CHAPTER – 10: THE HUMAN EYE AND THE COLOURFUL WORLD
CHAPTER – 10: THE HUMAN EYE AND THE COLOURFUL WORLD
VOLUME – 1
Section-A: Multiple Choice Questions
Question 1 to 12 are multiple choice questions. Only one of the choices is correct. Select and write the correct choice as well as the answer to these questions.
- A person can clearly see objects only when they are between 50 cm and 200 cm from his eye. What kind of lenses will he need to see objects at infinity clearly?
A. Convex lens of suitable focal length
B. Concave lens of suitable focal length
C. Bifocal lens
D. Cylindrical lens
ANS- B. Concave lens of suitable focal length - If the focal length of the eye lens increases, the eye will be unable to see:
A. Distant objects
B. Nearby objects
C. Both near and distant objects
D. Objects at infinity
ANS- B. Nearby objects - A ray of white light enters a glass prism. Which of the following is true?
A. Red light bends the most
B. Violet light bends the least
C. Red light bends the least
D. All colors bend equally
ANS- C. Red light bends the least - A student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his textbook. Which of the following statements is correct?
A. The near point of his eyes has receded away
B. The near point of his eyes has come closer to him
C. The far point of his eyes has come closer to him
D. The far point of his eyes has receded away
ANS- A. The near point of his eyes has receded away - A person cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power
A. + 0.5 D
B. – 0.5 D
C. + 0.2 D
D. – 0.2 D
ANS- B. –0.5 D - When we enter a dark room coming from outside, immediately the things inside the room do not appear clear to our eyes. This is because
A. Pupils do not open at all in the dark
B. Pupils take time to adjust.
C. Light travels slower in a dark room.
D. Pupils open very quickly in the dark.
ANS- B. Pupils take time to adjust - The image shows a light ray incident on a glass prism. The various angles are labelled in the image. Which angle shows the angle of incidence and angle of refraction, respectively?
A. A and D
B. B and E
C. C and F
D. D and F
ANS: Option A - Which option shows the correction of the defect of the eye?
ANS: Option D - When we enter a dark room coming from outside, immediately the things inside the room do not appear clear to our eyes. This is because
A. Pupils do not open at all in the dark
B. Pupils take time to adjust.
C. Light travels slower in a dark room.
D. Pupils open very quickly in the dark.
ANS: B. Pupils take time to adjust. - Twinkling of stars is due to atmospheric
A. Dispersion of starlight by water droplets of the atmosphere
B. Refraction of starlight by different layers of varying refractive indices
C. Scattering of light by dust particles of the atmosphere
D. Reflection of starlight by different layers of varying refractive indices
ANS: B. Refraction of starlight by different layers of varying refractive indices
Assertion-Reason Questions
Question No. 11 to 12 consist of two statements– Assertion (A) and Reason (R). Answer these questions by selecting the appropriate option given below:
- 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. A is true but R is false.
- D. A is false but R is true.
- Assertion (A): The human eye forms a real image on the retina.
Reason (R): The crystalline lens of the eye acts as a converging lens.
ANS- (a) Both Assertion and Reason are correct and Reason is correct explanation. - Assertion (A): Concave lens is used to correct myopia or short sightedness.
Reason (R): A concave lens of suitable focal length diverges the parallel rays from the distant objects as if they are coming from the far point of the myopic eye. This helps the eye lens to form a clear image at the retina.
ANS- (a) Both Assertion and Reason are correct and Reason is correct explanation.
Section-B: Very Short Answer Questions
- An eye specialist prescribes lenses of power +2 D to a person. What is the nature and focal length of the lens? How does it help the person?
ANS- Convex lens, focal length = 100 / 2 = 50 cm = 0.5 m. It helps focus light on retina for farsightedness (hypermetropia). - During a solar eclipse, an observer notices a ring of light around the Moon. Which optical phenomenon is responsible for this? Explain its working in the atmosphere.
ANS- Atmospheric refraction + diffraction (ring of light = annular eclipse effect). - How will the rainbow appear if the observer is in a plane flying above the rain-bearing clouds?
