Topper's Academia: Heat and Light Examination
Section A: Multiple Choice Questions (MCQ) (5 x 1 = 5 Marks)
Instructions: Choose the most appropriate option.
Which mode of heat transfer is the fastest? a) Conduction b) Convection c) Radiation d) Conduction and Convection equally
The image formed by a plane mirror is always: a) Real and Inverted b) Virtual and Magnified c) Virtual and Erect d) Real and Diminished
The S.I. unit of heat energy is: a) Calorie b) Celsius c) Watt d) Joule
Light travelling from air into glass (rarer to denser medium) bends: a) Away from the normal b) Towards the normal c) Parallel to the interface d) No bending occurs
Which mirror is used by dentists to see enlarged images of teeth? a) Plane mirror b) Concave mirror c) Convex mirror d) Cylindrical mirror
Section B: Assertion-Reasoning (A/R) (5 x 1 = 5 Marks)
Instructions: Two statements are given: an Assertion (A) and a Reason (R). Mark the correct option: a) Both A and R are true, and R is the correct explanation of A. b) Both A and R are true, but 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): Cooking pots often have wooden or plastic handles. Reason (R): Wood and plastic are poor conductors of heat.
Assertion (A): Sea breeze blows during the daytime. Reason (R): During the day, the air over the land is cooler than the air over the sea.
Assertion (A): The angle of incidence is always equal to the angle of reflection. Reason (R): This is the first law of reflection.
Assertion (A): Water is a poor choice for a coolant in car radiators. Reason (R): Water has a very high specific heat capacity.
Assertion (A): Dispersion of white light occurs when it passes through a prism. Reason (R): Different colours of light travel at different speeds in glass.
Section C: Short Answer Questions (SAQ) (Total 10 Marks)
State the two laws of reflection of light. (2 Marks)
Why do we wear light-coloured clothes in summer? Explain using the concept of heat transfer. (3 Marks)
Distinguish between a real image and a virtual image. (2 Marks)
Calculate the heat energy required to raise the temperature of 5 kg of water from 20∘C to 50∘C. (Specific heat capacity of water is 4200 J kg−1∘C−1). (3 Marks)
Section D: Case-Based Question (CBQ) (1 x 10 = 10 Marks)
Read the following case and answer the questions that follow.
Case Study: Optical Fibre and Heat Preservation
Optical fibres are thin strands of highly pure glass or plastic, used to transmit light signals over long distances. They work on the principle of Total Internal Reflection (TIR). For TIR to occur, light must travel from a denser medium to a rarer medium, and the angle of incidence must be greater than the critical angle. In a completely different context, a Thermos flask is designed to keep liquids hot or cold for long periods. Its key features include a double-walled glass vessel with a vacuum between the walls, silvered surfaces, and an insulating stopper. These features are meant to minimise heat transfer.
Based on the case study, answer the following questions:
a) What is the full form of TIR, and state the two necessary conditions for it to occur? (3 Marks) b) Name the three modes of heat transfer that the design features (vacuum, silvered walls, stopper) of the Thermos flask attempt to prevent. (3 Marks) c) Explain the role of the silvered surface in the thermos flask. (2 Marks) d) If the core of an optical fibre has a refractive index of 1.55, and the cladding has a refractive index of 1.50, is it possible for light to undergo TIR at the core-cladding boundary? Justify your answer. (2 Marks)
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