Science (English Medium) Class 12 - CBSE Question Bank Solutions for Physics

Deutrium was discovered in 1932 by Harold Urey by measuring the small change in wavelength for a particular transition in ¹H and ²H. This is because, the wavelength of transition depend to a certain extent on the nuclear mass. If nuclear motion is taken into account then the electrons and nucleus revolve around their common centre of mass. Such a system is equivalent to a single particle with a reduced mass µ, revolving around the nucleus at a distance equal to the electron-nucleus separation. Here µ = m_eM/(m_e + M) where M is the nuclear mass and m e is the electronic mass. Estimate the percentage difference in wavelength for the 1st line of the Lyman series in ¹H and ²H. (Mass of ¹H nucleus is 1.6725 × 10^–27 kg, Mass of ²H nucleus is 3.3374 × 10^–27 kg, Mass of electron = 9.109 × 10^–31 kg.)

The gravitational force between a H-atom and another particle of mass m will be given by Newton’s law: `F = G(M.m)/r^2`, where r is in km and ______.

A male voice after modulation-transmission sounds like that of a female to the receiver. The problem is due to ______.

An audio signal of 15kHz frequency cannot be transmitted over long distances without modulation because ______.

1. the size of the required antenna would be at least 5 km which is not convenient.
2. the audio signal can not be transmitted through sky waves.
3. the size of the required antenna would be at least 20 km, which is not convenient.
4. effective power transmitted would be very low, if the size of the antenna is less than 5 km.

A TV transmission tower antenna is at a height of 20 m. How much service area can it cover if the receiving antenna is (i) at ground level, (ii) at a height of 25 m? Calculate the percentage increase in area covered in case (ii) relative to case (i).

The work function for a metal surface is 4.14 eV. The threshold wavelength for this metal surface is ______.

Electromagnetic waves with wavelength

1. λ₁ is suitable for radar systems used in aircraft navigation.
2. λ₂ is used to kill germs in water purifiers.
3. λ₃ is used to improve visibility in runways during fog and mist conditions.

Identify and name the part of the electromagnetic spectrum to which these radiations belong. Also arrange these wavelengths in ascending order of their magnitude.

The graph shows the variation of photocurrent for a photosensitive metal

1. What does X and A on the horizontal axis represent?
2. Draw this graph for three different values of frequencies of incident radiation ʋ₁, ʋ₂ and ʋ₃ (ʋ₃ > ʋ₂ > ʋ₁) for the same intensity.
3. Draw this graph for three different values of intensities of incident radiation I₁, I₂ and I₃ (I₃ > I₂ > I₁) having the same frequency.

Why it is the frequency and not the intensity of the light source that determines whether the emission of photoelectrons will occur or not? Explain.

Arrange the following electromagnetic radiation in the ascending order of their frequencies:

X-rays, microwaves, gamma rays, radio waves

Write two uses of the following radiation.

X-rays

Write two uses of the following radiation.

Gamma rays

Electromagnetic waves of wavelengths λ₁, λ₂ and λ₃ are used in a radar system, in water purifiers and in remote switches of TV, respectively.

1. Identify the electromagnetic waves.
2. Write one source for each of them.

Give any two uses of infrared waves.

Read the following paragraph and answer the questions.

1. Which light beam has the highest frequency and why?
2. Which light beam has the longest wavelength and why?
3. Which light beam ejects photoelectrons with maximum momentum and why?

A series RL circuit with R = 10 Ω and L = `(100/pi)` mH is connected to an ac source of voltage V = 141 sin (100 πt), where V is in volts and t is in seconds. Calculate

1. the impedance of the circuit
2. phase angle, and
3. the voltage drop across the inductor.

Which one of the following electromagnetic radiation has the least wavelength?

Which of the following statements about a series LCR circuit connected to an ac source is correct?

How would the stopping potential for a given photosensitive surface change if the intensity of incident radiation was decreased? Justify your answer.

How would the stopping potential for a given photosensitive surface change if the frequency of the incident radiation were increased? Justify your answer.