Tag: physics

Questions Related to physics

To increase the magnification of a telescope 

physics wave optics resolving power of optical instruments resolution of optical instruments diffraction

  1. the objective lens should be of large focal length and eyepiece should be of small focal length.

  2. the objective and eyepiece both should be of large focal length.

  3. both the objective and eyepiece should be of smaller focal lengths

  4. the objective should be of small focal length and eyepiece should be of large focal length

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Correct Option: A
Explanation:
$m=\dfrac { { f } _{ o } }{ { f } _{ e } } $
from the above relation we can see that magnification is directly proportional to the focal length of objective lens.

Magnification of an object ($m$), is equal to

physics wave optics resolving power of optical instruments resolution of optical instruments diffraction

  1. $\cfrac {v+f}{f}$

  2. $\cfrac {vf}{v-f}$

  3. $\cfrac {f}{v+f}$

  4. None of these

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Correct Option: D
Explanation:

we know,mirror formula
$\cfrac {1}{f}=\cfrac{1}{v}+\cfrac{1}{u}, magnification(m)=\cfrac{-v}{u}$
$\cfrac{1}{u}=\cfrac{1}{f}-\cfrac{1}{v}$
$\cfrac{1}{u}=\cfrac{v-f}{fv}$
${u}=\cfrac{fv}{v-f}$
$(m)=\cfrac{-v}{u}$,substituting $u$.
$m=\cfrac{-v}{1}\times\cfrac{v-f}{fv}$
$m=\cfrac{f-v}{f}$

Calculate the limit of resolution of a telescope objective having a diameter of 200 cm, if it has to detect light of wavelength 500 nm coming from a star ; -

physics wave optics resolving power of optical instruments resolution of optical instruments diffraction

  1. $305 \times 10^{-9} $ radian

  2. $152.5 \times 10^{-9} $ radian

  3. $610 \times 10^{-9} $ radian

  4. $457.5 \times 10^{-9} $ radian

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Correct Option: A
Explanation:

Limit of resolution of telescope = $\dfrac{1.22 \lambda}{D}$
$\theta = \dfrac{1.22 \times 500 \times 10^{-9}}{200 \times 10^{-2}} = 305 \times 10^{-9}$ radian

In an electron microscope the accelerating voltage is increased from 20 kV to 80 kV, the resolving power of the microscope will change from R to

physics wave optics resolving power of optical instruments resolution of optical instruments diffraction

  1. $2 R$

  2. $\dfrac{R}{2}$

  3. $4R$

  4. $3R$

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Correct Option: A
Explanation:

Electron microscope is a microscope that can magnify very small details with high resolving power due to the use of electrons as the source of illumination. Since the wavelength of electrons are 100,000 times shorter than visible light the electron microscopes have greater resolving power
We have the Abbe's formula as resolution limit $d=\dfrac{0.61\lambda}{NA}$(NA is the numerical aperture)


The resolving power increases when d, the minimum distance that can be seen between two points in the image, decreases. Thus, according to the formula the resolving power is inversely proportional to the wavelength.

Resolving Power  $\propto \dfrac{1}{\lambda}$

A higher voltage will give the electrons a higher speed. Thus the electrons will have a smaller de Broglie wavelength according to the equation,  $\lambda=h/mv$

$\lambda\propto\dfrac{1}{\sqrt V}$

Thus we get Resolving power $\propto \sqrt{V}$

$ \implies\dfrac{R}{R'} = \sqrt{\dfrac{20}{80}} $

Thus, $R' = 2R$

An astronomical telescope, consists of two thin lenses set $36 cm$ a part and has a magnifying power $8$. Calculate the focal length of the lenses.

physics wave optics resolving power of optical instruments resolution of optical instruments diffraction

  1. $32 cm$

  2. $18 cm$

  3. $25 cm$

  4. $36 cm$

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Correct Option: A
Explanation:

Here, $f _o+f _e=36 cm$
$M=-8$ (magnifying power is negative)
Now, $M=\cfrac {f _o}{f _e}$
$\therefore -8=-8=-\cfrac {f _o}{f _e}$
$\Rightarrow f _o=8f _e$

From the equations, we have
$8f _e+f _e=36$ or $f _e=4\ cm$

Again, $f _o=8f _e=8\times 4=32\ cm$

Which among the following is the best conductor of electricity?

physics some natural phenomena transfer of charge lightning safety seismic waves and tsunami

  1. Oxygen

  2. Aluminium

  3. Carbon

  4. Wood

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Correct Option: B
Explanation:

Among oxygen, carbon, aluminium and wood, aluminium is a metal while others are non-metals. Metals are good conductors of electricity.

What happens when the lower end of the lightning conductor is not properly buried in the ground?

physics some natural phenomena transfer of charge lightning safety seismic waves and tsunami

  1. No earthing

  2. Building are protected

  3. Damage building and even life

  4. Both a & c

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Correct Option: D
Explanation:

Earthing is important to protect building from lightning. If proper earthing is not provide than it can damage building and dangerous for life also.

State whether given statement is True/False ?
Lightning conductor is made up of copper.

physics some natural phenomena transfer of charge lightning safety seismic waves and tsunami

  1. True

  2. False

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Correct Option: A
Explanation:

Copper wires allow electric current to flow without much loss of energy. Hence, copper is a good conductor of heat and electricity. This is why copper wires are used in mains cables in houses and underground. And, a thick copper strip is used for lightning conductors on tall buildings like church spires.

Why earthing is important?

physics some natural phenomena transfer of charge lightning safety seismic waves and tsunami

  1. To protect us from electrical shocks.

  2. Transfers charge from a charged body to the Earth.

  3. Both A and B.

  4. Neither A nor B.

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Correct Option: C
Explanation:

Earthing is the process of transferring charges to the Earth. During lightning, if earthing is not proper, we will get electrical shocks if lightning strikes.

Which of the following is safest way to protect yourself from lightning?

physics some natural phenomena transfer of charge lightning safety seismic waves and tsunami

  1. Run to an open field

  2. Open an umbrella for cover

  3. Take shelter under trees

  4. Squat low on ground

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Correct Option: D
Explanation:

Squat low on ground is safest way to protect yourself from lightning.