3B Scientific Light Box User Manual

LIGHT BOX & OPTICAL SET

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Contents

• 1 Light Box

• 1 set of 8 color cards

• 1 Plane mirror

• 1 Semi-circular mirror

• 1 Lens biconvex large

• 1 Rectangular slab

• 1 Prism 45° 45 °90°

• 1 Prism 60° 60 °60°

Introduction

The apparatus consists of a source of light rays and a variety of optical devices that reflect and refract
light. The set allows study of the phenomena of reflection and refraction and a variety of color
experiments.

The light source is located in a specially constructed light box the top view of which is shown below:

• 2 Slit former plates

• 1 set of 8 color filters

• 1 Parabolic mirror

• 1 Lens biconvex small

• 1 Lens biconcave

• 1 Semi-circular slab

• 1 Prism 60° 30° 90°

• Spare Lamp

For Color

Experiments

Light

Source

Box

Collimatin

For Experiments

On Reflection &

R

n

Fig. 1

Study of colors: The source-end of the box is fitted with a light bulb and has one front opening and

two side openings. The side openings are fitted with mirrored doors to reflect the light emerging from
the openings. All three openings are constructed so as to allow the color filters (provided in the kit) to
be fitted into them. Then, by adjusting the mirrors, the reflected side-rays can be swung back and forth
to overlap the center-beam from the front opening. A colorful pattern can be obtained on a screen
placed in front of the center-opening about 15-20 cm from the light-box.

Study of reflection & refraction: The other end of the box also has an opening in which a slit plate or
a color filter can be fitted. Between the source and the opening is a collimating lens, the position and
effect of which can be changed by an adjusting knob on the top of the light-box. The rays emerging
from the opening are used to perform various experiments in reflection and refraction.

While performing the experiment: The results of an experiment are obtained best in darkness, as the
rays are most clearly visible then.

The collimating lens should be adjusted to give a converging, parallel or diverging beam of light, as
desired. For strong convergence (or divergence), a convex (or concave) lens could also be used.

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The optical devices (mirrors, lenses, prisms, slabs) should be handled by their finger-grips, so that no
smudges or scratches are inadvertently left on the optical faces. The bases of the lenses and prisms are
specially finished to provide a background for obscuring the path of light rays.

The optical accessory being used should be placed on a plain sheet of paper on which its outline can be
marked by running a pencil around its perimeter. Rays can be traced by marking two points on each
ray (incident on the accessory and emerging from it) as far apart as possible. Its path, within the
accessory, can be traced by joining the incident and emergent point by a straight line (after removing
the accessory). The direction of propagation can be indicated by arrowheads on the ray paths.

Possible problems and their remedies:

Faint secondary rays emerging from the light box: These are caused by reflections from the

support-wire of the filament and can be removed by rotating the bulb-holder (on top) by 180° so that
the support-wire lies behind the filament.

Internally reflected rays inside the device: Faint, secondary reflected rays (in the case of refracting
surfaces) are normal, as there is some light always reflected at such surfaces, and can be neglected.

Sometimes, rays enter the device from above and reflect off the vertical surfaces, giving erroneous
results. These can be removed by

• Blanking off the top of the slits to shorten the rays (not very helpful).

• Moving the entire set-up farther from the light-box.

• Placing the entire set-up slightly higher than the surface on which the light-box rests.

Experiments with colors

The source end of the light-box is to be used for experiments in this section. Filters can be fitted in the
three openings and the mirrors can be adjusted so that a rich mixing of colors takes place on a screen
placed in front of the center-opening.

Colors of objects:

• Close the side-openings (use the mirrors) and one by one, place the filters in the center-opening

and observe the color on the screen in front of it. This will familiarize you with the various colors.
Next place cards on the screen and observe their colors in lights of different colors. Note your
observations in a table of the form:

Color of light falling on
plate →
Color of plate ↓

R

W R M O Y G C B V

M

O

Y

G

C

V

Index: W-White, R-Red, M-Magenta, O-Orange, Y-Yellow, G-Green, C-Cyan, B-Blue, V-Violet

• Next, observe these cards through colored filters and tabulate your results in a similar manner

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Color of filter used →
Color of plate in white
light ↓

R

W R M O Y G C B V

M

O

Y

G

C

V

Index: W-White, R-Red, M-Magenta, O-Orange, Y-Yellow, G-Green, C-Cyan, B-Blue, V-Violet

• After familiarizing yourself with the different colors, perform some experiments on your own by

observing a color card illuminated by light of one color and viewed in another color. Try to predict
your observation before performing the experiment.

