ICSE Class 9 Physics Sample Paper with Solutions Set 05

ICSE EXAMINATION

Sample Question Paper - 5

Physics

Time: 2 hours. Total Marks: 80

SECTION A (40 Marks)

Attempt all Questions from this Section.

Question 1

(i) Which of the following measures small length to highest accuracy [1 Mark]
a) metre scale
b) vernier callipers
c) screw gauge
d) none of the above

Answer: (c) screw gauge

Screw gauge measures small lengths to a higher accuracy (up to 0.001 cm or 0.01 mm) compared to a metre scale and vernier callipers.

Teacher's Note:
a) Remember the order of precision: metre scale (0.1 cm) < vernier callipers (0.01 cm) < screw gauge (0.001 cm).
b) Students often confuse vernier callipers and screw gauge; always check the least count.

(ii) Which of the following is the third equation of motion? [1 Mark]
a) v² = u² + 2at
b) v² = u² + 2st
c) v² - u² = 2as
d) S = ut+at²

Answer: (c) v² - u² = 2as

The third equation of motion relates velocity, acceleration, and displacement without time: \( v^2 - u^2 = 2as \).

Teacher's Note:
a) Recall the three equations of motion: \( v = u + at \), \( S = ut + \frac{1}{2}at^2 \), and \( v^2 - u^2 = 2as \).
b) Watch out for incorrect combinations of symbols in options like \( 2st \).

(iii) The property of bodies to resist a change in their state of rest or of motion is known as: [1 Mark]
a) Force
b) Acceleration
c) Velocity
d) Inertia

Answer: (d) Inertia

Inertia is the inherent property of a body by virtue of which it resists any change in its state of rest or uniform motion.

Teacher's Note:
a) Inertia is directly proportional to mass.
b) Do not confuse inertia with force, which is the external cause that changes or tends to change the state.

(iv) State the forecast when barometric pressure rises steeply [1 Mark]
a) The forecast is rainstorm
b) The forecast is dust storm
c) The forecast is dry weather with strong anticyclonic winds.
d) The forecast is fair weather

Answer: (c) The forecast is dry weather with strong anticyclonic winds.

A steep rise in barometric pressure indicates rapid increase in air density and pressure, predicting dry weather with strong anticyclonic winds.

Teacher's Note:
a) Sudden fall in pressure indicates a storm or cyclone.
b) Gradual fall indicates rain.

(v) Assertion: The zero error of a vernier calliper is always positive.
Reason: The zero error arises when the two jaws of the calliper are in contact but the zero mark of the vernier scale is not aligned with the main scale zero mark. [1 Mark]
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true and R is not the correct explanation of A
c) Assertion is false but reason is true.
d) Assertion is true reason is false.

Answer: (c) Assertion is false but reason is true.

Zero error can be positive or negative depending on whether the zero of the vernier scale lies to the right or left of the main scale zero when jaws are in contact.

Teacher's Note:
a) Positive zero error occurs when the vernier zero is to the right of the main scale zero.
b) Negative zero error occurs when it is to the left.

(vi) What is the unit of relative density? [1 Mark]
a) g cm^-3
b) kg m^-3
c) m³kg^-1
d) no unit

Answer: (d) no unit

Relative density is the ratio of two similar quantities (density of substance to density of water), hence it has no units.

Teacher's Note:
a) Ratios of identical physical quantities are always dimensionless and unitless.
b) Do not confuse relative density with absolute density which has units like kg m^-3.

(vii) The same body is immersed in two liquids A and B in succession. The extent to which the body sinks in liquid B is less than in liquid A. What are the conclusions that could be derived from such an observation? [1 Mark]
a) Density of liquid B is more than liquid A
b) Density of liquid A is more than liquid B
c) No such conclusion can be made
d) Density of the solid is less than the liquid in both

Answer: (a) Density of liquid B is more than liquid A

Since the body sinks less in liquid B, more upthrust is exerted by liquid B, implying that liquid B is denser than liquid A.

[Figure: Floating body sinking to different depths in two different liquids.]

Teacher's Note:
a) Law of floatation states that heavier displacement means lower density of liquid.
b) Less sinking means higher buoyant force and higher liquid density.

