OUTREACH PROGRAMME 2006
SECOND YEAR STUDENTS
Junior Certificate Science
To show that light travels in straight lines
What you need: 2 pieces of card with a hole in the centre.
What to do:
1. Hold the 2 pieces of card, one behind the other, so that
the holes in the centre line up.
2.
3. Look through the holes in the cardboard. Can you see
the person sitting beside you? Yes/No?
4. Now, move one of the pieces of card so that the holes
no longer line up. Can you see the person beside you?
Yes/ No?
What does this mean?
You should only be able to see the person beside you when the
holes in the centre of the card are in a straight line. This proves that
light travels in straight lines.
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To investigate the reflection of light by a plane mirror
What you need: plane (flat) mirror, upright pin, paper, ruler,
pencil, and protractor.
What to do:
1. Place the pin on a piece of paper opposite a plane
(flat) mirror, as in the diagram above.
2. Using your pencil, draw the Normal line, which is
at a right angle to the mirror, as shown.
3. Using a pencil, mark two or three points on the
ray of light going from the pin to the mirror (right
of Normal). Use your ruler to connect the points.
4. Now, look into the mirror from the left side of the
Normal line. You should be able to see the
reflection of the pin in the mirror. Using your
pencil, mark two or three points along the ray of
light coming from the mirror. These points should
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be in a straight line with the image of the pin that
you can see in the mirror.
5. Use your ruler to connect these points.
6. Now, measure angles i and r using a protractor.
7. Change the angle that the pin is making with the
mirror, and repeat steps 3-6.
What did you see?
Angle i
Angle r
1st Time
2nd Time
3rd Time
You should see that Angle i is the same (or very nearly the
same!) as Angle r each time.
This means that when the Angle of Incidence (i) is large, the
Angle of Reflection (r) is also large.
So, if the Angle of Incidence (i) gets smaller, so does the
Angle of Reflection (r).
This happens because light travels in straight lines.
So,
when light is reflected from a plane mirror, the Angle of
Incidence and the Angle of Reflection are always the same!
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Dispersion (Students sheet)
Aim:
Show in a simple experiment that white light is made up of
coloured light.
Experiment:
1. Turn on the white source in front of the screen.
2. Position the prism between the beam and the screen.
3. Rotate the prism until you see light on the screen.
Q1> What can you see on the screen?
Q2> What is the name of these colours?
Q3> What is dispersion?
Conclusions / Remember:
a. The breaking up of white light into its spectrum colours is called
dispersion.
b. The spectrum of light is made up of seven major colours:
RED – ORANGE – YELLOW – GREEN – BLUE – INDIGO –
VIOLET . (Richard of York Gives Battle In Vain)
c. The dispersion of white light by water droplets creates rainbows.
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