Tag: physics

Questions Related to physics

Two identical plano convex lenses are arranged in three different combinations as:
I)  the plane surfaces touching each other
II) the curved surfaces touching each other
III) the plane surface of one lens touching the curved surface of the other.  
Then the focal lengths of the combinations are in the ratio:

physics ray optics and optical instruments power of the lens thin lens combination of lenses

  1. 1 : 2 : 3

  2. 1 : 1 : 1

  3. 3 : 2 : 1

  4. 2 : 2 : 1

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Correct Option: B
Explanation:

The focal lengths of the combination of two lenses kept in any fashion is irrespective of their arrangement. 

Two plano convex lenses of same material and of focal lengths 20 cm & 5 cm should be arranged such that the combination is free from chromatic aberration. Then effective focal length of the combination will be:

physics ray optics and optical instruments power of the lens thin lens combination of lenses

  1. 20 cm

  2. 4 cm

  3. 12.5 cm

  4. 8 cm

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Correct Option: C
Explanation:

The effective focal length of the lens is, 


$ \dfrac{1}{f} = \dfrac{1}{{f} _{1}} + \dfrac{1}{{f} _{2}} - \dfrac{{f} _{1} + {f} _{2}}{2{f} _{1}{f} _{2}} $

Substituting the respective values, the effective focal length obtained is, $f = 12.5$

A convex lens has a focal length of $0.5 m$. It has to be combined with a second lens, so that the combination has a power of $1.5$ dioptre. Which of the following could be the second lens?

physics ray optics and optical instruments power of the lens thin lens combination of lenses

  1. A concave lens of focal length $2 m$

  2. Another convex lens of focal length $0.5 m$

  3. A concave lens of focal length $0.5 m$

  4. A convex lens of focal length $2 m$

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Correct Option: A
Explanation:

Power of combination $P = 1.5 $ dioptre

Power of convex lens having focal length of $0.5 m$,  $P _1 = \dfrac{1}{0.5}$ dioptre
Using $P = P _1 + P _2$
$\therefore$ $1.5 = \dfrac{1}{0.5} + P _2$
We get $P _2 = -0.5$ dioptre
Thus focal length of second lens $f _2 = \dfrac{1}{P _2}$ 
$\implies$ $f _2 =- \dfrac{1}{0.5} = -2 m$ (negative sign implies the lens must be a concave lens)
Thus second lens could be concave lens of focal length $2m$.

A combination of convex and concave lenses has power $ 4 D $ .If the convex lens has power $5 D $ focal length of the concave lense will be 

physics ray optics and optical instruments power of the lens thin lens combination of lenses

  1. $ 100 cm$

  2. $ -100 cm$

  3. $-1 cm$

  4. $-\dfrac{100}{9}cm$

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Correct Option: B
Explanation:

Power of combination 
$P=P _1+P _2$
$\Rightarrow   4=5+P _2$
$\Rightarrow  P _2=4-5=-1 D$
$\therefore   -1=P _2=\dfrac{100}{f(cm)}$
$\therefore   f=-100 cm$

Two identitical thin plano-convex glass lenses (refractive index 1.5 ) each having radius of curvature of 20 cm are placed with their convex surface in contact at the centre .The intervening space is filled with oil of refractive index 1.7.The focal length of the combination is 

physics ray optics and optical instruments power of the lens thin lens combination of lenses

  1. -20 cm

  2. -25 cm

  3. -50 cm

  4. 50 cm

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Correct Option: C
Explanation:

The intervening space will act as concave lens.

Thus, the net focal length of the combination of three lenses would be given as, $\dfrac{1}{f} = \dfrac{1}{f _1}+\dfrac{1}{f _2}+\dfrac{1}{f _3}$

Now by the lens makers' formula,
$\dfrac{1}{f _1}=(1.5-1)\bigg(\dfrac{1}{\infty}-\dfrac{1}{-20}\bigg)=\dfrac{1}{40}$
$\dfrac{1}{f _2}=(1.7-1)\bigg(\dfrac{1}{-20}-\dfrac{1}{20}\bigg)=-\dfrac{2.8}{40}$
$\dfrac{1}{f _3}=(1.5-1)\bigg(\dfrac{1}{20}-\dfrac{1}{\infty}\bigg)=\dfrac{1}{40}$
$\therefore \dfrac{1}{f}=-\dfrac{0.8}{40}\implies f=-50cm$

Option C is correct.

A lens of power $6D$ is put in contact with a lens of power $-4\ D$. The combination will behave like a:

physics ray optics and optical instruments power of the lens thin lens combination of lenses

  1. Divergent lens of focal length $25\ cm$

  2. Convergent lens of focal length $50\ cm$

  3. Divergent lens of focal length $20\ cm$

  4. Convergent lens of focal length $100\ cm$

Show answer
Correct Option: B
Explanation:

For a combination of lenses, net power of the combination is given by sum of individual powers i.e.

            Pnet = ${ P } _{ 1 }+{ P } _{ 2 }$
According to question
            Pnet = 6D - 4D = +2D
hence it will be convergent lens of focal length = $\dfrac { 100 }{ 2 } cm=50cm$

Physical independence of force is a consequence of 

physics force and newton's laws of motion first law of motion newton's first law of motion momentum and newton's laws

  1. First law of motion

  2. Second law of motion

  3. Third law of motion

  4. All of these laws

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Correct Option: A
Explanation:

Physical independence of force is a consequence of first law of motion.

Mark the correct option

physics force and newton's laws of motion first law of motion newton's first law of motion momentum and newton's laws

  1. The state of motion of an object is described by its speed only

  2. Many a time application of force does not result in a change in the state of motion of an object

  3. By sitting on a bicycle seat force is applied to the spring fixed to the seat of a bicycle

  4. All of the above

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Correct Option: D
Explanation:
$(i)$ option $A$ is correct, as if to know about the unifomity of a motion and the body is moving or not speed is used as the parameter
$(ii)$Option B is correct, as the acceleration of a body depends on the net force a applied not on single $10r$ individual forces.
$(iii)$ Option $C$ is correct, while sifting on the bicycle, the spring force allached to the seat balances weight.
Option $D$ 

Consider the following statements
I) When a body is travelling with uniform speed, no force is acting on it.
II) When a body is travelling with constant velocity, no force is acting on it.
Which of the above statements is/are true?

physics force and newton's laws of motion first law of motion newton's first law of motion momentum and newton's laws

  1. only I

  2. only II

  3. both I and II

  4. neither I nor II

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Correct Option: A

Newton's law of motion cannot be applicable to the particle moving at a speed comparable to the speed of

physics force and newton's laws of motion first law of motion newton's first law of motion momentum and newton's laws

  1. $Light$

  2. $Sound$

  3. $Rocket$

  4. $Bullet train$

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Correct Option: A
Explanation:

Newton's law of motion are applicable for matter which occupy some space and have some mass. It is not possible for any matter to travel with speed of light. Secondly the mass charges as the speed approaches speed of light. Newton's law is applicable only when mass is constant so light is correct option.