Time
Event
13:00
Welcome & Introduction
13:00 – 13:30
13:35 – 14:50
14:50 – 15:05
Lecture: Maths with Physics
Dr Lisa Jardine-Wright
Practical Session 1:
Lenses, Light & Telescopes
Break for Drinks
Practical Session 2:
15:05 – 15:55
Investigating our Universe - Gravity
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.
Dr Lisa Jardine-Wright, Cavendish Laboratory
1
December 2010
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
Dr Lisa Jardine-Wright, Cavendish Laboratory
2
December 2010
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
Dr Lisa Jardine-Wright, Cavendish Laboratory
3
December 2010
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
Dr Lisa Jardine-Wright, Cavendish Laboratory
4
December 2010
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
Dr Lisa Jardine-Wright, Cavendish Laboratory
5
December 2010
Practical 1: Lenses, Light & Telescopes
Safety: HOT COMPONENTS
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.
Equipment:







Optical bench with metre rule
1 lamp
Cork pointer
Card with pin hole
Card screen
2 lenses one labelled “15” one labelled “30”
Mirror
Aim:
To investigate how lenses can be used to form images.
different focal lengths we will make a simple telescope.
Using two converging lenses with
Background & Theory:
Converging lenses and light:
lens
focus
focus
Dr Lisa Jardine-Wright, Cavendish Laboratory
6
December 2010
A - Measuring the focal length of a lens:

The aim of this first part of the experiment is to measure the actual focal length of a lens.

The focal lengths of the lenses provided are nominally 15cm and 30cm (though some are quite
far from these values).

Choose the lens labelled 15.
Experiment:
1. Place the lamp at the end of the ruler as shown.
2. Stand the card with the pin-hole in it up against the end of the ruler, using the lamp to keep it
there.
3. Place the lens approximately 15cm from the hole with the mirror beyond it.
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).
Lamp
Pin-hole
Mirror
Lens
Metre Rule
Optical Bench
5. Once you see a clear image of the pin hole on the card read off the ruler the position of your
lens.
My lens is at __________________cm.
6. What do you think is happening to the light once it goes through the pin hole? Can you
draw a sketch?
7. What is the focal length of the lens, f?
f = _____________________________________
Dr Lisa Jardine-Wright, Cavendish Laboratory
7
December 2010
B – Lens Maker’s Formula:
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
Optical Bench
u
v
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.


Start with u about 30 cm and getting larger.
Then work from u = 30cm getting smaller until 15cm.
4. You will need to measure:


u
v
5. Take measurements filling in the table in the computer spreadsheet:
u (cm)
1/u (cm -1)
v (cm)
1/v (cm -1)
6. 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?
Gradient of graph = _____________________.
Intercept of graph = _____________________ cm-1 .
1
= ____________________cm.
int ercept
What do you notice about this last value?
Can you put together an equation relating 1/u, 1/v and the intercept?
This is called the Lens maker’s formula.
Dr Lisa Jardine-Wright, Cavendish Laboratory
8
December 2010
C - Magnifying glass:
Experiment
1. Reassemble the optical bench with the light source and viewing screen.
Lamp Cork
Graph Paper Screen
Lens
Metre Rule
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?
3. This is the normal situation used for a magnifying glass.
D - Making a telescope
1. Put the lamp, pointer, lens and screen as shown in the diagram below.
Lamp Cork
Graph Paper Screen
Lens
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
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.
What can you see?
_________________________________________________________________________
Dr Lisa Jardine-Wright, Cavendish Laboratory
9
December 2010
Practical 2: Investigating the Universe - Gravity
Aim:
To calculate the acceleration due to gravity using a simple
pendulum.
Brief experimentation:
Using the stopwatch to time the swing of the pendulum can you
answer the following two questions:
1. What happens to the length of time for 1 swing if you change
the length of the pendulum?
2.
What happens to the length of time for 1 swing if you
change the angle of the pendulum‟s swing?
Background:
If you drop an object it will always fall at the same rate no matter how heavy it feels. This means
that if we drop a heavy object and a light object from the same height they will land at the same
time (if they are not affected by air resistance or wind).
The rate at which objects fall to the surface of the Earth is known as, g, the acceleration due to
gravity. We will use a simple pendulum to find a value for g.
The time of the pendulums swing, or its period T, depends on the length of the string and the
acceleration due to gravity, g
4 2
T 
L
g
2
where T, is the time for one swing (that is once in both directions left & right or right & left), L, is the
length of the pendulum,  = 3.14159 and g is the acceleration due to gravity.
If we experiment with changing the length of the pendulum and timing its swing we can plot a
graph of L against T2.
If we compare
T 2  slope  L  intercept
The slope or gradient of this graph is then equal to ________________________.
The intercept of this graph (where the line crosses the y-axis) should be =
Dr Lisa Jardine-Wright, Cavendish Laboratory
10
December 2010
Experiment:
Our simple pendulum is made of a weight on the end of a string hung from a clamp
stand.
Task:
L
In pairs you are going to time how long it takes for the pendulum to complete a
swing (from one side to the other and back again). This is known as the
period of the pendulum which we give the label, T.
What happens if you make the string shorter or longer?
What happens to the period, T, if you make the angle of the swing larger?
To improve the accuracy of our experiment you will make 2 timings for 5 swings for different
lengths of string and put each of your results into the table (like that below) on the computer.
Length of
String
1. Timing for
5 swings
2. Timing for
5 swings
L (m)
T1
T2
Average
Timing for 5
swings
Average
Timing for 1
swing.
T3 =(T1+T2)/2
T = T3/5
Period
Squared
T2
0.10
0.20
0.30
0.40
Results & Analysis:
Now we are going to plot a graph of T2 (the time for one swing squared) against the length of the
string, L. using the computers and an excel spreadsheet
What do you notice about the points that you have plotted on your graph?
Task:
Using excel we will fit a straight line through the points that is a `best fit; to the data called a
“trendline”.
Should our „best fit line‟ go through (0,0)?
1. Calculate the gradient of your best fit line (see graph example below),
Slope = gradient = Height of your triangle = __________________________________
Base of your triangle
Dr Lisa Jardine-Wright, Cavendish Laboratory
11
December 2010
2. Use excel to find out the equation of your line and compare the equation with the slope you
calculated above
0.6
Length of String, L (m)
0.5
0.4
Height
0.3
0.2
Base
0.1
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Period Squared, T 2 (s2)
Conclusions:
The slope of your line is going to help us to calculate the acceleration due to gravity.
Slope =
4π2
=
acceleration due to gravity, g
So, g =
4π2 = ___________________
slope
4π2
g
My experiment measured g = __________________m s-2.
Dr Lisa Jardine-Wright, Cavendish Laboratory
12
December 2010