PHY205H1F Summer Physics of Everyday LifeClass 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

Recall from Chapters 25 and 26…

A changing electric field creates a field, which then changes in just the right way to recreate the field, which then changes in just the right way to again recreate the 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 RedCHECK YOUR NEIGHBOR

Color we see depends on frequency of light.

High Frequency,

Short

Low Frequency,

Long

𝑓

𝜆

Color

0.4𝜇m 0.7𝜇m

Selective Reflection

A red ball seen under white light.

Only 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.

Somehow, this mix looks “ ” to us.

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 RedCHECK 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 Universityhttp://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 soStare at the bird’s eye for 10 seconds without tilting your head.

What colour is the afterimage bird?A. orangeB. greenC. blueD. yellowE. None of

the above

Bird Was Red

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

What colour is the afterimage bird?A. redB. orangeC. blueD. violetE. None of

the above

Bird Was Green

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

What colour is the afterimage bird?A. redB. orangeC. yellowD. greenE. 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..

Why the Sky Is BlueFor small scattering particles, like nitrogen or oxygen molecules, 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 RedCHECK YOUR NEIGHBOUR

Doppler Shift for Light

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

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

Doppler Shift for Light

• 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 RedLight 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 RedCHECK YOUR NEIGHBOUR

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 RedCHECK YOUR NEIGHBOR

Law of ReflectionThe 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?

A B

C D

• 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 .

• 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

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

• 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 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 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.

airwater

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).

RainbowsRainbows 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 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 than speed of light in core.

Medical Fibrescopes

Video-laryngoscopy with a flexible fiberscope

Converging Lens

Focal Point

, f

NOTE: Focal length is defined for initially parallel rays.

Diverging Lens

Virtual Focal Point

Negative , -f

Rays appear to emerge from Virtual Focal Point

Diverging rays through a Converging Lens

, 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

, 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

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!)Rooms TBA

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

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