Color Combinations: By using the side-openings, up to three colors can be projected onto the screen.
Place the weakest color filter in the center-aperture and the stronger ones on the sides so that intensity
loss (due to reflection) is compensated.

Initially, take any set of filters and note down the different colors formed by their overlapping.

Next, try to find COMPLEMENTARY COLORS, i.e., combinations that reproduce white light. Note
down the colors and their complement (the complement of color A is the color, which on combining
with it gives white light).

Shadows: Remove the filters and close the side-openings. Place a pencil in front of the screen. A sharp
shadow within a faint one can be observed. If the faint shadow is too faint, use a tracing-paper to
diffuse the light coming from the light box.

The sharp shadow is called the UMBRA and is the region where no light falls. The faint shadow
enclosing the umbra is the PENUMBRA, the region where partial light falls.

The phenomenon of umbra-penumbra can also be observed in everyday-life in your own shadow.

Try to explain the phenomenon with a ray diagram.

Colored Shadows: Place a set of complementary colors in the openings and adjust the mirrors to
obtain the white light on the screen. Place a pencil in a patch illuminated by all three colors and
observe the shadows formed. Shift the pencil to patches illuminated by all three colors and observe the
shadows formed. Shift the pencil to patches illuminated by two colors and then by one color and
observe the shadows. The number of shadows formed is the same as the colors illuminating the patch,
while their colors are the same as a few of the patches.

The above phenomena occur with any set of colors (not necessarily complementary). Try to explain the
number of shadows and their colors. (A ray diagram will prove helpful here, too.)

Experiments on Reflection:

Reflection is a phenomenon where light falling on a surface bounces back, in accordance with the
following laws:

1. The angle of reflection is equal to the angle of incidence.

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2. The incident ray, normal ray and reflected ray, all lie in the same plane.

Besides these laws, there are other important features of reflection that we come across every day, i.e.,
lateral inversion, shaving mirrors, rear-view mirrors etc.

Reflection from a Plane Mirror: Adjust the collimating lens so that the resultant beam of light is
parallel. Allow a single ray to pass out of the light-box (use a slit plate). Mark its position on the paper.
Place the plane mirror in its path, at an angle. Mark the position of the mirror and that of the reflected
ray. Remove the mirror and draw a normal to the outline of the mirror at the point where the incident
and reflected ray meet the outline. Measure the angles between the normal and incident rays and
normal and reflected rays to obtain the angles of incidence and reflection. Check that the FIRST LAW
OF REFLECTION is verified.

Next, replace the slit plate by the multiple slit plate with three or four slits. As above, place the mirror
in the path of the rays and measure the various angles of incidence and reflection.

Next, place a concave (or convex lens) in the path of the rays to obtain diverging (or converging) rays
falling on the mirror. After plotting their paths, measure the various angles, and answer the following
questions:

• Do these observations match with what you expected?

• What happens to the parallel, converging and diverging rays after they are reflected?

Reflected Images in a Plane Mirror: Set up the apparatus so that multiple converging rays fall on the
mirror. On looking into the mirror, you will see that the incident rays and the images of the reflected
rays appear to form the complete straight line paths of the incident rays (and vice-versa). This can be
verified by lifting the mirror and observing the real converging rays.

• Are the image of the reflected focus and the real focus (in the absence of the mirror) the same

point?

• Join the real and reflected foci by a straight line. What angle does it make with the mirror?

• Why is the image of the reflected rays fainter than the actual rays observed by removing the

mirror?

Lateral Inversion: When you look at an object and then at its image in a mirror, the two are observed
not to be identical.

• What is different about them?

The above phenomenon is called lateral inversion.

Remove all slit-plates from the light-box and adjust the collimating lens for a parallel beam of light.
Stand a pencil in the left (or right) corner of the opening and then look at the image in the mirror.
Where does the pencil stand?

• From the above experiment can you explain why lateral inversion occurs?

Place a multiple slit-plate in the opening so that four parallel rays fall on the mirror. Hold the red filter
in front of two slits and the violet filter in front of the other two so that the two inner rays are stopped
by the filter frames and the outer rays are of different colors. Observe the image.