(viii) Aneroid barometer which directly indicates the altitude instead of atmospheric pressure is called [1 Mark]
a) Altimeter
b) Fortinmeter
c) Lactometer
d) Micrometer

Answer: (a) Altimeter

An altimeter is an aneroid barometer calibrated to read altitude directly based on atmospheric pressure variation with height.

Teacher's Note:
a) Used extensively in aircrafts for measuring height above sea level.
b) Lactometer is used for checking milk purity.

(ix) A body of weight W experiences an upthrust R in water. What will be the apparent weight of the body and apparent density of the body when W = R? [1 Mark]
a) Apparent weight = W-R; Apparent density = (\rho - 1) gcm^-3
b) Apparent weight=Zero; Apparent density = Zero
c) Apparent weight = R-W; Apparent density = (1 - \rho) gcm^-3
d) Apparent weight = W-R; Apparent density = (1000 - \rho) kg m^-3

Answer: (b) Apparent weight=Zero; Apparent density = Zero

When weight equals upthrust (\(W = R\)), the net downward force is zero, making apparent weight zero and apparent density zero.

Teacher's Note:
a) Apparent weight equals actual weight minus upthrust (\(W - R\)).
b) When buoyant force balances weight completely, the body floats fully submerged with zero apparent weight.

(x) What is the basis of grouping organisms into producers, consumers and decomposers? [1 Mark]
a) Manner in which they obtain sustenance from the environment.
b) Physiological Characters
c) Geographical distribution
d) Energy efficiency

Answer: (a) Manner in which they obtain sustenance from the environment.

Trophic classification is based on how organisms get their food and energy (autotrophs, heterotrophs, saprotrophs).

Teacher's Note:
a) Producers make their own food, consumers eat others, and decomposers break down dead matter.
b) This forms the functional basis of ecosystems.

(xi) Which trophic level has the greatest number of individuals? [1 Mark]
a) 1st trophic level
b) 2nd trophic level
c) 3rd trophic level
d) 4th trophic level

Answer: (a) 1st trophic level

The first trophic level consists of producers (plants), which are always greatest in number to support higher trophic levels in an energy pyramid.

Teacher's Note:
a) Energy decreases progressively from producers to top carnivores due to 10% law.
b) Consequently, population numbers also decrease upwards.

(xii) In a lateral inversion [1 Mark]
a) The left side of the object becomes the right side of the image and vice-versa.
b) The right side of the object remains the right side of the image.
c) The left side of the object remains the left side of the image.
d) None of the above

Answer: (a) The left side of the object becomes the right side of the image and vice-versa.

Lateral inversion is the apparent reversal of mirror image left and right when compared with the object.

Teacher's Note:
a) It is a characteristic property of plane mirror images.
b) Top and bottom remain unaffected in lateral inversion.

(xiii) Which among the following is true for a plane mirror? [1 Mark]
a) Angle of incidence is always equal to angle of reflection.
b) Angle of incidence is not always equal to angle of reflection.
c) Angle of incidence is the angle between the reflected ray and the incident ray.
d) Angle of incidence is the angle between incident ray and the mirror surface, incident ray.

Answer: (a) Angle of incidence is always equal to angle of reflection.

The first law of reflection states that the angle of incidence is always equal to the angle of reflection for any reflecting surface including a plane mirror.

Teacher's Note:
a) Angle of incidence is measured with the normal, not the mirror surface.
b) Both incident ray, reflected ray, and normal lie in the same plane.

(xiv) For which of the following ultrasound can be used? [1 Mark]
a) Detect defective foetus
b) As a tool in the treatment of muscular pain
c) Clean spiral tubes
d) All of the above

Answer: (d) All of the above

Ultrasound has diverse applications including medical diagnostics (foetal monitoring), physiotherapy (muscular pain relief), and industrial cleaning (spiral tubes).

Teacher's Note:
a) Ultrasound refers to sound waves with frequencies greater than 20,000 Hz.
b) High energy and directive property make it extremely useful across medical and industrial fields.

(xv) The correct relation is: [1 Mark]
a) 1J = 1C/1V
b) 1J = 1 V/1C
c) 1J = 1C x 1V
d) 1 J x 1C x 1V=1

Answer: (c) 1J = 1C x 1V

Since Work / Energy (Joule) = Charge (Coulomb) \times Potential Difference (Volt), \(1\text{ J} = 1\text{ C} \times 1\text{ V}\).

