Transcript Lightx - Mill-Park

WAVE-LIKE PROPERTIES OF LIGHT
OUTLINE
Waves
 Light
 Reflection and Refraction
 Diffraction and Interference
 The eye

WAVES
What is a wave?
A disturbance or oscillation that transfers energy

DESCRIBING WAVES

Examples of waves:
DESCRIBING WAVES
Energy is being transferred in all these waves
 The medium is the substance that a wave
needs for it to travel. Eg: Sound needs
air/water to travel through. It cant travel
through empty space.

Question: List the mediums for the wave types
you listed previously
TYPES OF WAVES
There are two types of waves:
 Transverse: Particles in the medium are
vibrated perpendicular to the direction of the
wave


Longitudinal: Particles in the medium are
vibrated parallel to the direction of the wave
TYPES OF WAVES
Draw a longitudinal wave and a transverse
wave as seen in the slinky
 Label with the following terms: Peak, trough,
amplitude, wavelength, compression,
rarefaction

TRANSVERSE AND LONGITUDINAL

Because transverse and longitudinal waves
behave the same, to make things simple we will
draw all our waves as transverse from now on.
DRAWING WAVES ON A GRAPH
WAVE PROPERTIES

Amplitude: The height of the wave (from the centre)
Units: m

Wavelength: The length from one crest to the next.
Units: m

Frequency: How many full wavelengths pass you every
second. Units: Hz, s-1

Time Period: The time it takes for a full wavelength to
pass. Units: s
QUESTIONS
Question 1. Which of the following statements is
true?
A. Period is the measurement of the length of a wave
B. The Amplitude of a wave is dependent on its
frequency
C. The amplitude of a wave is distance from the
bottom of a trough to the top of a peak
D. The amplitude of a wave is from the middle to the
peak of a wave
QUESTIONS
Question 2. On the graph provided draw a wave
that has an amplitude of 2 units, and a
wavelength of 4 units
WAVE EQUATIONS

The frequency and time period are related 𝑓 =
1
𝑇
𝑑
𝑡

If the speed of a wave is given by 𝑣 =

But we know, the wavelength of the wave λ
and the time period T , we have the equation
𝜆
of the speed of the wave being
𝑣=
𝑇

And
𝑣 = 𝜆𝑓
WAVE EQUATION QUESTIONS
Question 3. A boy watching water waves pass him
notices that 2 waves pass him every second.
What is the frequency? What is the time period?
Question 4. The same boy takes a photo of the
waves and finds that the distance between the
peak of one wave and the trough is 2.1m. What is
the wavelength.
Question 5. Use your answers to question 3 and
4 to find the wave speed
QUESTIONS
Question 6. When lightning strikes, we see the flash
almost immediately. However, the sound of the thunder
takes time to reach us. The speed of sound in air is
330ms-1. If I started counting after I saw lightning, and the
thunder arrived 7 seconds afterwards, how far away was
the lightning strike?
Question 7. Which have a longer wavelength: AM radio
stations (such as 927kHz Radio Sport) or FM radio
stations (such as 107.5MHz Triple J)?
Question 8 [Extension] A wave travels a distance of 50
times it wavelength in 10 seconds. What is it’s frequency?
WAVE EQUATION ANSWERS
1.
2.
3.
4.
5.
6.
f=2Hz. T = 0.5s
λ=4.2m
8.4ms-1
2.33km (2330m)
AM stations
5Hz
SUPERPOSITION OF WAVES

Principle of Superposition: When two waves
meet, the resulting wave is the sum of the
two individual waves
SUPERPOSITION OF WAVES
Constructive Interference: When two waves
sum to give a resulting wave with a larger
amplitude than the original waves
 Eg
 Destructive Interference: When two waves sum
to give a resulting wave with a smaller
amplitude than the original waves
 Eg

SUPERPOSITION OF WAVES
After the waves have passed through each
other… they continue on unaffected
 Eg.

DIFFRACTION

Diffraction is when waves bend around objects
DIFFRACTION
Longer wavelength waves diffract more than
shorter wavelength waves.
 This explains why AM stations can be tuned in
from behind a hill, but FM stations cant.
 It also explains why you can still get cell phone
reception in the Burnley tunnel!