• Are the rays from the left side of the light-box and the image on the left side the same color?

Look at yourself in a plane mirror. Do you observe any vertical inversion?

• Why does vertical inversion not occur?

If you observe the image of the word MAXAYAXAM in the mirror, what do you expect to see?

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Similarly, if you place the mirror on the dotted line and look at the image of the word below from the
bottom of the page, what do you see?

--------------------------­ DI-OXIDE

An object is said to be SYMMETRICAL about an axis parallel to the mirror surface if the two halves
of the object are mirror images of each other.

• Are the above bold words symmetrical? If so, about which axes?

Multiple Reflections - Single Mirror: This experiment requires knowledge of refraction.

Allow a single ray to fall on the plane mirror placed very close to the light-box. Examine the reflected
rays closely.

• How many rays do you observe? Which one is the brightest and which one is the faintest?

• Explain how the images occur?

(HINT: Use the theory of refraction and the fact that the reflecting surface of the mirror is at the
back).

• What happens to the rays as the angle of incidence is

- Decreased from 45° to 0 °?

- Zero?

- Increased from 45 °to 90°?
To study an angle of incidence of 90°, place the mirror so that its side completely covers the slit
and the mirror is parallel to the ray.

• What do you observe at the other side of the mirror?

Rotate the mirror keeping the slit covered by one end of the mirror at all times. What happens?

The above phenomenon is used in OPTICAL FIBERS where light follows a curved path within the
fiber.

Shift the mirror forward and allow the ray to fall on the side of the mirror and observe the mirror from
above.

• How does the above phenomenon occur?

(HINT: Internal reflection)

Multiple Reflections - Multiple Mirrors: Borrow a plane mirror from another light box set. Place the
two mirrors at right angles to each other. Allow a single ray to fall on one of the mirrors. Observe the
ray reflected from the other mirror - it should be parallel to the incident ray.

• Using geometry, explain why this should be so.

• Is this true for all angles of incidence?

• Look into the corner of the mirrors. What do you observe?

If a large plane mirror is available, place it on the table with these two mirrors on it so that all are
mutually pair-wise perpendicular.

• What do you observe in the triple corner?
The above experiment was performed successfully on the moon as well.

• What can you say about the nature of light?
Examine the reflectors at the back of a car. What shape are the dimples in them?

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Ref

Rotation of a Plane Mirror: Aim a single ray at the mirror and note the incident ray, mirror position
and reflected ray. Rotate the mirror slightly around the incident point and reflected ray. Measure the
angle of rotation of the mirror (∠M) and that of the reflected ray (∠R). Repeat this for different angles.

• Derive a relation between ∠M and ∠R.

The above method is used in various instruments to amplify small changes.

Concave Mirror - Focus, Center of Curvature:

• Aim a set of parallel rays at the center of the mirror such that they are parallel to the mirror’s axis

of symmetry. Note the point where they converge - this is the FOCUS. The distance of the focus
from the center of the mirror is called the FOCAL LENGTH (f). The center of curvature lies at a
distance of 2f from the mirror along this line. The distance of 2f is the radius of curvature, ‘r’ of
the mirror.

• Place a concave lens (of short focal length) in front of the slits to produce diverging rays. Shift the

mirror until the reflected rays retrace their paths. The CENTER OF CURVATURE is the point
where the incident rays appear to diverge from and the RADIUS OF CURVATURE is the distance
of the point from the mirror.

• If, in the above experiment, you are unable to obtain a position where the reflected rays retrace

themselves, use

2111

=+=

rvuf

where, f = focal length of the mirror

u = distance between the mirror and the point from where incident rays appear to diverge
v = distance between the mirror and the point where reflected rays meet

(All distances are measured along the axis of symmetry)

lections From a

Concave Mirror - Caustic Curve:

SPHERICAL ABERRATION is the phenomenon in which parallel rays far away from the center of
the mirror do not focus onto the same point as those near the center, giving a distorted image.

To study aberration, allow four parallel rays to fall on the mirror and mark the points where two
adjacent rays meet. Shift the light-box to obtain another set of parallel rays, ensuring that the new set is
parallel to the old one and do the same. Join these points free-hand to obtain a curve called the
CAUSTIC CURVE.

Spherical Concave

Mirror

Fig. 2

X

C F

• What causes the caustic curve?

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