball seen

under red light.

A red ball seen

under green

light.

There is no

source of red

light to reflect!

Mixing Colored Light

The spectrum of sunlight is a graph of

brightness versus frequency.

2

A beam of white light, when shone through a

prism, will end up many different colours.

What will happen if a green laser is shone

through a prism?

A. It will spread into a small range of

colours.

B. It will change to a single colour other

than green.

C. It will stay green.

Why Sunsets Are Red

CHECK YOUR NEIGHBOR Rods and cones contain stacks of membranes. Rhodopsin is a transmembrane protein complex.

Cone Rod

Light Light

Rhodopsin

Retinal

(pigment)

Opsin

(protein component)

The retinal molecule inside rhodopsin changes shape when retinal absorbs light.

Light

trans conformation

(activated)

Opsin

cis conformation

(inactive)

0.5 µm

Opsin

Slide courtesy of Ross Koning, Biology Department, Eastern Connecticut State University

http://plantphys.info/sciencematters/vision.ppt Slide from http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colcon.html

Response Curves for the three types of cones in

the retina of the human eye.

Use both eyes, stare at area shown in blue for 15 seconds or so What do you see on this blank white slide? Blink if needed!

This is called an “after image”

Does it move around as you move your gaze?

Use both eyes, stare at the parrot’s eye for 15 seconds or so Stare at the bird’s eye for 10 seconds without tilting your head.

What colour is

the afterimage

bird?

A. orange

B. green

C. blue

D. yellow

E. None of

the above

Bird Was Red Use both eyes, stare at the parrot’s eye for 15 seconds or so Stare at the bird’s eye for 10 seconds without tilting your head.

What colour is

the afterimage

bird?

A. red

B. orange

C. blue

D. violet

E. None of

the above

Bird Was Green

3

Use both eyes, stare at the parrot’s eye for 20 seconds or so Stare at the bird’s eye for 10 seconds without tilting your head.

What colour is

the afterimage

bird?

A. red

B. orange

C. yellow

D. green

E. None of

the above

Bird Was Blue

Additive Primary Colours (light bulbs)

and Subtractive Primary Colours (ink)

Discussion Question

• Why is this square red?

A. The light bulbs in the projector emit light with

blue frequencies

B. The light bulbs in the projector emit light with

green frequencies

C. The light bulbs in the projector emit light with

red frequencies

D. Both A and B

Discussion Question

• Why is the red folder in Harlow’s hand

red?

A. The pigments in the paper absorb light

with blue frequencies

B. The pigments in the paper absorb light

with green frequencies

C. The pigments in the paper absorb light

with red frequencies

D. Both A and B

• When the colour yellow is seen on the

screen of your computer, what are the

lights being activated in that region of your

screen?

A. Mainly yellow

B. Blue and red

C. Green and yellow

D. Red and green

Discussion Question : Using the

colour wheel..

• If an object is blue, which of the coloured

lights below would make the object appear

most black?

A. blue

B. cyan

C. yellow

D. magenta

Discussion Question : Using the

colour wheel..

• If an object is red, which of the coloured

lights below would make the object appear

not appear red?

A. red

B. orange

C. white

D. cyan

Discussion Question : Using the

colour wheel..

4

Why the Sky Is Blue For small scattering particles, like nitrogen or oxygen molecules, higher frequency blue light is scattered much more readily than lower frequency red light.

Why the Sky Is Blue

𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦

If molecules in the sky scattered orange light

instead of blue light, the sky would be

A. orange.

B. yellow.

C. green.

D. blue.

Why Sunsets Are Red

CHECK YOUR NEIGHBOUR

Doppler Shift for Light

• When a light source is moving away from you, the spectrum is shifted toward the red.

• When a light source is moving toward you, the spectrum is shifted toward the blue.

Doppler Shift for Light

• The Doppler shift can be observed in the headlights of cars on the highway.

• The cars moving away from you appear more red, while the cars moving toward you appear more blue-ish or white.

Doppler Shift for Light

• The Doppler shift can be observed in the headlights of cars on the highway.

• The cars moving away from you appear more red, while the cars moving toward you appear more blue-ish or white.

wthhhhhhhhhhhh??

Doppler Shift for Light (yes, really!)

• The Doppler shift can be observed with carefully obtained spectra of very fast moving objects like stars

• There is a slight shift in “absorption lines”

Why Sunsets Are Red

Light that is least scattered is light of low

frequencies, which best travel straight through air. If molecules in the sky scattered orange light

instead of blue light, sunsets would be

A. orange.

B. yellow.

C. green.

D. blue.

Why Sunsets Are Red

CHECK YOUR NEIGHBOUR

5

Why Clouds Are White

• Clouds are clusters of various sizes of water droplets

Why Clouds Are White

Size of clusters determines scattered

cloud color. • Tiny clusters produce bluish clouds.

• Slightly large clusters produce greenish clouds.

• Larger clusters produce reddish clouds.

• Overall result is white clouds.

• Slightly larger clusters produce a deep gray.

• Still larger clusters produce raindrops.

What does a white sky indicate?

A. The atmosphere has mostly particles smaller

than the wavelength of light

B. The atmosphere has mostly particles larger

than the wavelength of light

C. The atmosphere has a mixture of particle sizes

D. The atmosphere has a lot of water vapour

E. The atmosphere is filled with pollutants.

Why Sunsets Are Red

CHECK YOUR NEIGHBOR

Law of Reflection The angle of reflection equals the angle of incidence.

