Transcript Reflection and refraction

Light, optics & colour
Vision – how do humans see?
Discussion and demonstrations
Activity: Euclid’s model of vision
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What arguments did Euclid give for his model?
How would you try to persuade Euclid that the eye too is a
‘receiving organ’?
How would you counter his first argument, about the difficulty
of finding a pin?
Summarise the conditions necessary for you to be
able to see an object.
Starting points
Prior learning:
Primary school children are introduced to the
distinction between primary (luminous) and secondary (non-luminous)
sources of light. They are also encouraged to think of light, rather
than darkness, as an entity.
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Some correctly use the terms translucent, transparent and opaque.
Misconceptions:
Students may equate light with its source (light
is in a lamp, or in the Moon).
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They may continue to think of both light and darkness as states of
being, so they do not explain shadows in terms of light propagation.
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‘Common sense’ suggests that vision involves light (or something)
emanating from the eye (rather than light entering the eye).
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Many people think that magnifying lenses make light brighter behind
the lens than in front of it, and that mirror images are located on their
surface (not behind it).
Teaching challenges
What is often lacking is any notion of light travelling in
space.
Understanding virtual images, which are a kind of optical
illusion, relies on a correct understanding of vision.
Students are familiar with mixing coloured paints, but need
to be persuaded this involves colour subtraction. By
comparison they may have little practical experience of
adding coloured lights or using coloured filters, which
require different rules.
A general model for radiation
SOURCE
MEDIUM
DETECTOR
journey: may involve transmission, reflection,
refraction, partial absorption
detector: absorption at the journey’s end
Refraction
Ray of light changes direction as its speed
changes at a boundary.
Ratio of sini / sinr is constant, called the
refractive index.
sini
wavespeed1
=n=
sin r
wavespeed 2
Class experiment: Plot a graph of sini against sinr to test data
collected. This should produce a straight line through the origin.
Equation for a straight line is y = mx + c
Light travels more slowly in glass than in air i.e. wave, not particles.
Dispersion of white light
Speed of light in glass depends on frequency
(colour).
Dispersion of white light
With a diffraction grating
Light sources
filament lamp
a fluorescent lamp
(these differ)
• 700 nanometres
• 400 nanometres
• 0.7 thousandths of a millimetre • 0.4 thousandths of a millimetre
Photo credit http://home.comcast.net/~mcculloch-brown/astro/spectrostar.html
Astronomical spectroscopy
Spectra of stars at different points in the sky
When a source is moving …
Moving source of sound - Doppler effect
Local spiral galaxy rotating
Cosmological redshift
Beyond the visible
detecting infrared radiation
detecting UV radiation
modern astronomy: collecting radiation from
across the whole electromagnetic spectrum
Modelling light
The journey from light source to detector can be
thought of in three different ways.
– rays
– waves
– photons
Pinhole camera
This 1544 diagram shows how to
safely view a solar eclipse.
Shadows … and ray streaks
Demonstrations and discussion about rays.
In pairs:
1 Explain shadow formation using a ray model.
Use words from this list: emit, transmit,
absorb, reflect.
2 Sketch a diagram to show how ray streaks
are made by a ray-box with slit openings.
Write an explanation of why these are not
rays.
Reflection
For light,
angle of incidence i = angle of reflection r
angles measured with respect to a ‘normal’
Waves and particles are reflected in
exactly the same way.
Total internal reflection
At a boundary between two optical media, typically some
light is reflected from the surface and some is
refracted into it e.g. the multiple images seen in glass
shop-fronts.
Going from a slower medium (e.g. glass) to a faster
medium (e.g. air), when the angle of incidence is
greater than a critical angle, all of the light is internally
reflected.
sin i
1

sin r nglass
Two kinds of lenses
1. Converging, convex, positive.
1. Diverging, concave, negative.
focal length, f, and power, P, describe how much a lens bends
light. Units of f metres, of P dioptres.
1
f 
P
Describing images
real image: Converging rays arrive at the image position, so an
image will be formed on a screen placed there.
virtual image: Diverging or parallel rays make light appear to
come from the image position. The eye creates an image but it
cannot be captured on a screen placed at the apparent
position of the image.
[To understand this you must first understand how the eye works.]
Compared to the object:
• upright or inverted?
• larger, same size or smaller?
What happens if …
• the convex lens is removed?
• a cardboard mask covers half of the lens?
• the screen is moved forward or back?
Converging lens, real image
C21 ppt P7.3 Ray diagrams
Virtual Physics Laboratory simulation
Don Evans Refraction by lenses presentation
A caution: For convenience, ray diagrams showing
image formation use just 2 or 3 rays. Lenses act on
a cone of light.
Diverging lens, virtual image
Human eye with 20:20 vision
Long sight
freezeray.com
Short sight
freezeray.com
Seeing a real image
Seeing a virtual image
Virtual image with a plane mirror
In pairs,
Do McDermott experiment 2.4
Practical session
• Image formation with a lens, comparing short
cameras and long cameras
• Experiments with a fan of rays
• Pinhole camera and lens camera
• Depth of field for a camera
• Law of refraction
• Model eye demonstration with flask
• The lens formula
(all from the Practical Physics website)
Astronomical telescope
Practical Physics: Making a telescope
Spherical aberration
Arecibo radio telescope, Puerto Rico
Reflecting telescopes
use a parabolic mirror
Two kinds of colour
1. Coloured lights (emission is additive)
2. Pigments and filters (absorption is
subtractive)
Demonstration and discussion.
Virtual Physics Laboratory simulation.
Phet simulation Color Vision, with
accompanying question sheet.
Colour – a teaching order
In pairs:
Given these three things to teach, in what order would
you choose to present them? Discuss.
colour filters
and/or
pigments
the appearance of
objects under different
colours of light
white light, pigments
(primary, secondary
colours)
Keep encouraging students to use the terms transmit,
absorb, reflect.
Support, references
talkphysics.org
SPT 11-14 Light & sound
1 Seeing with light
2 Modelling light with ray diagrams
3 Reflection and refraction
4 Colours of two kinds
Practical Physics website, Optics topic
Freezeray.com
David Sang (ed, 2011) Teaching secondary physics ASE / Hodder