Teacher's Note:
a) Recall potential difference \(V = W/Q\), hence \(W = Q \times V\).
b) Dimensional consistency is crucial for verifying such unit relations.

Question 2

(i) Complete the following by choosing the correct answers from the bracket: [6 Marks]
a) The SI unit of luminous intensity is __________ [luminous/ candela/joules].
b) To reflect sound waves from any surface, it should have dimensions __________ [equal or less/ equal or greater/Always greater] than the wavelength of the sound wave.
c) The sound which is produced due to a mixture of several frequencies is called __________ [Noise/ Tone/Note].
d) Between a football and stone of same size but different masses, stone being heavier the inertia of __________ [Football is greater / Stone is greater/ Both the objects is equal.]
e) Black or dull surfaces are __________ [poor/good] reflectors, but __________ [poor/good] absorbers of heat.

Answer:
a) candela
b) equal or greater
c) noise (Note: Official key shows 'note', but noise/note context in acoustics usually implies complex mixtures; however, following standard textbook definitions, a mixture of random frequencies is noise while a pleasant blend of notes forms a chord. Following the marking key: note)
d) Stone is greater
e) poor, good

Teacher's Note:
a) Candela is one of the seven base SI units.
b) Inertia depends directly on mass, hence stone has greater inertia.

(ii) The walls of a barber shop are covered with a plane mirror and two movie films are made - one recording the movements of the barber and the other of his mirror image. From viewing the films later, can an observer differentiate between the object and the image? [2 Marks]

Answer:
Yes, the observer can differentiate if they know whether the barber is left-handed or right-handed, because in a plane mirror, left appears as right due to lateral inversion.

Teacher's Note:
a) Plane mirrors produce laterally inverted images.
b) Without knowing original handedness, differentiation is impossible.

(iii) A ray of light is incident on a plane mirror at an angle of incidence of 50°. What is the angle (a) of reflection (b) between the incident ray and the mirror (c) between the reflected ray and the mirror (d) of deviation (angle between the directions of the incident ray and the reflected ray)? [2 Marks]

[Figure: Ray diagram showing angle of incidence 50° on a plane mirror.]

Answer:
(a) Angle of reflection = 50°
(b) Angle between incident ray and mirror = 90° - 50° = 40°
(c) Angle between reflected ray and mirror = 90° - 50° = 40°
(d) Angle of deviation = 180° - (50° + 50°) = 80°

Teacher's Note:
a) Angle of incidence equals angle of reflection.
b) Deviation formula for reflection is \( \delta = 180^{\circ} - 2i \).

Question 3

(i) Calculate the number of seconds in a year. Take 1 year = 365 days. [2 Marks]

Answer:
1 year = 365 days
1 day = 24 hours = \( 24 \times 3600\text{ s} = 86400\text{ s} \)
Total seconds in a year = \( 365 \times 86400 = 3,15,36,000\text{ seconds} \) (or \( 3.15 \times 10^7\text{ s} \)).

Teacher's Note:
a) Always show intermediate conversion steps (days to hours, hours to minutes, minutes to seconds).
b) Scientific notation is preferred for large numbers.

(ii) Calculate the frequency of oscillation of Second's pendulum. Does it depend upon amplitude of oscillation? [2 Marks]

Answer:
Time period of a second's pendulum \( T = 2\text{ s} \).
Frequency \( f = \frac{1}{T} = \frac{1}{2} = 0.5\text{ Hz} \).
No, frequency does not depend upon the amplitude of oscillation for small amplitudes.

Teacher'sNote:
a) Time period of second's pendulum is always 2 seconds.
b) Frequency is reciprocal of time period.

(iii) Under what condition, the balance is in equilibrium? [2 Marks]

Answer:
A physical balance is in equilibrium when the moment of the weight of an object on one pan is equal to the moment of the standard weights on the other pan about the central knife-edge (beam is horizontal).

Teacher's Note:
a) Based on the Principle of Moments (Clockwise Moment = Anticlockwise Moment).
b) The pointer must oscillate equally on both sides of the zero mark.

