Model Question

Sample Paper 2

Part 3

These sample papers have been taken from CBSE Board

Question 20:

(a) State the laws of refraction of light.

Answer: Laws of Refraction of Light

1. The incident ray, refracted ray and normal to the interface of given two transparent media, all lie in same plane.
2. The ratio of sine of angle of incidence and sine of angle of refraction is always constant for the light of given colour and for the pair of given media.

The Second Law of Refraction is also known as Snell’s Law of Refraction.

That is, (text(Sin) i)/(text(Sin) r)= constant

(b) What is meant by absolute refractive index of a medium?

Answer: When one medium is taken as vacuum and speed of light is taken in it, then the refractive index of second medium with respect to vacuum is called Absolute Refractive Index and it is generally denoted by n2.

Thus, n_2=text(Speed of light in vacuum)/text(Speed of light in given medium)

(c) The absolute refractive index of diamond is 2.42. If the absolute refractive index of glass is 1.5 and the speed of light in glass is 2 × 108 m/s, calculate the speed of light in diamond.

n_2=text(Speed of light in vacuum)/text(Speed of light in given medium)

Or, 1.5=text(Speed of light in vacuum)/(2xx10^8)

Or, Speed of light in vacuum =1.5xx2xx10^8=3xx10^8 m/s

Now, for diamond:

2.42=text(Speed of light in vacuum)/text(Speed of light in given medium)

Or, 2.42=(3xx10^8)/text(Speed of light in diamond)

Or, Speed of light in diamond = (3xx10^8)/(2.42)=1.24xx10^8 m/s

OR

(a) State the laws of reflection of light.

Answer: Laws of reflection of light:

1. The angle of incidence and angle of reflection is equal.
2. The incident ray, reflected ray and normal to the point of reflection lie in the same plane.

(b) List any two characteristics of the images formed by plane mirrors.

(c) An object is placed in front of a spherical mirror of focal length –20 cm at a distance of 30 cm. At what distance from the mirror a screen be placed in order to obtain a sharp image of the object on it?

Answer: Given: f = -20 cm, u = -30 cm

Image distance can be calculated as follows:

1/v-1/u=1/f

Or. 1/v=1/f+1/u

Or, 1/v=-1/(20)-1/(30)

Or, 1/v=(-3-2)/(60)=-5/(60)

Or, v=-(60)/5=-12 cm

Question 21:

(a) Define electric power. Express it in terms of potential difference ‘V’ and resistance ‘R’.

Answer: Rate of transfer of electrical energy through a circuit in unit time is called electric power. The SI unit of electric power is watt (W). It can be expressed in terms of V and R as follows:

1W=1 text(volt)xx 1 text(ampere)

=1V\xx1A

(b) An electric motor takes 5.0A from an electric source of 220 V. Determine the power of the motor and calculate the energy consumed by the motor in kWh, when it runs for 5 hours.

Answer: Given: Current I = 5 A, V = 220 V, time = 5 hours = 5 × 60 × 60 s

Power =VxxR

=220xx5=1100 W

Now power consumed in 5 hours

=1100 W\xx5xx3600=19800000 W

=1.98xx10^7 kWh

Question 22: State observations of the reactions of dil. HCl with the following:

(i) Blue litmus paper

Answer: Blue litmus paper turns red

(ii) Red litmus paper

(iii) Zinc-granules

Answer: Strong effervescence is observed which happens due to formation of hydrogen gas during the reaction between zinc and hydrochloric acid.

Zn + 2HCl → ZnCl2 + H2

(iv) Solid NaHCO3

Answer: When hydrochloric acid reacts with sodium hydrogen carbonate, the reaction gives sodium chloride and carbon dioxide.

HCl + NaHCO3 → NaCl + H2 + CO2

Question 23: You have two samples of water marked A and B. How would you distinguish between the two if one sample is of soft water and the other is of hard water?

Answer: Samples of soft water and hard water can be distinguished with the help of a test of foam formation. For this, soap is used with particular sample of water. Amount of foam is checked. After that, the time for which the foam does not disappear is also checked. Soap easily forms foam in soft water compared to in hard water. So, this apparently simple test helps us in differentiating between samples of soft and hard water.

Question 24: Your friend is preparing a temporary mount of a leaf peel to observe stomata. At which stage of preparation, should he use a stain and glycerine. State the role of these two liquids.

Answer: Stain imparts color to various membranes in the cell and thus helps in marking the boundaries of cells. Glycerine prevents drying up of specimen.

Question 25: When a student observes a slide showing different stages of binary fission in Amoeba, he generally observes four stages of the process. Describe these stages in a proper sequence.

Answer: Four stages in binary fission of amoeba are as follows:

Mother cell: The mature mother cell can be seen by presence of distinct nucleus.

Division of Nucleus: Some cells show nucleus dividing into two.

Division of cytoplasm: Some cells show division of cytoplasm. It is clear by appearance of two smaller cells joined somewhere in the middle.

Daughter cells: Daughter cells are smaller than mother cell and no two daughter cells are joined to one another.

SECTION-B

Question 26: A student has to determine the focal length of a given concave mirror by focusing a distant object. Answer the following questions:

(i) Which two precautions should he observe while selecting the object?

Answer: The object should be not too large nor too small.

(ii) Which distance should he measure to determine the focal length and why?

Answer: We know that when object is at infinity, it’s highly diminished and inverted image is formed at focus of concave mirror. Hence, the student should measure the distance between image of a distant object and the mirror to measure the focal length.

Question 27: List four precautions which a student observes while performing the experiment of determining the equivalent resistance of a combination of two resistors connected in series.

Answer: Precautions to be observed to determine equivalent resistance of two resistors in series:

• Ends of wires should be properly cleaned with sandpaper.
• Plug key should be in ON position only for a short duration.
• Battery should be fully charged before the experiment
• Connections should be tightly fit.

OR

In the experiment of studying the dependence of potential difference (V) across a resistor on the current (I) passing through it a student has tabulated a set of six observations. If, now he draws a graph between V and I, what will be the shape of the graph and what conclusion can be drawn from it?

Answer: The graph of V (potential difference) versus I (electric current) is always an upwardly slanting straight line. This shows that potential difference varies directly as electric current.