© 2010 Pearson Education, Inc.

Mirror

• A dentist uses a mirror to

look at the back of a

second molar (A).

• Next, she wishes to look

at the back of a lateral

incisor (B), which is 90°

away.

• By what angle should she

rotate her mirror?

A. 90°

B. 45°

C. 180°

A

B

Virtual Image Formation

No light rays actually pass through or even near the image, so it is “virtual”.

Two plane mirrors form a right angle.

How many images of the ball can the

observer see in the mirrors?

A. 1

B. 2

C. 3

D. 4

• Which picture is most likely a mirror image of

Harlow? • Alice looks at Bob’s image a mirror and sees he has a

red shoe on the foot to Alice’s left.

• Then she asks Bob to turn and face her, so she can

compare the image to what Bob looks like in real life.

• Bob takes a couple of steps forward, turns around and

faces Alice.

Virtual Image Formation

• Alice notes that the red shoe is now

on the foot on the right.

• Alice concludes: “Mirrors reverse left

and right, not up and down.” Is this

true? Can you see any flaws in

Alice’s reasoning?

mirror real

6

Bob in Mirror

Bob turns to

face Alice

• Bob chooses to rotate around a vertical axis,

and therefore he looks flipped left-to-right.

• But if Bob wants to turn to face Alice, is there

any other way to do it?

Virtual Image Formation

Bob in Mirror

Bob stands on his

head to face Alice!

• If Bob had chosen to face Alice by standing on

his head, he would have been flipped up-to-

down, and not left-to-right!

Virtual Image Formation • What really happens is the image is reversed

front-to-back

Refraction

When light bends in going obliquely from

one medium to another, we call this process

refraction.

Cause of Refraction • Bending of light when it passes from one medium

to another

• Caused by change in speed of light

Dry Land

Mud

Dry Land

1

2

A soldier wants to get from point 1 to

Point 2 in the shortest amount of time.

Marching through mud is much

slower than marching on dry land.

Which might be the best path?

A B C

Refraction

Light travels slower in glass than in air, so it

minimizes the time it spends in the glass.

© 2010 Pearson Education, Inc.

A fish swims below the surface of the water.

An observer sees the fish at:

A. a greater depth than it really is.

B. its true depth.

C. a smaller depth than it really is.

air

water

Dispersion

• Process of separation of light into colors arranged by frequency

• Components of white light are dispersed in a prism (and

in a diffraction grating).

7

Rainbows

Rainbows are a result of dispersion by many drops.

• Dispersion of light by a single drop

Rainbows

Antisun

Radius of circle is about 41°

Double-

rainbow

The second rainbow

has blue on the top,

and a radius of about

53°

Total Internal Reflection

• Total reflection of light traveling within a medium that strikes the boundary of another medium at an angle at, or greater than, the critical angle

Discussion Question

• Light waves with speed v1 are incident upon the flat

surface of a material in which they have speed v2.

• For what condition is total internal reflection possible?

A. v2 > v1

B. v2 < v1

C. v2 = v1

D. All of the above

An Optical Fibre

Speed of light in cladding is higher than speed

of light in core.

Medical Fibrescopes

Video-

laryngoscopy

with a flexible

fiberscope

Converging Lens

Focal Point

Focal length, f

NOTE: Focal length is defined

for initially parallel rays.

Diverging Lens

Virtual Focal

Point

Negative

Focal length, -f

Rays appear to emerge

from Virtual Focal Point

8

Diverging rays through a Converging Lens

Focal length, f

If an object emits rays at the focal point, they

end up being parallel on the other side of the

converging lens.

What will happen to the rays emerging to the

right of the lens if the face is moved a little

further away from the lens?

A. They will remain parallel.

B. They will diverge (spread out).

C.They will converge (toward a focus).

f Real Image Formation

Focal length, f

Light rays actually pass through the image, and a screen can be placed there, so it is “real”.

image

The Camera

• A camera “takes a picture” by

using a lens to form a real, inverted

image on a light-sensitive detector

in a light-tight box.

• We can model a combination lens

as a single lens with an effective

focal length (usually called simply

“the focal length”)

• A zoom lens changes the effective

focal length by varying the spacing

between the converging lens and

the diverging lens.

This is the end!!!!

The final exam, will be:

Wed. June 26 at 1:00pm sharp! (not 1:10!)

EX100

Test will begin promptly at 1:00 and will be 2 hours

long.

Please bring a calculator, and, if you wish, a letter-

sized aid sheet upon which you may write anything

you wish

This is the end!!!!

The final exam will cover the entire course, including all of the assigned reading plus tutorial materials and what was discussed in class.

Approximately even spread over the course material

Final Exam: First Page Please Fill Out the Online Survey For This

Course!!!!!!

• Your feedback is vital to me. I promise I will read every

word you type in the online evaluations.

• The online system is open now (you should have received

an email already)

• The deadline is Sunday June 23 – please don’t forget!!

Keep in Touch!!

• My office hours are Mondays and Wednesdays: 10-

11AM, Fridays: 9-10AM.

• I will have an extra office hour Tuesday June 25

from 2-3pm.

• I am in MP121-B and my land-line is 416-946-4071

(please don’t send text messages to my land-line)

• You can always email me (

jharlow@physics.utoronto.ca ) with any questions,

or if you’d like to make an appointment outside my

office hours.

• Good luck on the exam and have a great rest of the

summer!