ANS- Observer may see a complete circular rainbow. - A person is suffering from both myopia and hypermetropia. Explain how this is possible and what type of lens would be required to correct this defect.
ANS- This is possible in the case of presbyopia, where the lens loses its flexibility and ability to focus on both near and far objects. A bifocal lens would be required to correct this defect. - Explain how the human eye adjusts to changes in light intensity. What is the role of the iris and pupil in this process?
ANS- The human eye adjusts to changes in light intensity through the iris and pupil. The iris controls the amount of light entering the eye by adjusting the size of the pupil.
Section-C: Short Answer Questions
- A rainbow is a natural spectrum appearing in the sky after a rain shower. What role do water droplets play in its formation?
ANS- \( \text{Sunlight} \rightarrow \text{refraction (entry)} \rightarrow \text{reflection (inside raindrop)} \rightarrow \text{refraction + dispersion (exit)} \). - Rahul observes that during foggy mornings, car headlights appear to be scattered and less effective. Explain scientifically why this happens. What change in the type of headlight can improve visibility?
ANS- Tiny water droplets scatter light \( \rightarrow \) beam spreads \( \rightarrow \) reduced visibility. Solution: use yellow light, fog lights with focused beams. - A student complains of blurred vision while reading a book but can see distant objects clearly. Name the defect and draw a diagram showing how this defect can be corrected using an appropriate lens.
ANS- Hypermetropia; corrected with convex lens (diagram: convex lens converging rays onto retina). - A person is working on a computer for an extended period. What are the potential risks to their eye health, and how can they mitigate these risks?
ANS- Potential risks include eye strain, dry eyes, and blue light exposure. Mitigation strategies include following the 20-20-20 rule, adjusting display settings, and taking regular breaks. - Explain how the human eye adapts to changes in light intensity. What are the limitations of this adaptation, and how can we protect our eyes in extreme lighting conditions?
ANS- The human eye adapts to changes in light intensity through the iris and pupil. However, there are limitations to this adaptation, and prolonged exposure to extreme lighting conditions can cause eye damage. Protection measures include wearing sunglasses or protective eyewear.
Section-D: Long Answer Questions
- Observe the diagram and answer the following questions:
(a) Name the defect of vision shown in figure. What will be the optical experience of the person suffering from this defect?
(b) Give two possible reasons for this defect of eye in human beings.
(c) Draw a labeled diagram to show how the defect is rectified by using suitable lens. - A) Light of wavelength 600 nm is refracted by a prism of glass having refractive index 1.5. Calculate the speed of this light in glass (Speed of light in vacuum = \( 3\times10^8 \text{ m/s} \)). What does this tell you about dispersion?
ANS- \( v = c / n = 3\times10^8 / 1.5 = 2\times10^8 \text{ m/s} \). Shows light slows \( \rightarrow \) dispersion.
B) In a science fair, a student shows a setup where white light passes through two identical prisms placed upside down with respect to each other. Explain the outcome of this arrangement. What concept does it demonstrate?
ANS- Spectrum from 1st prism recombined by 2nd \( \rightarrow \) white light. Concept: recombination of spectrum. - A) If the Earth had no atmosphere, what would be the color of the sky and the appearance of the Sun at sunrise?
ANS- Sky: black. Sun: white at sunrise.
B) Describe the phenomenon of scattering of light and its effect on the color of the sky. How does the scattering of light change during sunrise and sunset?
ANS- Scattering of light is the phenomenon where shorter wavelengths of light are scattered more than longer wavelengths. This is why the sky appears blue during the day. During sunrise and sunset, the light travels through more atmosphere, scattering shorter wavelengths and making the sky appear red.
Section–E: Case-based Questions
- In an experiment, Paheli used an equilateral triangular glass prism and projected a narrow beam of white light source from one side of the surface of the prism. She placed a screen on the other side and saw many colours appearing as patches on the screen. But when she used a red light source, she could only see a red patch on the screen. Similarly she used a blue and green light source and could only see one colour patch on both occasions.
(i) What explanation can Paheli give to her friends to explain this phenomenon?
(ii) Why she could not see any other colour when the red light was used?
