|
INTRODUCTION TO
OPTICAL EXPERIMENTS
Objectives
1. Applying lens formula, calculate focal
length of convex lenses.
2. Using an improvised semi-dark box and a parallel rays setup, measure the focal length of assorted lenses
and mirrors.
3. Confirm laws of reflection and laws of
refraction.
3. Disperse white light using a glass
prism or an acrylic prism or a CD.
Materials
and tools (6 groups)
Big black polyethylene plastic bags or
sheets, six big boxes (e.g. 40cm x 30cm opening and 25 cm deep plastic box),
six dry cell holders, electric cables, eighteen D size dry cells, 2.5V torch
bulbs plus some spare bulbs, six torch bulb holders, clothes pegs, A4 plain
paper, protractor pattern sheets, graph pattern sheets, sheets of black
paper, six 250ml beakers, water, six 30cm rulers, colour pencils, some
pair of scissors, adhesive tape and six calculators
Lenses and mirrors
Twelve convex lenses, six concave lenses,
six plane mirrors, six convex mirrors, six concave mirrors, six rectangular
parallelepiped acrylic solid blocks, glass prisms, acrylic prisms, and six
compact discs
Basic techniques
|
|
|
|
|
In order to make mirrors and lenses
stand on the table we can use pegs.
|
Connect
three D dry cells in series in order to make the light source of optical
experiments.
|
|
|
|
|
|
The
drawing above shows the overview of an improvised semi-dark box. We
must wrap four faces of the plastic box with black materials (e.g. black
plastic bags). Add water in the beaker 4cm deep.
|
Set a
beaker with water and the light source unit like the picture above.
Place white sheet or other materials on the bottom. We must check
which pattern or plain sheet we should choose for each experiment.
|
Looking
into the box from the opening, adjust the distance between the beaker and
the bulb in order to make parallel light rays.
|
Activities
1. Focal
length measurement of convex lenses using lens formula
1)
Fix a sheet of white
paper on the back side of a plane mirror and trim the paper along the edges
of the mirror. The white paper side is called white board.
2)
Place the white board perpendicular to the table
using two pegs.
3)
Check the approximate focal length of the convex lens
using the sun (or other light source) and plain paper.
4)
Place the convex lens facing its surface toward the
white board. Distance between the lens and the board must be longer
than the approximate focal length.
5)
Place a torch bulb as shown on the diagram and turn
on the bulb using a dry cell.
6)
Adjust the distance between the lens and the board,
in order to make a clear image of the bulb filament on the board.
7)
Measure the distance “a” and “b”.
8)
Calculate the focal length (f) of the lens using lens
formula.
2.
Focal length measurement of assorted lenses and mirrors
Convex
lens
|
|
|
1)
Set up an
improvised semi-dark box and make clear parallel rays.
2)
Place a graph
pattern sheet or a ruler on the bottom of the box.
3)
Place a convex lens
like the drawing and the picture above.
4)
Measure the focal
length “f”.
|
Concave
lens
|
|
|
1) Place a sheet
of plain paper on the bottom of the box and set a concave lens on the paper
using a peg.
2) Trace
the diverged light paths and the position of the lens on the paper.
3) Remove
the paper from the box and extend the light path lines to find the intersection
of the lines.
|
Convex mirror
|
|
|
1) Place a sheet
of plain paper on the bottom of the box and set a convex mirror on the
paper using a peg.
2) Trace
the diverged light paths and the position of the mirror on the paper.
3) Remove
the paper from the box and extend the light path lines to find the
intersection of the lines.
|
Concave mirror
|
|
|
1)
Place a graph
pattern sheet or a ruler on the bottom of the box and set a concave mirror
using a peg.
2) Measure the focal
length.
|
3.
Measurement of incident and reflection angles
|
|
|
1)
Place a protractor
pattern sheet, plain mirror with pegs on the bottom of the box.
2)
Read and record the
angle of incidence (i) and the angle of reflection (r). Use the
brightest beam of light to observe.
|
4. Measurement of refraction angles
|
|
i1 &
i2:
angles of incidence
r1 & r2:
angles of refraction
|
|
|
1) Put a
sheet of plain paper on the bottom of the box and place a rectangular
parallelepiped acrylic solid block on it.
2) Mark the
clearest light path and edges of the block on the paper.
3) Remove
the paper from the box and trace the path. Add normal lines.
4) Measure
i1, r1, i2 and r2.
|
5.
Dispersion of white light
|
|
1)
Put a sheet of black paper on the bottom of the
box.
2)
Put a glass or acrylic prism on the paper.
Place the prism in the light rays and start rotating it slowly in order to
see the dispersed colours.
3)
If you do not have the glass prism, set a CD with a
peg on the bottom of the box and try to disperse the light.
|
assignment sample
INTRODUCTION
TO OPTICAL EXPERIMENTS
|
Names of group member (fill in block
letters)
|
|
Index number
|
First name
|
Middle name
|
Last name
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
1.
Focal length measurement of convex lenses using lens formula
|
|
|
|
Approximate estimate of focal length (cm)
|
a (cm)
|
b (cm)
|
Focal length (cm)
|
|
Lens No. ( )
|
|
|
|
|
|
Lens No. ( )
|
|
|
|
|
2.
Focal length measurement of assorted lenses and mirrors
|
Convex lens
|
Concave lens
|
Convex mirror
|
Concave mirror
|
|
Lens No. ( )
|
Lens No. ( )
|
Lens No. ( )
|
Lens No. ( )
|
|
|
|
|
|
|
f
=
cm
|
f
=
cm
|
f
=
cm
|
f
=
cm
|
|
3. Incident and reflection angles
|
4. Refraction angles
|
|
|
|
|
Incident
angle ( )
Reflection
angle ( )
|
i1=
r1=
i2=
r2=
|
|
5. Dispersed colour of light by prism
Draw using colour pencils
|
|
|
|
