Q8 of 26 Page 1

(a) Why are infra-red waves often called heat waves ? Explain.

(b) What do you understand by the statement, ‘‘Electromagnetic waves transport momentum’’ ?

a) Infrared rays are readily absorbed by the (water) molecules in most of the substances and hence increases their thermal motion. Due to increase in thermal motion temperature of the substance also increases and heat is emitted from the substance. Thus, infra-red waves are often called heat waves.


b) Electromagnetic waves can set (and sustain) charges in motion. Hence, they are said to transport momentum. Electromagnetic waves are also known to exert “radiation pressure‟. This pressure is due to the force associated with rate of change of momentum. Hence, EM waves transport momentum.


More from this chapter

All 26 →
6

Two electric bulbs P and Q have their resistances in the ratio of 1 : 2. They are connected in series across a battery. Find the ratio of the power dissipation in these bulbs.

 7

A 10 V cell of negligible internal resistance is connected in parallel across a battery of emf 200 V and internal resistance 38Ω as shown in the figure. Find the value of current in the circuit.


OR


In a potentiometer arrangement for determining the emf of a cell, the balance point of the cell in open circuit is 350 cm. When a resistance of 9 Ω is used in the external circuit of the cell, the balance point shifts to 300 cm. Determine the internal resistance of the cell.

 9

If light of wavelength 412.5 nm is incident on each of the metals given below, which ones will show photoelectric emission and why ?

 10

A carrier wave of peak voltage 15 V is used to transmit a message signal. Find the peak voltage of the modulating signal in order to have a modulation index of 60%.