SECTION B: ACTIVITIES

ACTI VITY BI
AIM

To identify a diode, an LED, a resistor and a capacitor from a mixed collection of

such items.

APPARATUS AND MATERIAL REQUIRED

A multimeter, a mixed collection of diode, LED, resistor and capacitor.
THEORY
when
Diode. A two terminal device which conducts current when forward biased and not
reverse biased. It does not emit light during its conduction.
conducts current when forward
LED. A light emitting diode is a two terminal device which
conduction.
biased and not when reverse biased. It emits a characteristic light during its
Resistor. A two terminal device which conducts equally in both directions.
resistance to dc but has a finite reactance
Capacitor. A two terminal device which offers infinite
multimeter shows a large current initially (for
for ac. When connected across a dc source, a
capacitor initially draws a charge.
C>u) which decreases to zero quickly. This is because the
DIAGRAMS D
Violet -Orange
Red Silver

Carbon resistor
Dr Diodes
LED
1000 uF
6V
0.47 uf
Paper Electrolytic
Mica CAPACITORS

diodes, LED and capacitors.

Fig. 1 Diagrams of carbon resistor,
PROCEDURE
components. lf a component has a set of three colour
1. Look for the colour bands on the given component is a resistor.
bands followed by asilver or gold band, then the
into common and positive terminals of the
2. Insert the black and red leads (or probes) highest range (0-M2).
multimeter. Turn its selector switch to resistance mode -
component one by one. Note the direction of
O. louch the two probes to the two ends of each component
deflection in the multimeter. Interchange the positions of two probes for each
and again note the direction of deflection.
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 LAB MANUAL PHYSICS-XII

4. If the multimeter shows an equal deflection in both directions, then the component is aresisio
light from h.
5. 1f amultimeter shows deflection in one direction without any emission of
diode
component and no deflection in the opposite direction, then the component is a
light from h
6. If the multimeter shows deflection in one direction alongwith the emission of
is an LED
component and no deflection in the opposite direction, then the component
7. If the multimeter does not show any deflection on connecting its probes either way to a
capacitor is laroe
component, then the component is acapacitor. But if the capacitance of the
the multimeter will show a large deflection initially which gradually decreases to zero.
8. Record all your observations in a tabular form.
OBSERVATIONS
Table B1 : State of conduction of each component

Item code State of conduction of a component Identified component

A Conducts equally in both directions
B Conducts in one direction without emission of light
Conducts in one direction with emission of light
D Does not conduct, gives an initial deflection which
decays to zero

RESULT
From the mixed grouping of components, the components marked A, B, Cand Dhave been
identified as resistor, diode, LED and capacitor respectively.
PRECAUTIONS
1. Whilechecking the conduction state of any component, clean its leads properly.
2. Use the selector switch of the multimeter in resistance mode with highest range option.
3. While testing any component, avoid touching the metal end of either of multimeter probe.
Body resistance in parallel with the component resistance may create confusion about the
conduction state of the component.

VIVA VOCE
1. What is aresistor ? 5. How do the conduction states of an ordinary diode
Any material that has some resistance is called a and an LED differ ?
resistor. An ordinary diode conducts in forward biasing
2. What is a linear resistor ? without any emission of light while an LED conducts
A linear resistor is one which obeys Ohm's law or for in forward direction with emission of light. Both do
not conduct in reverse biasing.
which V-lgraph is a straight line passing through the
origin. 6. How does a capacitor behave towards dc?
3. What is a non-ohmic device ? A capacitor does not conduct dc. But a capacitor of
large capacitance shows an initial deflection in the
A device which does not obey Ohm's law is called a
ammeter which decays to zero quickly. This is due to
non-ohmic device. Semiconductor diodes, LEDS, etc;
are non-ohmic devices. charging of the capacitor.
4. Does an ohmic resistor conduct equally for both 7. How does a capacitor behave towards ac ?
forward and reverse biasing ? A capacitor conducts ac because its capacitive
Yes, an ohmic resistor conducts equally when current is is finite against ac.
passed in one direction and then in opposite direction. 2C)
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 Section B: ACTIVITIES

ACTI VIT Y B2
AIM
Use of multimeter to see the unidirectional flow of current in case of a diode
and an
LED and check whether a given electronic component (e.g., diode) is in working order.
APPARATUS AND MATERIAL REQUIRED
A multimeter, a diode, LED, sand paper.