(iii) What is the measurement of angle of prism in an equilateral triangle?
(iv) Arrange the following component of white light in increasing order of wavelength. Blue, Red, Green, Violet.
ANS-
i) White light disperses \( \rightarrow \) spectrum; single color light \( \rightarrow \) no dispersion.
ii) Monochromatic light \( \rightarrow \) no dispersion.
iii) 60°
iv) Violet < Blue < Green < Red - Whether the color of the ocean and the color of the sky are related? Is this the same reason? But they occur independently of each other. In both the cases, the preferential absorption of long wave length light gives rise to blue. The Ocean looks blue, the blue wave length returned predominantly than longer wave length. Note that this effect only works if the water is very pure.
A. Which phenomenon of light is the reason for blue color of sky?
B. The intensity of scattered light will be more in ______ (shorter / longer) wavelength
C. Say Yes or No for the following:
(i) Red color of sun during sunset is due to scattering
(ii) Twinkling of stars is due to scattering of light
ANS- A) Rayleigh scattering
B) Shorter wavelength
C) i) Yes ii) No (due to refraction)
CHAPTER – 11: ELECTRICITY
VOLUME – 1
Section A: Multiple Choice Questions (1 Mark Each)
- Which device is used to measure electric current in a circuit?
a) Voltmeter
b) Ammeter
c) Galvanometer
d) Thermometer
ANS: b) Ammeter - In which arrangement do resistors give minimum resistance?
a) Series
b) Parallel
c) Both a and b
d) None
ANS: b) Parallel - What is the SI unit of electric charge?
a) Volt
b) Ampere
c) Coulomb
d) Ohm
ANS: c) Coulomb - Which of the following represents Ohm’s Law?
a) \( V = IR \)
b) \( P = VI \)
c) \( I = Rt \)
d) \( Q = It \)
ANS: a) \( V = IR \) - Which material is used in the filament of electric bulbs?
a) Copper
b) Aluminium
c) Tungsten
d) Nichrome
ANS: c) Tungsten - A wire has resistance R. If it is stretched to double its length, cross sectional area reduced to half. What will be the new resistance?
a) R/2
b) 2R
c) 4R
d) R
ANS: c) 4R - Fuse is always connected in –
a) Parallel
b) Series
c) Diagonal
d) None
ANS: b) Series - Electric power is equal to –
a) I/R
b) \( IR^2 \)
c) \( V \times I \)
d) V/I
ANS: c) \( V \times I \) - A current of 1 A flows through a wire for 10 seconds. How much charge flows?
a) 10 C
b) 5 C
c) 1 C
d) 100 C
ANS: a) 10 C - Which device works on heating effect of current?
a) Fan
b) Bulb
c) Battery
d) Switch
ANS: b) Bulb
Assertion-Reason Questions
- Assertion: In a parallel circuit, total resistance decreases.
Reason: Current divides through each resistor.
Correct option: a) Both A and R are true, and R is the correct explanation. - Assertion: Copper is used for household wiring.
Reason: It has low resistivity and is a good conductor.
Correct option: a) Both A and R are true, and R is the correct explanation.
Section B: Short Answer Questions (2 Marks Each)
- Why is tungsten used in electric bulbs?
ANS: High melting point and glows without melting. - Define 1 volt. Write relation between work and charge.
ANS: 1V = 1J/1C; \( V = W/Q \). - State two factors affecting resistance of a conductor.
ANS: Length and cross-sectional area. - Why is parallel connection preferred in household circuits?
ANS: Same voltage across devices and independent operation. - State two disadvantages of series circuits.
ANS: One device failure stops entire circuit; voltage divides.
Section D: Short Answer Questions (3 Marks Each)
- In the given circuit calculate the (i) equivalent resistance ii) current iii) power consumed in watts in the resistor of 2Ω :
ANS:
Equivalent resistance = \( R_1 + R_2 = 1 \Omega + 2 \Omega = 3 \Omega \)
\( I = V / R = 6 V / 3 \Omega = 2 \text{ A} \)
Electric power, \( P = I^2 \times R = (2 \text{ A})^2 \times 2 \Omega = 4 \times 2 \text{ W} = 8 \text{ W} \) - A bulb is rated 60W, 220V. Calculate: (a) Current (b) Resistance
ANS:
\( I = P/V = 60/220 \approx 0.27 \text{ A} \)
\( R = V/I = 220/0.27 \approx 814.8 \Omega \) - How is the resistivity of alloys compared with those of pure metals from which they may have been formed?