(iv) Ratio of the velocities of two bodies thrown in upward direction is 2:5. Prove that the ratio of their height attained will be h₁:h₂ = 4:25. [2 Marks]

Answer:
Given \( \frac{u_1}{u_2} = \frac{2}{5} \).
Using equation \( v^2 - u^2 = 2as \), at highest point \( v = 0 \) and \( a = -g \).
Thus, \( 0 - u^2 = 2(-g)h \implies h = \frac{u^2}{2g} \).
Therefore, \( \frac{h_1}{h_2} = \frac{u_1^2}{u_2^2} = \left(\frac{2}{5}\right)^2 = \frac{4}{25} \).

Teacher's Note:
a) Maximum height is proportional to the square of initial velocity.
b) Acceleration due to gravity \( g \) remains constant for both bodies.

(v) Name two greenhouse gases. Will these gases increase or decrease the average temperature of the earth? [2 Marks]

Answer:
Two greenhouse gases: Carbon dioxide (CO₂) and Methane (CH₄).
These gases will increase the average temperature of the earth by trapping outgoing infrared radiation.

Teacher's Note:
a) Greenhouse gases absorb terrestrial radiation and warm the atmosphere.
b) Water vapour is also a major greenhouse gas.

(vi) Above diagram represents a simple pendulum. A simple pendulum is a heavy point mass suspended from a rigid support by a massless and inextensible string. Answer the following questions: [3 Marks]
a) What is the length of given simple pendulum?
b) What is the value of amplitude of simple pendulum?
c) What are the factors affecting the time period of simple pendulum?

[Figure: Diagram of a simple pendulum with support S, string, bob, amplitude x, and length parameters y and z.]

Answer:
a) Length of pendulum is \( y + z \) (length of string plus radius of bob).
b) Amplitude of simple pendulum is \( x \) (maximum displacement from mean position).
c) Factors affecting time period: (i) length of pendulum, (ii) acceleration due to gravity.

Teacher's Note:
a) Effective length is measured from point of suspension to the centre of gravity of the bob.
b) Time period is independent of mass and amplitude for small oscillations.

(vii) In riveting boiler plates, red hot rivets are used. Why? [2 Marks]

Answer:
Red hot rivets are used because when they are inserted and hammered tightly, upon cooling they contract and hold the metal plates together with immense force.

Teacher's Note:
a) This is a practical application of thermal expansion and contraction.
b) Contraction creates a leak-proof and extremely tight joint.

SECTION B (40 Marks)

Attempt any four Questions from this Section

Question 4

(i) Given diagram shows a screw gauge. In one measurement, the final position of the scale is as shown in the diagram. The circular scale has 50 divisions. [3 Marks]
a) What is the least count of the screw gauge?
b) If 40th division of the circular scale coincides with the main scale line, what is the final reading?
c) What do you mean by back-lash error of a screw gauge?

[Figure: Screw gauge diagram showing main scale and circular scale with 40th division coinciding.]

Answer:
a) Least Count = Pitch / Total number of divisions = \( 1\text{ mm} / 50 = 0.02\text{ mm} = 0.002\text{ cm} \).
b) Final Reading = MSR + (CSR \times L.C.) = \( 17\text{ mm} + (40 \times 0.02\text{ mm}) = 17\text{ mm} + 0.80\text{ mm} = 17.80\text{ mm} = 1.780\text{ cm} \).
c) Back-lash error is the error due to wear and tear of screw threads, causing the tip not to move immediately when the direction of rotation is reversed.

Teacher's Note:
a) Always convert units properly between mm and cm.
b) Back-lash error can be avoided by turning the screw in only one direction while taking measurements.

(ii) In a physical balance, [3 Marks]
(a) State the principle on which it works.
(b) What is measured by physical balance?
(c) What is the role of a plumb line?
(d) What is the role of base screws?
(e) State two requirements for a good balance.

Answer:
(a) Principle of moments (in equilibrium, clockwise moment = anticlockwise moment).
(b) Mass of a body.
(c) To check whether the balance is vertical.
(d) To make the base board horizontal.
(e) Two requirements: (1) Both arms must be of equal length, (2) Both pans must be of equal weight.

Teacher's Note:
a) Physical balance compares mass, not weight.
b) Equal arm length ensures true mass measurement independent of location.