THEORY
1. To check the unidirectional flow of current through a junction diode/LED. When a
junction diode/LED is forward biased, a substantial current ( few mA) willflow through it.
When a junction diode/LED is reverse biased, a negligible current ( few A) will flow
through it.
2. To check whether a diode is in working order. A junction diode offers a low resistance (a
few 2 to k) during forward biasing and it offers a very high resistance (r M2) during
reverse biasing. Thus the working of a junction diode can be examined by measuring its
resistance in forward and reverse biased conditions.

WORKING DIAGRAM
Diode R

RE
p-n
+ Red probe
- Black probe

K 6V variable
dc battery
Fig. 2 To check unidirectional flow of current through a diode.

PROCEDURE
(a) To check unidirectional flow of current through a junction diode/LED
1. As shown in Fig. 2, connect the junction diode, a resistance box, a 6 V variable dc battery
and a plug key K in series. Adjust the battery to minimum voltage.
2. Set multimeter in current measuring mode at a suitable range of mA (starting from high
current
current range). Take out a suitable resistance R from the resistance box so that the
K and note the value of urrent
flows within the range chosen. Insert the plug in the key
flowing in the circuit.
3. Increase the forward bias in steps of 0.2 V. Note the current in each case. Beyond a certain
forward bias.
applied voltage, current increases rapidly with the increase in
4. Reverse the terminals of the junction diode so that its pend is at lower potential and rend is
at hicher notential. Observe the current in multimeter on uA scale. Negligible current
reading will indicate the unidirectional teature of the diode. Increase in reverse bias will
show negligible change in reverse current.
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 LAB MANUAL PHYSICS-XII

5. Replace the diode by LED and repeat steps 1to 4. It will be seen that LED allows the flow
when the
current only when it is forward biased. LED starts glowing applied
exceeds 1.2 V. Initially, it glows faintly. lts glow becomes brighter and brighter as the voltage
is gradually increased. voltage
(b) To check whether the junction diode is in working order or not
6. Set the multimeter in resistance measuring mode.
7. Touch the two probes of the multimeter to the two end terminals of the junction diode. Note
the diode resistance. Reverse the diode connections. Again note the diode resistance. If the
diode resistance is low in one case and high in the other case or Vice versa, then the diode is

in working order.
8. If the diode resistance is low both during forward and reverse biasings, then the diod :
short-circuited. If the diode resistance is very high both during forward and reu
biasings, then the diode junction is discontinuous or open. In both of these situations #he
diode is not in working order. Record the observations in a tabular form.
OBSERVATIONS
A.For unidirectionalflow of current through the junction diode
Table B2: When the junction diode is connected in forward bias
Forward bias voltage Forward current
S.No.
(V) (mA)
1
2
3.
4
5

Table B3: When the junction diode is connected in reverse bias

Reverse bias voltage Reverse current
S.No.
(V) (uA)
1.
2.
3.

5.

B. For unidirectional flow of current through the LED

Table B4: When the LED is connected in forward bias
S.No. Forward bias voltage Forward current
(mA)
1.
2

3.
4
5

184
 Section B: ACTIVITIES

Table B5:When the LED is connected in

reverse blas
S.No. Reverse bias voltage Reverse current
(V) (uA)
1.
2
3.
4
5.