OR
If the conductor A is made of copper and the conductor C is made of constantan (alloy of copper and nickel), then which one of the two will have more electrical resistance and why?
ANS:
Resistivity of alloys is generally higher than its constituent metals. Therefore, alloy has more resistance. - Show how you would connect three resistors each of resistance 6 Ω, so that the combination has a resistance of 9 Ω. Also justify your answer.
ANS: Connect two in parallel (\( \frac{1}{6} + \frac{1}{6} = \frac{2}{6} \Rightarrow R_p = 3 \Omega \)) and one in series (\( 3 \Omega + 6 \Omega = 9 \Omega \)). - State Joule’s Law of heating and give two uses.
ANS: \( H = I^2Rt \). Uses: Electric heater, iron.
Section E: Long Answer Questions (5 Marks Each)
- (i) How is electric current related to the potential difference across the terminals of a conductor?
(ii) Why should an ammeter have low resistance?
(iii) Two V-I graphs A and B for series and parallel combinations of two resistors are as shown. Giving reason state which graph shows (a) series, (b) parallel combination of the resistors.
ANS:
(i) \( V=IR \)
(ii) To measure the entire current passing through the circuit, the ammeter should have low resistance.
(iii) Series: \( R_s = R_1 + R_2 \) (Maximum resistance). Parallel: \( R_p \) (Minimum resistance). Since slope of I-V graph represents 1/R (or V-I represents R, assuming V on y-axis graph B has higher slope/resistance, A has lower).
Note: Based on standard V-I graph where V is x-axis and I is y-axis, Slope = I/V = 1/R. Higher slope means lower resistance. So B (higher slope) is Parallel, A (lower slope) is Series. If V is y-axis, B is Series.
From text: So, \( R_A > R_B \) (This implies graph A corresponds to higher resistance/Series if V is y-axis). - What is a fuse? Why is it necessary? Write 2 precautions & 1 use.
ANS: Fuse protects from overload by melting. Precautions: Use correct rating, switch off supply before changing. Use: In home main circuits. - A fuse wire melts at 5 A. If it is desired that the fuse wire of same material melt at 10 A, then whether the new fuse wire should be of smaller or larger radius than the earlier one? Give reasons for your answer.
ANS:
Let the resistance of the wire be R, heat produced \( H \propto I^2R \).
To prevent melting at higher current, resistance must be lower (or heat dissipation capacity higher).
\( R \propto \frac{1}{A} \propto \frac{1}{r^2} \).
So, at 10A, the new fuse wire of same material and length needs a larger radius than the earlier one to handle more current without melting immediately.
Section E: Case Based Study (4 Marks Each)
- Riya is studying household circuits. She observed that when multiple appliances like fans, lights, and TV are used in her house, all of them work independently and receive equal brightness or speed.
1) Which bulb glows brighter? 100W or lower? (Assuming implied context).
2) Why are bulbs connected in parallel?
3) What is the Voltage across each bulb? Will both work if one fails?
ANS:
1) 100W (in parallel, P is proportional to rating).
2) To get full voltage and work independently.
3) 220V. Yes, because connected in parallel. - Raj wanted to test how much electricity his iron box consumes. He checked the label which read '220V, 1000W'. He connected it to a power supply...
1) What is the Power consumed?
ANS: \( P = V \times I = 220 \times 5 = 1100 \text{ W} \) (Note: Text says 1000W label, calculation uses 5A current. \( 220 \times 5 = 1100 \)).
2) What is the Function of fuse?
ANS: Breaks circuit when current exceeds limit.
3) What if higher-rated fuse used? Which effect of current is used?
ANS: Won’t melt \( \rightarrow \) appliance damage/fire risk. Heating effect.
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