3B Scientific Light Box - Manual (Page 2)

2
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 ↓
W R M O Y G C B V
R
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