C. For working condition of the junction diode

Table B6 : Measurement of the diode resistance with
multimeter
S.No. Biasing condition of the diode Resistance Nature of resistance
1
(2)
Forward biasing Low/Very high
2.
Reverse biasing Low/Very high
RESULT
1. The unidirectional feature of a junction diode and LED has been established, both conduct
only when forward biased and not when reverse biased.
2. The given junction diode is in working order as it offers low resistance in forward
biasing
and high resistance in reverse biasing/The given junction diode is not in working order as it
offers low (or high) resistance both during forward and reverse biasing.
PRECAUTIONS
1. For measuring resistance of any component, its leads should be cleaned properly with a
sand paper.
2. For measuring current and resistance, appropriate selection of function switch and range
Switch should be made.
3. The polarity probe leads should be connected to the proper polarities for mneasurements in de
circuits.
4. While measuring resistance of any component, avoid direct touching of the metallic ends of
the multimeter leads. Body resistance in parallel with the component resistance will affect
the resistance measuremernt.
5. Each time when we select a resistance scale of different range, we should set the pointer to
zero using »zero Adj. knob'.
VIVAVOCE
1 What doyou mean by unidirectional flow of current reverse biasing ( M), then the diode will be in
in a junction diode/LED ? working order.
A junction diode/LED conducts current easily when 3. What is difference between normal junction diode
forward biased and does not conduct current in and an LED ?
reverse biasing. This is the unidirectional feature of When forward biased, a junction diode conducts
current flow of a junction diode/LED. current without emitting any light. An LED when
How do you check whether a diode in working forward biased conducts current and glows
order or not ?
brighter and brighter as the forward bias voltage is
If junction diode offers low resistance (few 2 to kQ) increased.
in forward biasing and a very high resistance in

185
 LAB MANUAL PHYSICS-X|

ACTIVIT Y
B3
AIM
To study the effect of intensity of light (by varying distance of the source) on
an LDR.

APPARATUS AND MATERIAL REQUIRED

LDR, 3WLED bulb with holder, an iron stand for the bulb, a battery eliminator (12 V), a plug
key, milliammeter (0-500 mA), voltmeter (0-10 V), a resistance of 47 2, a half metre scale,
connecting wires and a piece of sand paper.
THEORY
Alight dependent resistor or aphotoresistor is alight sensitive device whose resistance decreases with the
increase in intensity of incident light. It is made from a semiconductor material cadmium sulphide,
CdS [other materials being CdSe, PbS, PbSe, InSb].
The basic structure of an LDR and its symbols are shown in Fig. 3. A snake like or zigzag pattern
of the light sensitive material is deposited on an insulating substrate such as ceramic. Such a
pattern provides desired resistance and power rating. Also this zigzag pattern separates a thin metal
film into two areas for which two low resistance metal contacts are made. A thin transparent
coating on the top surface provides a window for the incident light.
Transparent coating over entire surface
Electrode 1 Electrode 2
(Thin metal film)

Metal
-Photoconductive
material over
contact
top surface
Ceramic
substrate

+-Wire
terminal

(a) (b)

Fig. 3(a) Basic structure (b) Symbol of an LDR.

When light of certain minimum frequency falls on the LDR, the absorbed photons give bound
electrons enough energy to jump to the conduction band where they are free to conduct
electricity. This increases the conductance or lowers the resistance of the device. Atypical LDR
has a high resistance of several M2 in total darkness and just a few hundred ohms in
bright light.

186
 Seclion B: ACTIVITIES

CIRCUIT DIAGRAM
To ac
mains

3 W LED

47 SQ K mA
LDR

Iron
stand

12 V

Fig. 4Circuit to study the effect of intensity of ight on an LDR.

PROCEDURE
1. Draw the circuit diagram as shown in Fig. 4 and assemble the apparatus on the working
table accordingly.
2. Make neat and tight connections by connecting a 12 V battery eliminator in series with the
LDR, milliammeter mA, plug key Kand a 47 SQ resistor. Connect the voltmeter Vin parallel
with the LDR.
3. Keeping the LED lamp switched off, insert the plug in the key K.
4. Note the readings of voltmeter and milliammeter. Then calculate the value of LDR resistance R'.
5. In order to take into account the background illumination, the reference resistance R' is
added to all further measurements of resistance R of LDR.
6. Clamp a 3 W LED lamp on a rigid iron stand facing LDR. Adjust the lamp normally above
the LDR at a distance of 10 cm with the help of half metre scale.
7. Switch on the LED lamp and note the voltmeter and milliammeter readings.
8. Repeat the activity by adjusting the LED lamp at distances of 15 cm, 20 cm, 25 cm, and 30 cm
from the LDR. Record the reading of voltmeter and milliammeter in each case, calculate the
resistance of LDR at different distances of the lamp.
OBSERVATIONS AND CALCULATIONS
Range of voltmeter =0 to
Least count of voltmeter = V

