CBSE Class 10 Science Physics Light Eye Electricity Magnetic Effects and Environment Assignment with Answers Set 03

Answer: Laws of reflection: (1) The angle of incidence equals the angle of reflection. (2) The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
Laws of refraction: (1) The incident ray, the refracted ray and the normal lie in the same plane. (2) Snell's law: for light passing from one medium to another, sin i / sin r = constant, called the refractive index of the second medium with respect to the first.

Teacher's Note: Use the key term in your answer.

Answer: Refractive index: the ratio of the speed of light in vacuum to its speed in the medium, n = c/v.
Centre of curvature: the centre of the sphere of which the mirror or lens surface is a part.
Focal length: the distance between the pole (or optical centre) and the principal focus.
Optical centre: the central point of a lens, through which a ray passes without deviation.
Pole: the centre of the reflecting surface of a spherical mirror.
Principal axis: the straight line through the pole and the centre of curvature of a mirror (or through the optical centre of a lens and its centres of curvature).
Aperture: the diameter of the reflecting surface of a spherical mirror.

Teacher's Note: Give an example if the question allows.

Answer: Beyond C: the image is formed between F and C; it is real, inverted and smaller than the object.
Between P and F: the image is behind the mirror; it is virtual, erect and enlarged.
At infinity: the image is formed at F; it is real, inverted and very small, like a point.

Teacher's Note:

Answer: Beyond 2F₁: the image is between F₂ and 2F₂; real, inverted and diminished.
Between F₁ and 2F₁: the image is beyond 2F₂; real, inverted and enlarged.
At F₁: the refracted rays are parallel, so the image is at infinity; it is real, inverted and highly enlarged.

Teacher's Note:

Answer: f = −20 cm, v = −15 cm. From 1/v − 1/u = 1/f:
1/u = 1/v − 1/f = −1/15 + 1/20 = −1/60, so u = −60 cm.
Magnification m = v/u = (−15)/(−60) = 0.25. Height of image = 0.25 × 6 = 1.5 cm.
The object is 60 cm from the lens, and the image is virtual, erect and 1.5 cm tall.

Teacher's Note: Learn this point and write it in your own words.

Answer: u = −15 cm, f = −10 cm, h = 4 cm. From 1/v + 1/u = 1/f: 1/v = −1/10 + 1/15 = −1/30, so v = −30 cm.
m = −v/u = −(−30)/(−15) = −2. Height of image = −2 × 4 = −8 cm.
The screen should be placed 30 cm in front of the mirror. The image is real, inverted and 8 cm tall.

Teacher's Note: Use the key term in your answer.

Answer:

Teacher's Note: Give an example if the question allows.

Answer: Astigmatism: a defect in which the cornea is not perfectly spherical, so lines in one direction look blurred; corrected by a cylindrical lens.
Presbyopia: loss of the eye's power of accommodation with age.
Dispersion: splitting of white light into its seven colours when it passes through a prism.
Scattering: the spreading of light in all directions by small particles in its path.
Accommodation: the ability of the eye lens to change its focal length to see near and far objects.
Atmospheric refraction: the bending of light by the layers of air of different density in the atmosphere.

Teacher's Note: Write a short, clear answer.

Answer: (a) Twinkling of stars: starlight is refracted by air layers of changing density, so the apparent position of the star keeps changing and its brightness flickers.
(b) Rainbow: sunlight is dispersed by raindrops, which act as small prisms; it is refracted, internally reflected and refracted again, and we see the colours spread in a band.
(c) Blue colour of sky: air molecules scatter short wavelengths (blue) much more than long wavelengths (red), so the blue light reaches our eyes from all directions.
(d) Early sunrise and delayed sunset: because of atmospheric refraction, we can see the Sun about 2 minutes before it actually rises and 2 minutes after it sets.

Teacher's Note: Remember this for your exam.

Answer: u = −25 cm, v = −60 cm. 1/f = 1/v − 1/u = −1/60 + 1/25 = 7/300, so f = 300/7 ≈ 42.9 cm = 0.43 m.
Power P = 1/f = +2.33 D. A convex lens of power +2.33 D is needed.

Teacher's Note: Learn this point and write it in your own words.

Answer: Cornea: the transparent front layer, where most of the refraction of light takes place.
Iris: the coloured muscle that controls the size of the pupil.
Pupil: the opening through which light enters the eye; it controls the amount of light.
Retina: the screen at the back of the eye where the image forms; its rods and cones change the light into electrical signals sent to the brain.

Teacher's Note: Use the key term in your answer.

Answer: Resistance: the property of a conductor that opposes the flow of current. R = V/I; its SI unit is the ohm (Ω).
Resistivity: the resistance of a conductor of unit length and unit area of cross-section; ρ = RA/l, with the unit Ω m.
Ohm's law: at constant temperature the current through a conductor is directly proportional to the potential difference across it, V = IR.