DIFFRACTION
LIGHT
Is light a wave or particle?
 Light rays – always travel in a straight line
 Converging


Diverging

Parallel
LENSES AND MIRRORS
PRAC: EXPLORING LIGHT
Draw annotated diagrams for what happens when
light …
a) Strikes a convex mirror
b) Strikes a concave mirror
c) Passes through a convex lens
d) Passes through a concave lens
e) Passes through a prism
f) Passes though a rectangular block at an angle
g) Another object of your choice
LIGHT - REFLECTION

Law of reflection: Angle of incidence = angle or
reflection eg
𝜃𝑖
Normal
Mirror
𝜃𝑟

Prac: How much of yourself can you see in a
mirror?
REFLECTION, TRANSMISSION, ABSORPTION
Reflection: Light rays bounce off the object
 Transmission: Light rays pass through the
object
 Absorption: Light rays are killed by the object

QUESTIONS
Question 9. Complete the diagrams below for
light hitting the mirrors
QUESTIONS
Question 10. An observer stands at point P.
Which objects (A, B, C, D) can they see in the
mirror?
REFRACTION
REFRACTION

Pyrex Stirring Rod
REFRACTION
Law of refraction:
 When light enters a slower medium, eg water, it
bends towards the normal
 When light enters a faster medium, eg air, it
bends away from the normal

REFRACTION – SNELL’S LAW
Refractive Index: Describes how light travels
through a medium
 Symbol: n
 Space: n = 1.0000
 Air: n = 1.003
 Water: n = 1.33
 Glass: n = 1.5

REFRACTION – SNELL’S LAW
𝑛𝑖 𝑠𝑖𝑛𝜃𝑖 = 𝑛𝑟 𝑠𝑖𝑛𝜃𝑟
Example: A light beam strikes a piece of glass at
an angle of 45o to the normal.
a) Draw this situation
b) What is the angle between the refracted light
beam and the normal?
PRAC: SNELL’S LAW
Measure and calculate the angles of refraction
for the following two glass shapes
Angle of Incidence
Angle of Refraction
Angle of Incidence
Angle of Refraction
20
??
20
??
50
??
40
??
50
??
CRITICAL ANGLE AND TOTAL INTERNAL
REFLECTION
θr
θi
Air
Glass
𝑛𝑖 𝑠𝑖𝑛𝜃𝑖 = 𝑛𝑟 𝑠𝑖𝑛𝜃𝑟
𝑠𝑖𝑛90 = 1
𝑛𝑟 = 𝑛𝑖 𝑠𝑖𝑛𝜃𝑖
𝑛𝑟
−1
𝜃𝑐 = 𝜃𝑖 = sin
𝑛𝑖
CRITICAL ANGLE AND TOTAL INTERNAL
REFLECTION

Any incidence angle larger than the critical
angle leads to the light being reflected.
Air
Glass
REFRACTION IN LIFE
Mirage
 Atmosphere: Green flash

REFRACTION IN LIFE

Fibre Optics
QUESTIONS
Question 11. Explain, with the help of a
diagram, why you need to aim below the fish
when you go spear fishing
Question 12. Calculate a, b and c
40
b
a
c
Question 13. Calculate the critical angle
between water and air.
LIGHT

James Maxwell (the great Scot!) formulated
that light is made of oscillating electric and
magnetic fields. He also calculated the speed
of light at 3 x 108 ms-1
SPEED OF LIGHT

How do we measure the speed of light?
ELECTROMAGNETIC SPECTRUM
QUESTIONS
The speed of light is 3x108 ms-1
1nm = 10-9 m
Question 14. Convert 650nm into m
Question 15. Convert 1.064x10-6 m to nm
Question 16. Calculate the frequency of red light
(wavelength = 650nm)
PRAC: “SEEING IR LIGHT”
PRAC: SPEED OF LIGHT IN A MICROWAVE
f = ____________
 λ = ____________
 v = ____________

ROYGBIV
Red
 Orange
 Y ellow
 Green
 Blue
 I ndigo
 V oilet

633nm
532nm
Decreasing Wavelength
Increasing Frequency
Increasing energy
445nm
ROYGBIV
The three Primary Colours are
 Red, Green, Blue. All other colours on our TV
can be made from these
 Why? Because we only have receptors for Red,
Green and Blue in our eyes!

Colour
Green
Yellow
Yellow
Red
Cyan
Magenta
Cyan
Magenta
Blue
DISPERSION
Shorter wavelength light (blue) refracts at a
slightly larger angle than longer wavelength
light (red).
 This can be seen in a prism
 The separation of white light into its colours is
called DISPERSION

DISPERSION – RAINBOWS!
So to see a rainbow, the sun always has to be
behind you!
 What is a moonbow?

