Tag: thin lens

Questions Related to thin lens

Two lenses of power $-15D$ and $-5D$ are in contact will each other. The focal length of the combination:

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

  1. $-20\ cm$

  2. $-10\ cm$

  3. $+20\ cm$

  4. $+10\ cm$

Show answer
Correct Option: A
Explanation:

Given that,

lenses power 

$P _{1}=-15\ D$

$P _{2}=-5\ D$

We know that,

$P=\dfrac{1}{f}$

Now,

  $ P={{P} _{1}}+{{P} _{2}} $

 $ P=-15-5 $

 $ P=-20 $

Now, the focal length is

  $ f=\dfrac{1}{P} $

 $ f=\dfrac{1}{-10} $

 $ f=0.02\,m $

 $ f=-20\,cm $

Hence the focal length is -$20\ cm$

There are two thin symmetrical lenses, one is converging with a refractive index  $2$ and the othe other is diverging with a refractive index $1.5$. Both lenses have same radius curvature of $10 cm$. The lenses were put together and submerged in water. What is the focal length of the system of water .The refractive index of water is $\cfrac{4}{3}$

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

  1. $40 cm$

  2. $\cfrac{40}{3} cm$

  3. $\cfrac{20}{3} cm$

  4. $-\cfrac{40}{3} cm$

Show answer
Correct Option: D

A convex lens of focal length 10 cm is in contact with a concave lens. The focal length of the combination is numerically equal to that of the concave lens. The focal length of the concave lens is :

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

  1. 10 cm

  2. 15 cm

  3. 5 cm

  4. 20 cm

Show answer
Correct Option: C

A diverging lens of focal length $-10cm$ is moving towards right with a velocity $5m/s$. An object, placed on principal axis is moving towards left with a velocity $3m/s$. The velocity of image at the instant when the lateral magnification produced is $1/2$ is: (All velocities are with respect to ground)

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

  1. $3m/s$ towards rigtht

  2. $3m/s$ towards left

  3. $7m/s$ towards rigtht

  4. $7m/s$ towards left

Show answer
Correct Option: A

Lenses applied in achromatic combination having dispersive power in ratio of $5:3$ if focal of length of the concave lens is $15cm$, then focal length of the other lens will be:

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

  1. $-9 cm$

  2. $+9 cm$

  3. $-12 cm$

  4. $+12 cm$

Show answer
Correct Option: B
Explanation:

Given,

$\omega _1:\omega _2=5:3$
$f _1=-15cm$
We know that, 
$\dfrac{\omega _1}{\omega _2}=-\dfrac{f _1}{f _2}$. . . . . . .(1)
$\dfrac{\omega _1}{\omega _2}=\dfrac{5}{3}$. . . . . .(2)
From equation (1) and equation (2), we get
$-\dfrac{f _1}{f _2}=\dfrac{5}{3}$
$f _2=-\dfrac{3f _1}{5}$

$f _2=-\dfrac{3\times (-15cm)}{5}=+9cm$
The correct option is B.

Concave and convex lenses are placed torching each other. Ratio of magnitude of their power is $2:3$. The focal length of the system is $30\ cm$. Focal lengths of individual lenses are 

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

  1. $-75, 50$

  2. $-15, 10$

  3. $75, 50$

  4. $75, -50$

Show answer
Correct Option: C

When a telescope is adjusted for normal vision, the distance of the objective from the eye-piece is found to be 80 cm. The magnifying power of the telescope is 19. What are the focal lengths of the lenses ?

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

  1. 61 cm, 19 cm

  2. 76 cm, 4 cm

  3. 40 cm, 40 cm

  4. 50 cm, 30 cm

Show answer
Correct Option: B

Two plano- concave lenses of glass of refractive index $1.5$ have radii of curvature of $20$ and $30\ cm$. They are placed in contact with curved surfaces towards each other and the space between them is filled with a liquid of refractive index $(4/3)$. Find the focal length of the system :

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

  1. divergent lens, $f = 72\ cm$

  2. convergent lens, $f = 72\ cm$

  3. divergent lens, $f = 32\ cm$

  4. convergent lens, $f = 32\ cm$

Show answer
Correct Option: D

Which values for $K, L, M$ and $N$ will make the following paragraph true?
When an object of size $7\ cm$ is placed at the distance of $K$ in front of a $L$ of focal length $M,$ the image will be produced at the distance of $N$ in front of the mirror.

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

  1. $\mathrm { K } - 27\ \mathrm { cm } ; \mathrm { L-concave \ mirror}; \mathrm { M } - 18\ \mathrm { cm } ; \mathrm { N } - 36\ \mathrm { cm }$

  2. $\mathrm { K } - 18\ \mathrm { cm } ; \mathrm { L-concave\ mirror}; \mathrm { M } - 18\ \mathrm { cm } ; N - 54\ \mathrm { cm }$

  3. $\mathrm { K } - 27\ \mathrm { cm } ; L-concave\ mirror; \mathrm { M } - 18\ \mathrm { cm } ; N - 54\ \mathrm { cm }$

  4. $None\ of\ these$

Show answer
Correct Option: C

If two lenses of power + 1.5 D and +1.0 D are placed in contact, then the effective power of combination will be :

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

  1. 4.5 D

  2. 2.5 D

  3. 5.4 D

  4. 4.2 D

Show answer
Correct Option: B
Explanation:

Given,


$P _1=+1.5 D$


$P _2=+1.0 D$

The effective power of the combination,

$P=P _1+P _2$

$P=(+1.5)+(+1.0)$

$P=+2.5D$

The correct option is B.