(iii) A weather forecasting plastic balloon of volume 15 m³ contains hydrogen of density 0.09 kg/m³. The volume of equipment carried by the balloon is negligible compared to its own volume. The mass of the empty balloon alone is 7.15 kg. The balloon is floating in the air of density 1.3 kg/m³. Calculate: [4 Marks]
(a) Mass of hydrogen in the balloon.
(b) Mass of hydrogen and balloon.
(c) If mass of equipment is x kg, write down the total mass of hydrogen, the balloon and the equipment.
(d) Mass of air displaced by balloon.

Answer:
(a) Mass of hydrogen = Volume \times Density = \( 15\text{ m}^3 \times 0.09\text{ kg/m}^3 = 1.35\text{ kg} \).
(b) Mass of hydrogen and balloon = \( 1.35 + 7.15 = 8.50\text{ kg} \).
(c) Total mass including equipment = \( (x + 8.5) \text{ kg} \).
(d) Mass of air displaced = Volume of balloon \times Density of air = \( 15\text{ m}^3 \times 1.3\text{ kg/m}^3 = 19.5\text{ kg} \).

Teacher's Note:
a) Upthrust equals weight of displaced air.
b) For floatation, total weight must equal upthrust.

Question 5

(i) An electron moving with the speed of 5 \times 10⁴ m/s enters into an electric field and attains a uniform acceleration of 10¹⁵ m/s² in the direction of motion. In how much time, will it attain a speed twice of its initial speed? In this time, how much distance will it cover? [3 Marks]

Answer:
Given \( u = 5 \times 10^4\text{ m/s} \), \( a = 10^{15}\text{ m/s}^2 \), \( v = 2u = 10^5\text{ m/s} \).
Time \( t = \frac{v - u}{a} = \frac{10^5 - 5 \times 10^4}{10^{15}} = \frac{5 \times 10^4}{10^{15}} = 5 \times 10^{-11}\text{ s} \).
Distance \( S = ut + \frac{1}{2}at^2 = (5 \times 10^4)(5 \times 10^{-11}) + \frac{1}{2}(10^{15})(5 \times 10^{-11})^2 \)
\( = 25 \times 10^{-7} + 12.5 \times 10^{-7} = 37.5 \times 10^{-7}\text{ m} = 3.75 \times 10^{-6}\text{ m} \).

Teacher's Note:
a) Apply standard kinematic equations carefully with exponential terms.
b) Keep track of powers of 10 during addition and multiplication.

(ii)
(a) Explain with the help of an example whether the velocity or the acceleration of a body give the direction of motion.
(b) In the given figure, velocity-time graph of a body moving in a straight line is shown. Find the displacement and the distance travelled by the body in 6 s. [4 Marks]

[Figure: Velocity-time graph showing three rectangular/triangular regions above and below time axis up to 6 s.]

Answer:
(a) Velocity gives the direction of motion (e.g., upward motion has upward velocity and direction). Acceleration does not necessarily indicate direction of motion; it only indicates the rate of change of velocity.
(b) Displacement = Sum of areas of portions with proper signs = \( (4 \times 2) - (2 \times 2) + (2 \times 2) = 8 - 4 + 4 = 8\text{ m} \).
Distance travelled = Sum of areas ignoring signs = \( (4 \times 2) + (2 \times 2) + (2 \times 2) = 8 + 4 + 4 = 16\text{ m} \).

Teacher's Note:
a) Area under v-t graph represents displacement.
b) Total distance considers absolute values of area, whereas displacement accounts for vector directions.

(iii) A body is projected vertically upwards with a velocity of 98 m/s. Find (i) the maximum height attained by the body and (ii) time taken by body to reach the highest point. (Take g = 9.8 m/s²) [3 Marks]

Answer:
Given \( u = 98\text{ m/s} \), \( g = -9.8\text{ m/s}^2 \), \( v = 0 \).
(i) Using \( v^2 - u^2 = 2gs \):
\( 0 - (98)^2 = 2(-9.8)S \implies S = \frac{98 \times 98}{2 \times 9.8} = 490\text{ m} \).
(ii) Using \( v = u + gt \):
\( 0 = 98 + (-9.8)t \implies t = \frac{98}{9.8} = 10\text{ s} \).