POLARISATION
The polarisation of light describes the direction
that the electric field is oscillating in
 Light from the sun can have any polarisation
 Polarisation filters can block out one of
polarisation directions (PRAC) – These are in
polarised glasses

POLARISATION DIAGRAM

Consider a wave on a string trying to get
through a wire mesh

In a similar way, optical devices called
polarisers only allow light with a specific
polarisation to pass through them.
DOUBLE SLIT INTERFERENCE
YOUNG’S DOUBLE SLIT
Before the double slit experiment, people
thought that light was a particle
 The surprising result from the double slit
experiment is that you get spots on the screen
where there is no light at all
 This can only be explained if light is a wave.
These dark spots are where waves destructively
interfere with each other and cancel out.

DIFFRACTION AND INTERFERENCE

𝑧=
𝜆𝐿
𝑑
PRAC: FIND THE WIDTH OF A HUMAN HAIR
Use a laser and the formula from the previous
slide to find the width of a human hair.
 LASER SAFETY.
 1) Lasers are hazardous to eyes.
 2) Keep lasers propagated at a level below the
chest

QUESTIONS
Question 17. As light travels from air to water
does it speed up or slow down?
Question 18. Light with a wavelength of 590nm
passes through a medium. Its frequency is 3.81 x
1014 Hz. Determine whether the medium the light
travels through is air
Question 19. List these forms of EM radiation in
order of increasing frequency: X-rays, microwaves,
light waves, radio waves.
QUESTIONS
Question 20. What is the critical angle for light
passing from
a. Diamond (n= 2.42) to air
b. Glass (n= 1.5) to air
c. Water (n= 1.33) to air
Question 21. A laser passes through a double slit. A
screen is placed 1m away. The distance between
bright spots on the screen is 0.325m. The
wavelength of the laser is 650nm. What is the
distance between the slits?
ASSESSMENT










Make an A3 poster (in pairs) that explains one of the
following
SLR Camera
Pinhole Camera
Pepper’s Ghost (Tupac)
Laser
Human Eye (and defects)
Optical Illusions
Radar
The electromagnetic spectrum
Sonic Boom
EXTRA SLIDES
REFRACTION
GEOMETRIC OPTICS
When dealing with mirrors and lenses we don’t
worry about light being a wave – it’s too
complicated.
 We just assume it’s a ray (something that
travels in straight lines)

PLANE MIRRORS
What we see is a virtual image.
 The virtual image is not really there. Our eyes
and brain make us think there is candle behind
the mirror
 The distance from the candle to the mirror is
the same as the distance from the virtual
image to the mirror

QUESTIONS
Q 7, 8, 9. Pg 290
 Extension - Periscope

CURVED MIRRORS – CONCAVE
A concave mirror is a converging mirror
 Con CAVE
 For a far away object, it produces a real image

RAY DIAGRAMS
Concave mirror is part of a sphere
 Principal Axis goes through middle
 Centre of curvature in middle of sphere
 Focal length is half of radius of curvature

Focal Length (f)
Focal Point (Focus) (F)
Principle Axis
Centre of Curvature (C)
Radius of Curvature (R)
CONCAVE MIRRORS

Rule 1: A ray that travels through the centre of
curvature returns on that same path
CONCAVE MIRRORS

Rule 2: A ray that travels through the focal
point, is reflected parallel to the principal axis.
CONCAVE MIRRORS

Rule 3: A ray that travels parallel to the
principal axis is reflected through the focal
point.
CONCAVE MIRRORS

Rule 3: A ray that travels parallel to the
principal axis is reflected through the focal
point.
CONCAVE MIRRORS
We a have formed a:
 Real Image (it can be projected onto paper)
 An inverted image
 A diminished image


Magnification. 𝑀 =
𝑖𝑚𝑎𝑔𝑒 ℎ𝑒𝑖𝑔ℎ𝑡
𝑜𝑏𝑗𝑒𝑐𝑡 ℎ𝑒𝑖𝑔ℎ𝑡
CONCAVE MIRRORS

Questions.
CONVEX MIRRORS

Same rules as with concave mirrors, but
QUESTIONS
CONVEX LENSES
Ray tracing
 Forming Images (with outside lght)

CONCAVE LENSES
Ray Tracing
 The lens equation: 1/f = 1/di+ 1/do
 Magnification = Hi/Ho = -di/do

QUESTIONS






1) Explain, with the help of a diagram, the difference
between a transverse wave and a longitudinal wave?
2) Name one example of a transverse wave and one
example of a longitudinal wave
3) Draw a diagram to show diffraction of a wave
4) Water has a refractive index of n=1.33. Air has a
refractive index of n=1. Complete the diagram…
5) Use Snell’s Law (𝑛1 𝑠𝑖𝑛𝜃1 = 𝑛2 𝑠𝑖𝑛𝜃2 ) to work out the
angle
6) Describe what is meant by the critical angle