Tag: physics

Quizzes (598) Questions (13441)

Questions Related to physics

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

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:

  1. 1 : 2 : 3

  2. 1 : 1 : 1

  3. 3 : 2 : 1

  4. 2 : 2 : 1
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

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

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

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:

  1. 20 cm

  2. 4 cm

  3. 12.5 cm

  4. 8 cm
Reveal answer Fill a bubble to check yourself
C Correct answer
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, $f = 12.5$

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

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?

  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 $2 m$
Reveal answer Fill a bubble to check yourself
A Correct answer
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 $2m$.

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

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 

  1. $ 100 cm$
  2. $ -100 cm$
  3. $-1 cm$
  4. $-\dfrac{100}{9}cm$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

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

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

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 

  1. -20 cm

  2. -25 cm

  3. -50 cm

  4. 50 cm
Reveal answer Fill a bubble to check yourself
C Correct answer
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.

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

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

  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 $100\ cm$
Reveal answer Fill a bubble to check yourself
B Correct answer
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 = $\dfrac { 100 }{ 2 } cm=50cm$

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

Mark the correct option

  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
Reveal answer Fill a bubble to check yourself
D Correct answer
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 $D$ 

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

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

  1. $Light$
  2. $Sound$
  3. $Rocket$
  4. $Bullet train$
Reveal answer Fill a bubble to check yourself
A Correct answer
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.