Teacher's Note:
a) At maximum height, final velocity becomes zero.
b) Time of ascent equals time of descent under uniform gravity.

Question 6

(i)
(a) State Newton's third law of motion.
(b) John pushes a wall with a force of 20 N towards the east, what force will be exerted by the wall on John?
(c) In the following figure, a block of weight 10 N is hanging from a rigid support by a thread. Find:
1. The force exerted by block on the thread.
2. The force exerted by the thread on the block. [4 Marks]

[Figure: Block of weight 10 N hanging from a support by a thread.]

Answer:
(a) To every action, there is always an equal and opposite reaction.
(b) The wall will exert a force of 20 N towards the west.
(c) 1. The force exerted by block on the thread is 10 N downwards.
2. The force exerted by the thread on the block is 10 N upwards.

Teacher's Note:
a) Action and reaction act on two different bodies.
b) Magnitude is identical, directions are opposite.

(ii) Mention three disadvantages of construction of large dams for generating hydroelectric power: [3 Marks]

Answer:
1. Uprooting and displacement of people from their native places.
2. Disruption of plant and animal life (loss of flora and fauna / biodiversity).
3. Disruption and damage to regional ecosystems.

Teacher's Note:
a) Large dams cause environmental and social challenges.
b) Submergence of large land areas destroys forests.

(iii) A 3 kg stone is weighed first with a physical balance and then by a spring balance at the pole and at the equator. Where will the weight be maximum? [3 Marks]

Answer:
The mass measured by physical balance remains constant (3 kg) everywhere. However, the weight measured by a spring balance will be maximum at the pole because the value of acceleration due to gravity (\(g\)) is maximum at the poles due to the Earth's oblateness and lower centrifugal force.

Teacher's Note:
a) Mass is constant; weight depends on local \(g\).
b) Earth is flattened at poles and bulges at equator, making polar \(g\) greater than equatorial \(g\).

Question 7

(i) Explain the effect of ozone depletion. [3 Marks]

Answer:
Ozone depletion allows harmful ultraviolet (UV) radiations from the sun to reach the Earth's surface. This causes skin cancer, eye cataracts, suppression of human immune systems, reduction in crop yields, and disruption of marine ecosystems.

Teacher's Note:
a) Ozone layer in the stratosphere acts as a protective shield.
b) CFCs (chlorofluorocarbons) are the primary culprits in ozone destruction.

(ii)
(a) What is the use of thermos flask?
(b) Draw a labeled diagram of thermos flask.
(c) What contribution does the vacuum between the two walls give to the functioning of a thermos flask?
(d) What is the function of the two shining walls of the glass vessel in the thermos flask? [4 Marks]

[Figure: Labeled diagram of a thermos flask showing double-walled glass bottle, vacuum, silvered surfaces, cork, and outer case.]

Answer:
(a) It is used for keeping hot liquids hot and cold liquids cold for a sufficiently long time.
(b) [Figure: Thermos flask diagram with labels: metallic cover, cork, double-walled bottle, vacuum, silver polish, tin metal case, spring pad.]
(c) The vacuum between the two walls completely checks heat transfer by conduction and convection.
(d) The shining silvered walls prevent heat transfer by radiation by reflecting thermal rays.

Teacher's Note:
a) A thermos flask minimizes all three modes of heat transfer: conduction, convection, and radiation.
b) Labeling must clearly show silvering and vacuum space.

(iii) Draw a simple diagram showing the energy flow in the food chain. [3 Marks]

[Figure: Diagram showing energy flow from Sun to Producer, Primary consumer (Krill), Secondary consumer (Small fish), and Tertiary consumer (Large fish / Heron / Man).]

Answer:
Sun (Light Energy) → Producer (Plants) → Primary Consumer (Herbivores) → Secondary Consumer (Carnivores) → Tertiary Consumer (Top Carnivores).

Teacher's Note:
a) Energy flow in an ecosystem is unidirectional.
b) Only about 10% of energy is transferred from one trophic level to the next.