Range of milliammeter =0 to mA

Least count of milliammeter = mA

Observations for background illumination

V
Reading of voltmeter=
Reading of milliammeter : mA = A
V
LDR resistance for background illumination, R'=
187
 LAB MANUAL PHYSICS-XI|

Table B7:Variation in resistance of LDR with distance

Observed resistance
Distance of LDR Voltmeter Milliammeter Actual resistance of
S.No. from the lamp reading V reading I of LDR R = LDR R = R, +R'
(cm) (V) (mA) (2) (2)
1 10

2 15
3 20
4. 25
5 30

RESULT

1. As distance of LDR from the source increases, intensity of light decreases and resistance of
LDR increases.

PRECAUTIONS
1. All the connections should be neat and tight.
2. LDR must be placed normally to the light source so that angle of incidence of light rays
remains same throughout the experiment.
3. A suitable protective resistance must be connected in series with LDR to prevent it from
damage.
4. Resistance of LDR for background illumination must be taken into account.

VIVA VOCE
1, What is an LDR ? bands of the photosensitive material. These electrons
A light dependent resistor is a device whose become free to conduct electricity. This increases the
resistance decreases with the increase in intensity of conductance or decreases the resistance.
light incident upon it. 5. How does the intensity of light vary with the distance
from the source?
2. Name some mnaterials used for nmaking LDRs.
Semiconductor materials like CdS, PbS, CdSe, PbSe Intensity of light is inversely proportional to the
and InSb. square ofthe distance from the source.
6. How are LDRs different from photodiodes and
3. How does the resistance of an LDR change with the phototransistors ?
light intensity ? Even though LDRs are made from semiconductor
At low light intensities, an LDR has a high resistance. materials, yet they are different from photodiodes
As the intensity of incident light increases, the and phototransistors because they are simply passive
resistance of an LDR decreases.
devices and do not have p-n junctions.
4. Why does the resistance of an LDR decrease with the 7. Name some
increase in intensity of incident light ? applications of LDRs.
As the intensity of incident light increases, more and LDRs are used as light sensors in alarm clocks, street
more electrons are knocked off from the valence lights, light intensity meters and burglar alarm
circuits.

188
 Section B: ACTIVITIES

ACTIVIT Y B4
AIM

1o observe refraction and lateral deviation of a beam of

light incident obliquely on a
glass slab.
APPARATUS AND MATERIAL REQUIRED
A rectangular glass slab (preferably of larger size), a
drawing board, white paper sheet,
cello-tape/drawing pins, alpins, protractor, ruler, sharp pencil and eraser.
THEORY
Figure 5 shows the path ABCD of a ray suffering refraction through a rectangular glass slab
PORS. It is seen that
Angle of incidence i= Angle of emergernce e

Air
B
Glass

ig. 5 Refraction through a glass slab.

Thus the emergent ray CD is parallel to the incident ray AB But the emergent ray gets laterally
displaced with respect to the incident ray. The perpendicular distance between the incident and
emergent rays, when light is incident obliquely on arefracting slab with parallel faces, is called laterial shift
or lateral displacement.
It is given by
t cosi
d=
COS I
sin(i -r) =tsin i 1 (u-sin i)2
=t sin 90= t
Clearly, dmax
The lateral shift produced by a glass slab increases with
() the increase in thickness t of glass slab,
(ii) the increase in the value of angle of incidence i, and
(ii) the increase in refractive index å of the glass slab.

