# Model Question

## Sample Paper 1

### Part 3

These sample papers have been taken from CBSE Board

Question 20:

(a) What is presbyopia? State its cause. How is it corrected?

Answer: The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is called Presbyopia. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bifocal lenses. A common type of bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision.

(b) Why does the Sun appear reddish early in the morning? Explain with the help of a labelled diagram.

Answer: Red colour scatters the least and hence travels the farthest. During early morning, the sunlight has to travel a longer distance to reach us. During this, the blue colour is scattered almost completely and the sun and the sky appear reddish.

Question 21:

(a) What are magnetic field lines? How is the direction of magnetic field at a point in a magnetic field determined using field lines?

Answer: The imaginary lines of magnetic field around a magnet are called field line or field line of magnet. Outside the magnet, the direction of magnetic field line is taken from north pole to South Pole. Inside the magnet, the direction of magnetic field line is taken from south pole to north pole.

(b) Two circular coils ‘X’ and ‘Y’ are placed close to each other. If the current in the coil ‘X’ is changed, will some current be induced in the coil ‘Y’? Give reason.

Answer: When a conductor is set to move inside a magnetic field or a magnetic field is set to be changing around a conductor, electric current is induced in the conductor. When the current in the coil ‘X’ is changed it changes the magnetic field around ‘X’. Due to this, some current is induced in coil ‘Y’ by way of magnetic induction.

(c) State ‘Fleming’s right hand rule”.

Answer: Fleming’s Right Hand Rule: If the right hand is stretched in a way that the index finger, middle finger and thumb are in mutually perpendicular directions, then the thumb shows the direction of movement of the conductor, index finger shows the direction of magnetic field and the middle finger shows the direction of induced current in the conductor. The directions of movement of conductor, magnetic field and induced current can be compared to three mutually perpendicular axes, i.e. x, y and z axes.

### Section – B

Question 22: In the laboratory of a school, the sample of hard water is not available “to study the comparative cleaning capacity of a sample of soap in soft and hard water”. Which salt from the laboratory can be added to tap water to make it hard? In the experiment how is cleaning capacity of soap compared?

Answer: Salts of calcium and magnesium can be added to tap water to make it hard water. Cleaning capacity of soap is compared by comparing the time it takes the foam to disappear in case of different samples of water.

Question 23: In a school laboratory the students are studying the properties of ethanoic acid through certain experiments. How can they test its acidic nature? Give two tests.

Answer: Acidic nature of ethanoic acid can be tested by following methods:

• By using blue litmus paper: Blue litmus changes to red in presence of acid.
• By using sodium hydroxide to carry out neutralization reaction. If the beaker (containing the reaction mixture) becomes warm, it indicates reaction has taken place.

Question 24: Draw a labelled diagram of the experimental set up for the study of liberation of carbon dioxide gas during respiration.

Answer: Do It Yourself: You can make the diagram of a test tube a straw; showing air being blown by mouth through the straw. Alternately, you can make the diagram of conical flask with germinating seeds kept in it. The flask should be connected to a test tube containing lime water.

Question 25: Draw diagrams showing reproduction in yeast in proper sequence.

Question 26: While tracing the path of a ray of light passing through a rectangular glass slab a student tabulated his observations. If in his experiment for two readings he takes two values of ∠i as 40 º and 50º, what should be the correct values of ∠e and ∠r in each case?

Answer: We know that emergent ray is parallel to the incident ray in case of light entering a particular medium and exiting it.

So, ∠i=∠e

OR

Draw a labelled ray diagram for the formation of image by a convex lens of focal length 15 cm when the object is placed at a distance of 25 cm from the lens. Determine the size of the image formed, if size of the object is 4 cm.

Given, u = -25 cm, f = 15 cm

Image distance can be calculated as follows:

1/v-1/u=1/f

Or, 1/v+1/(25)=1/(15)

Or, 1/v=1/(15)-1/(25)

Or, 1/v=(5-3)/(75)=2/(75)

Or, v=(75)/2=37.5 cm

Now, size of image can be calculated as follows:

v/u=(h_2)/(h_1)

Or, (37.5)/(-25)=(h_2)/4

Or, h_2=(37.5)/(-25)xx4=-6 cm

Question 27: You have following material:

An ammeter (0-1A), a voltmeter (0-3V), a resistor of 20 Ω, a key, a rheostat, a battery of 3 V and seven connecting wires.

Using this material draw a labelled circuit diagram to study the dependence of potential difference (V) across a resistor on the current (I) passing through it.