Teacher's Note: Give an example if the question allows.

Answer: Each appliance gets the full voltage. Each works independently, so if one fails, the others continue to work. Each can be switched on and off separately. The total resistance is lower, so appliances of different current rating can be used.

Teacher's Note: Write a short, clear answer.

Answer: (a) Tungsten has a very high melting point (about 3380 °C) and high resistivity, so it glows white-hot without melting.
(b) Alloys have higher resistivity than pure metals and do not oxidise (burn) easily at high temperature, so they last long.

Teacher's Note: Remember this for your exam.

Answer: When the wire is folded in two, its length becomes half and the area of cross-section becomes double. R = ρl/A, so the new resistance R′ = ρ(l/2)/(2A) = R/4 = 1 Ω.

Teacher's Note: Learn this point and write it in your own words.

Answer: The heat produced in a resistor is directly proportional to the square of the current, to the resistance and to the time for which the current flows: H = I²Rt.

Teacher's Note: Use the key term in your answer.

Answer: Resistance depends on the length of the conductor (directly), the area of cross-section (inversely), the material and the temperature. Resistivity depends only on the nature of the material and the temperature, not on the shape or size.

Teacher's Note: Give an example if the question allows.

Answer: Fleming's left-hand rule: stretch the thumb, forefinger and middle finger of the left hand at right angles to each other. If the forefinger points along the magnetic field and the middle finger along the current, the thumb shows the direction of force (motion) on the conductor.
Fleming's right-hand rule: with the right hand, if the forefinger points along the magnetic field and the thumb along the motion of the conductor, the middle finger shows the direction of the induced current.
Maxwell's right-hand thumb rule: imagine holding the conductor in the right hand with the thumb along the current; the curled fingers show the direction of the magnetic field lines.

Teacher's Note: Write a short, clear answer.

Answer: Electric motor: a current-carrying conductor placed in a magnetic field experiences a force; it changes electrical energy into mechanical energy. Generator: when a conductor moves in a magnetic field, a current is induced (electromagnetic induction); it changes mechanical energy into electrical energy.

Teacher's Note: Remember this for your exam.

Answer:

Teacher's Note: Learn this point and write it in your own words.

Answer: Solenoid: a coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder.
Electromagnet: a soft iron core placed inside a solenoid, which becomes a strong magnet when current flows.
Commutator: a split ring that reverses the direction of current in the coil of a DC motor every half turn.
Slip rings: two complete rings used in an AC generator to connect the coil with the brushes without reversing the current.
Overloading: too many appliances connected to one circuit, so that the current exceeds its safe limit.
Short-circuiting: the live and neutral wires touch each other, so that resistance becomes very small and the current very large.

Teacher's Note: Use the key term in your answer.

Answer: (a) Armature: the coil of insulated wire wound on a soft iron core, which rotates in the magnetic field.
(b) Brushes: carbon strips that touch the split ring and pass the current from the battery to the coil.
(c) Split ring (commutator): reverses the direction of current in the coil after every half rotation, so that the coil keeps rotating in the same direction.

Teacher's Note: Give an example if the question allows.

Answer: Food chain: a chain of organisms in which each eats the one below it, and the energy passes along, e.g. grass → deer → lion.
Food web: many food chains linked together in an ecosystem.
Biological magnification: the increase in the concentration of harmful chemicals, such as pesticides, at each higher level of a food chain.

Teacher's Note: Write a short, clear answer.

Answer: Only 10% of energy passes to the next trophic level. The deer had 100 × 10 = 1000 J, and the plant had 1000 × 10 = 10,000 J. So 10,000 J was available to the producer.

Teacher's Note: Remember this for your exam.

Answer: Ozone (O₃) is a molecule of three oxygen atoms. It forms in the upper atmosphere when UV rays split oxygen molecules (O₂) into atoms, which combine with other oxygen molecules: O₂ → O + O; O + O₂ → O₃.
Cause of depletion: chlorofluorocarbons (CFCs) used in refrigerators, aerosols and foams.
Effects: more UV rays reach the earth, which can cause skin cancer, cataracts and damage to crops and aquatic life.

Teacher's Note: Learn this point and write it in your own words.

Answer: A natural water body is a self-sustaining ecosystem with producers, consumers and decomposers in balance, and the wastes are recycled. An aquarium is not a complete ecosystem, with few organisms and no proper decomposers, so wastes build up and it needs regular cleaning.

Teacher's Note: Use the key term in your answer.

Answer: Decomposers, such as bacteria and fungi, break down dead plants and animals and their waste into simple substances that return to the soil. Without them dead matter would pile up, nutrients would not be recycled to the producers and life would end in the ecosystem.

Teacher's Note: Give an example if the question allows.