Transcript LIGHT Refraction • Refraction Fact #1: As light goes from one medium to another, the velocity CHANGES! • Refraction Fact #2: As light.

LIGHT
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!
Refraction – Going from Air to Water
• If a light ray goes from a LOW refractive Index
(FAST MEDIUM) to a HIGH refractive index
(SLOW MEDIUM), it’s speed DECREASES and
the angle BENDS TOWARDS the normal
Refraction – Going from Water into Air
• If you are going from a HIGH refractive index
to a LOW refractive index, your speed
INCREASES and the angle BENDS AWAY the
normal
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.
Total internal reflection
• happens when the angle between the light ray
and the normal is greater than the critical
angle.
• The critical angle is formed when the light ray
travels along the boundary between the two
substances.
• Optical fibres use multiple total
internal reflections to transmit light.
• 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.
Apparent Depth Prac
• Coin and Fish
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 didn’t.
• 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 of
higher density, the light rays refract
towards the normal.
• 3 a
From the tip, to the eye.
• 4 a
From the coin to the eye.
Lenses
• There are two main types of lenses:
• convex lenses—these 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
Lenses – An 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.
Movement of Light through Lenses
CONVEX LENS
• In a convex lens, an incoming ray parallel to the
principal axis is refracted through the principal
focus (F).
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).
Movement of Light through a Lens
• The distance from the centre line (plane) of the
lens 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.
Focal Length
• The greater the curvature of a lens, the more
it bends light and hence the shorter the focal
length.
Image type and Location
• Convex lenses produce two different types of
images, depending on where the object is
located.
• If the object is at a distance greater than the
focal length of the lens, a real image is
formed.
• A real image can be projected onto a screen
Convex Lenses
• If the object is at a distance less than the focal
length of the lens, a virtual image is formed.
• This image can’t be projected onto a screen.
Concave Lenses
• Concave lenses produce only virtual images.
Finding the focal length
• Rays coming into a lens from a distant object are
almost parallel and form an image very close to
the focus.
• We can then measure the distance from lens to
image to determine the focal length of the lens.
Images in a Convex and Concave Lens Prac
• AIM: To investigate the image formed by different convex
lens and concave lens
• Complete Convex prac – using 2 different convex lenses
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 may
be ‘caught’ on a screen) using a concave lens.
2) Explain how the image changes as the object-to-lens distance is
varied.
Answers
Optical instruments
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Telescopes
Telescopes make small, far objects appear larger.
Two lenses are used.
The objective lens produces a real, inverted image
just inside the focus of a second lens, called the
eyepiece lens. The image produced by the first lens
now acts as the object for the second lens. Because
the first image is inside the focus of the second lens,
the second image (the one seen by the telescope
user) is virtual and enlarged compared to the first
one
Eye ball prac
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