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Sponsored by:
Programme – 2ndApril 2009
Time
13:00
13:00 – 13:50
13:50 – 14:55
15:00 – 15:15
15:20 – 15:55
NOTES:
Event
Welcome & Introduction
Dr Lisa Jardine-Wright
An Introduction to Light
Dr Lisa Jardine-Wright
Practical Session:
Lenses, Light & Telescopes
Break for Drinks
Extreme Waves in Astronomy
Dr Lisa Jardine-Wright
Please note that students are not allowed to take
food or drink into the lecture theatre or
laboratories. Refreshments will be provided for
the break.
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NOTES:
Introduction to Light
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4. REMOVE THE LAMP AND THE VIEWING SCREEN
5. Put the 30cm focal-length lens about 10cm after the original
screen position, and look through it from a position just part
the end of the optical bench. Adjust its position, if necessary,
until the image comes into focus.
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What can you see?
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6. How might you estimate the magnification of your telescope?
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C - Magnifying glass:
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Experiment
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1. Reassemble the optical bench with the light source and
viewing screen.
Lamp Cork Lens
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Graph Paper
Screen
Metre Rule
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Optical Bench
2. Now REMOVE THE LAMP AND VIEWING SCREEN and put
your eye up to the end of the ruler to look through the lens.
What do you see?
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3. This is the normal situation used for a magnifying glass.
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D - Making a telescope
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1. Put the lamp, pointer, lens and screen as shown in the
diagram below.
Graph Paper
Lamp Cork
Lens
Screen
Metre Rule
Optical Bench
2. Move the lens so that your image is about two times the size
of your object (magnification of 2). Write down the focussed
position of the screen.
3. Screen is at ______________________ cm
16
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5
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-1
(1/u)/ (cm )
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1/u versus 1/v
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(1/v)/ (cm -1 )
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7. Using your measurements of u and v in cm, plot 1/u
against 1/v and calculate the value of the gradient and the
intercept with the “y”-axis?
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-1
Gradient of graph = _____________________ cm .
-1
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Intercept of graph = _____________________ cm .
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1
= ____________________cm.
int ercept
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What do you notice about this last value?
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Can you put together an equation relating 1/u, 1/v and the
intercept?
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This is called the Lens maker’s formula.
Use this space for calculations:
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7
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2. Put the small pointer and cork at the zero mark at the lamp
end of the rulers
3. For about six different positions of lens, find the image by
determining when the object is in focus on the viewing screen.
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Start with u about 30 cm and getting larger.
Then work from u = 30cm getting smaller until 15cm.
4. You will need to measure:
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u
v
6. Take measurements filling in the table below:
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u (cm)
v (cm)
1/u (cm -1)
1/v (cm -1)
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5. Once you see a clear image of the pin hole on the card read
off the ruler the position of your lens.
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My lens is at __________________cm.
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6. What do you think is happening to the light once it goes
through the pin hole? Can you draw a sketch?
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7. What is the focal length of the lens, f?
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f = _____________________________________
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B – Lens Maker’s Formula:
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Experiment:
1. Set up the optical bench with the light source as before,
but this time without the card with the pin-hole, and move
the lamp further away from the rulers.
Lamp
Lens
Cork
Graph Paper
Screen
Metre Rule
u
12
v
Optical Bench
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Practical : Lenses, Light & Telescopes
Safety: HOT COMPONENTS
focus
Please be aware that during this experiment the lamps
may get quite hot and are relatively bright.
Handle them carefully and please take care not
shine them into each others eyes.
A - Measuring the focal length of a lens:
Equipment:
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Optical bench with metre rule
1 lamp
Cork pointer
Card with pin hole
Card screen
2 lenses one labelled “15” one labelled “30”
Mirror
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The aim of this first part of the experiment is to measure the
actual focal length of a lens.
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The focal lengths of the lenses provided are nominally 15cm
and 30cm (though some are quite far from these values).
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Choose the lens labelled 15.
Experiment
1. Place the lamp at the end of the ruler as shown.
Aim:
2. Stand the card with the pin-hole in it up against the end of the
ruler, using the lamp to keep it there.
To investigate how lenses can be used to form images. Using
two converging lenses with different focal lengths we will make a
simple telescope.
3. Place the lens approximately 15cm from the hole with the
mirror beyond it.
Background & Theory:
4. Turn the lamp on and adjust the position of the lens/mirror so
that a focussed image of the hole is formed on the card close
to the hole. (The mirror reflects the image back on to the
card).
Converging lenses and light:
lens
Lamp Pin-hole
Lens
Mirror
Metre Rule
focus
Optical Bench
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