Download - Light Refraction and Lenses
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LIGHT
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Refraction
Refraction Fact #1: As light goes from one
medium to another, the velocity CHANGES!
Refraction Fact #2: As light goes from one
medium to another, the path CHANGES!
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RefractionGoing from Air to Water
If a light ray goes from a LOW refractive Index(FAST MEDIUM) to a HIGH refractive index
(SLOW MEDIUM), its speed DECREASES and
the angle BENDS TOWARDS the normal
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RefractionGoing from Water into Air
If you are going from a HIGH refractive indexto a LOW refractive index, your speed
INCREASES and the angle BENDS AWAY the
normal
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There is one exception
That is when light hits head-on,
perpendicular to the boundary. The light does
not bend but its speed still changes.
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Total internal reflection
happens when the angle between the light rayand the normal is greater than the criticalangle.
The critical angle is formed when the light raytravels along the boundary between the twosubstances.
Optical fibres use multiple total
internal reflections to transmit light.
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Copy the diagrams into your workbook.
Draw a normal wherever the light rays enter a
new substance.
Demonstrate what will happen to the rays as
they enter and exit from the substances by
continuing the ray through the shape and out
the other side.
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Apparent Depth Prac
Coin and Fish
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Why does it look bent away from
normal and not towards??
Light from the lower part of the ruler is travelling
into a region of lower optical density (air), and so
has been bent away from the normal.
The image in the water is actually an illusion
an
image our brain constructs based on where light
from the lower part of the ruler appears to come
from. It assumes that the light travelled in a perfectly
straight line, even though it didnt.
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Part B: Refraction in water - ANSWERS
1 a Refract; towards.
b Emerge; away from. c Incident; a glass surface; no.
2 When light rays at an angle to the surface
travel from a medium of low density to one ofhigher density, the light rays refract
towards the normal.
3 a From the tip, to the eye.
4 a From the coin to the eye.
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Lenses
There are two main types of lenses:
convex lensesthese curve outwards and are
fatter in the middle
concave lenses
these curve inwards (a little
like a cave) and are thinner in the middle.
Convex lenses Concave Lenses
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LensesAn application of refraction
There are 2 basic types of lenses
A converging lens (Convex)
takes light rays and bring
them to a point.
A diverging lens (concave) takes
light rays and spreads them
outward.
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Movement of Light through Lenses
CONVEX LENS
In a convex lens, an incoming ray parallel to theprincipal axis is refracted through the principal
focus (F).
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Concave Lens
In a concave lens, an incoming ray parallel to
the principal axis is refracted so that it
appears to come from the principal focus (F).
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Movement of Light through a Lens
The distance from the centre line (plane) of thelens to the principal focus is called the focal
length of the lens.
A ray passing through the centre of either type
of lens is unaffected.
As with all images, rays of light that come from
a part of the object come together again at
that same part of the image.
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Focal Length
The greater the curvature of a lens, the moreit bends light and hence the shorter the focal
length.
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Image type and Location
Convex lenses produce two different types ofimages, depending on where the object is
located.
If the object is at a distance greaterthan thefocal length of the lens, a real image is
formed.
A real image can be projected onto a screen
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Convex Lenses
If the object is at a distance less than the focallength of the lens, a virtual image is formed.
This image cant be projected onto a screen.
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Concave Lenses
Concave lenses produce only virtualimages.
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Finding the focal length
Rays coming into a lens from a distant object arealmost parallel and form an image very close to
the focus.
We can then measure the distance from lens toimage to determine the focal length of the lens.
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Images in a Convex and Concave Lens Prac
AIM: To investigate the image formed by different convex
lens and concave lens Complete Convex pracusing 2 different convex lenses
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Questions CONVEX
1) What happened as the object was brought closer to the lens?
2) When does;
a a real image (on a screen) is obtained
b a virtual image (one that cannot be caught on a
screen) is obtained c no image is obtained
CONCAVE -
1) Assess whether it is possible to form a real image (one that maybe caught on a screen) using a concave lens.
2) Explain how the image changes as the object-to-lens distance isvaried.
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Answers
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Eye ball prac
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