189
 LAB MANUAL PHYSICS-XII

RAY DIAGRAMS
A N A, N N3
P
i=40° i=40o
P
i=40°
p
B B:M

R
t

D,

R
D
(a (b) (c)

500 As P, 60
P
P
B

Ds
(d .(e)
Fig. 6 Lateral displacements for different i and t.
PROCEDURE

1. Fix a white sheet of paper on the drawing board with the help of cello-tapeor drawing pins.
2. Place the glass slab breadthwise on the white sheet [Fig. 6(a)]. Mark its boundary PQRS with a
sharp pencil.
3. Remove the glass slab. Take a point B, on face AB Draw normal B, N, on PQ. With the help of a
protractor, draw an inident ray A, B, making an angle of incidence of 40° with the normal B, N,.
4. Again, place the glass slab within its boundary PQRS. Fix two alpins P, and P, vertically on
the incident ray A, B,, about 8 to 10 cm apart.
5. Looking into the slab from the opposite face SP, position the eye in sucha way that the feet of
pins P and P, appear to be one behind the other. Now fix pins P, and P, vertically in line
with pins P, and P, as viewed through the slab.
6. Remove the alpins and encircle the pin-pricks. Remove the slab andcomplete the path of the
ray of light A, B, C D, Draw perpendicular CE on A, B, produced and measure the length
of C, E,. This gives a measure of lateral displacement d.
7. Place the glass slab lengthwise as shown in Fig. 6(b). Repeat the experiment again for the same
angle of incidence of 40°. Measure the lateral displacement.
8. Place the glass slab thicknesswise as shown in Fig. 6(c). Repeat the experiment again for the
same angle of incidence of 40°, Measure the lateral displacement.
9. By placing the glass slab lengthwise as shown in Figs. 6(d) and (e), repeat the experiment twice
for angles of incidence of 50° and 60. Measure the lateral displacement in each case.

190
 Section B ACTIVITIES

10. Measure the length, breadth and thickness of the glass slab
using a ruler. Record all your
observations in a tabular form.
OBSERVATIONS AND CALCULATIONS
Least count of the
protractor = degrees
Least count of the ruler mm cm ; Length of the glass slab, t, Cm
Breadth of the glass slab, t, = Cm ; Thickness of the glass slab, t3 = Cm

Table B8 : Variation of lateral displacement with angle of incidence i and

thickness t of glass slab
Thickness of glass Angle of incidence Angle of Lateral
No. slab traversed Difference i~e
emergence displacement Ratio
(cm) i(degrees) (degrees) t
e(degrees) d (cm)
1
40°
d,
d
2. t 40°
t

3 40° ...

4 t 50°
5 t, 60

RESULT
1. As difference i~ eis small, so i =e When light refracts through a glass
slab, the emergent ray
is parallel to the direction of incident ray.
2. Within the limits of experimental error, from observations 1, 2 and 3 we see that the ratio
d|t =constant. The lateral displacement of the emergent ray is directly proportional to the
thickness of the glass slab (for constant i).
3. From observations 2, 4 and 5, we note the lateral displacement of the
emergent ray increases
with the increase in the angle of incidence i(for constant f).
PRECAUTIONS
1. The boundary of the glass slab should be marked with a sharp pencil.
2. Alpins should be fixed vertically and about 8 to 10cm apart.
3. The feet of the alpins and not their heads should be adjusted in the same straight line.
4. Just after removing an alpin, encircle the pin-prick with a sharp pencil.
5. The angle of incidence should lie between 30° and 60°.

VIVA VOCE
1. What is lateral shift in refraction ? 3. For what angle of incidence, the lateral shift
The sidewise shift in the path of light on emerging produced by a parallel sided glass slab is zero ?
from a refracting medium with parallel faces is called For i= 0° lateral shift is zero.
lateral shift. 4. For what angle of incidence, the lateral shift
2, On what factors does the lateral shift
depend ? produced by a parallel sided glass slab is maximum ?
Lateral shift depernds on arngle of incidence, the For i =90°, lateral shift is maximum and is equal to
refractive index and thickness of the refracting the thickness of the glass slab.
medium.

191
Dec 3