Question 8

(i)
(a) Select the luminous objects from the following: Candle flame, stars, moon, red hot wire of heater, polished surface, and firefly.
(b) In a room, the light is not reaching directly, even then it is illuminated. Why?
(c) What will be the colour of the sky for space travellers? [4 Marks]

Answer:
(a) Luminous objects: Candle flame, stars, red hot wire of heater, and firefly.
(b) The room is illuminated due to diffused reflection (scattering) of light from walls and objects.
(c) The sky appears black/dark for space travellers because there is no atmosphere to scatter sunlight.

Teacher's Note:
a) Luminous objects emit their own light; non-luminous reflect light.
b) Rayleigh scattering causes the blue sky on Earth, which is absent in vacuum.

(ii) In what way, a point source should be placed in front of a concave mirror to get the parallel beam and the divergent beam. [3 Marks]

[Figure: Ray diagrams for concave mirror showing source at focus for parallel beam and source between pole and focus for divergent beam.]

Answer:
1. To get a parallel beam, the point source must be placed at the principal focus of the concave mirror.
2. To get a divergent beam, the point source must be placed between the pole and the principal focus of the concave mirror.

Teacher's Note:
a) Rays originating from the focus become parallel after reflection.
b) Rays originating between pole and focus diverge after reflection, forming a virtual magnified image.

(iii) Compare the frequencies of two waves X and Y while velocity and wavelength of X are 5 \times 10³ m/s and 25 m respectively and for Y, 4 \times 10³ m/s and 20 m respectively. [3 Marks]

Answer:
Given for wave X: \( \lambda_X = 25\text{ m} \), \( v_X = 5 \times 10^3\text{ m/s} \).
Given for wave Y: \( \lambda_Y = 20\text{ m} \), \( v_Y = 4 \times 10^3\text{ m/s} \).
Frequency \( f = \frac{v}{\lambda} \).
\( f_X = \frac{5 \times 10^3}{25} = 200\text{ Hz} \).
\( f_Y = \frac{4 \times 10^3}{20} = 200\text{ Hz} \).
Ratio \( f_X : f_Y = 200 : 200 = 1 : 1 \).

Teacher's Note:
a) Wave equation is \( v = f\lambda \).
b) Both waves have identical frequencies despite different velocities and wavelengths.

Question 9

(i) A and B are two negatively charged and insulated conductors as shown in the figure. State, with reason, which conductor will tend to lose charge. [3 Marks]

[Figure: Two negatively charged insulated conductors A (spherical) and B (pear-shaped with a sharp point).]

Answer:
Conductor B will tend to lose charge faster. The reason is that surface charge density is higher at sharp points (action of points), resulting in a stronger electric field and leakage of charge into the surrounding air.

Teacher's Note:
a) Charge density is inversely proportional to the radius of curvature (\( \sigma \propto 1/r \)).
b) Sharp points accumulate high charge concentration leading to corona discharge.

(ii) Draw a labelled diagram of Leclanche cell. Why is it not suitable for continuous use? [3 Marks]

[Figure: Labeled diagram of Leclanche cell showing carbon rod, zinc rod, glass jar, ammonium chloride solution, and manganese dioxide depolarizer.]

Answer:
It is not suitable for continuous use because MnO₂ (solid depolarizer) is a slow depolarizer. It cannot oxidize hydrogen gas to water as fast as it is formed in the reaction. Consequently, polarization occurs due to hydrogen accumulation on the carbon anode, dropping the emf. It must be allowed to rest intermittently.

Teacher's Note:
a) Leclanche cell is best suited for intermittent work (e.g., electric bells, doorbells).
b) Polarization causes internal resistance to increase rapidly during continuous discharge.

(iii)
(a) What is the general law of attraction and repulsion between magnetic poles?
(b) What defines the direction of the magnetic field?
(c) The middle region of a bar magnet is:
1. A north pole
2. A north seeking pole
3. Unmagnetized
4. Magnetized
(d) Name two magnetic substances. [4 Marks]

Answer:
(a) Like poles repel and unlike poles attract each other.
(b) The direction of the magnetic field at any point is the direction of force experienced by a hypothetical north pole placed at that point.
(c) 3. Unmagnetized (or region of zero magnetic strength / neutral region).
(d) Iron and Steel (or Nickel, Cobalt).

Teacher's Note:
a) Magnetic poles always exist in pairs (monopoles do not exist).
b) Centre of a bar magnet has practically zero attractive force.