Tag: thin lens

Questions Related to thin lens

A convex lens of focal length 10 cm is being placed in contact with a concave lens of focal length 20 cm. 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. 20 cm

  3. 10 cm

  4. -10 cm

Show answer
Correct Option: B

A point object is placed at a distance of $15 cm$ from a convex lens. The image is formed on the other side at a distance of $30cm$ from the lens. When a concave lens is placed in contact with the convex lens, the image shifts away further by $30 cm$. Calculate the focal lengths of the concave and convex lenses.

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

  1. $10 cm, 60 cm$

  2. $ 20 cm, 30 cm$

  3. $60 cm, 10 cm$

  4. $30 cm, 20 cm$

Show answer
Correct Option: C

Two plano-convex lenses of glass of refractive index $1.5$ have radii of curvature $20\ cm$ 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 $4/3$. The focal length of the combination is

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

  1. $48\ cm$

  2. $72\ cm$

  3. $12\ cm$

  4. $28\ cm$

Show answer
Correct Option: B

An achromatic combination of a concave and convex lenses  has power 5D. If the power of convex lens is 4 D then the magnitue of focal length of concave lens is 

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

  1. 10 cm

  2. 200 cm

  3. 100 cm

  4. 20 cm

Show answer
Correct Option: B

Two lenses  of power  6 D and -2D are combined  to form a single lens.  The focal length of  this  lens will be 

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

  1. $\frac{3}{2}m$

  2. $\frac{1}{4}m$

  3. 4 m

  4. $\frac{1}{8}m$

Show answer
Correct Option: C

A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of the combination is :

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

  1. -1.5 dioptres

  2. -6.5 dioptres

  3. +6.5 dioptres

  4. +6.67 dioptres

Show answer
Correct Option: A

A convex lens is in contact with a concave lens. The magnitude of the ratio of their focal lengths is 2/3. Their equivalent focal length is 30 cm. What are their individual focal lengths?

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

  1. -15, 10

  2. -10, 15

  3. 75, 50

  4. -75, 50

Show answer
Correct Option: A
Explanation:

Let focal length of convex lens be $f$


Then focal length of the concave lens $ =  - 1.5f$


Equivalent focal length $ =  \dfrac{- 1.5 f \times f }{ (-0.5f) }= 3f$
 
=>$ f = 10 cm$

So individual focal lengths are 10 cm and -15 cm

A convex lens of focal length $f _1$ and a concave lens of focal length $f _2$ are placed in contact. The focal length of the combination is

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

  1. $(f _1 + f _2)$

  2. $(f _1 - f _2)$

  3. $\displaystyle \frac{f _1 f _2}{f _2 - f _1}$

  4. $\displaystyle \frac{f _1 f _2}{f _2 + f _1}$

Show answer
Correct Option: C
Explanation:

$\displaystyle \frac{1}{f} = \frac{1}{f _1}+ \frac{1}{- f _2} = \frac{f _2 - f _1}{f _1 f _2} ;        f = \frac{f _1f _2}{f _2 - f _1}$

A lens of power +2 diopter is placed in contact with a lens of power -1 diopter. The combination will behave like

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

  1. a convergent lens of focal length 50 cm

  2. a divergent lens of focal length 100 cm

  3. a convergent lens of focal length 100 cm

  4. a convergent lens of focal length 200 cm

Show answer
Correct Option: C
Explanation:

$P = P _1 + P _2 = + 2 - 1 = +1$ diopter, lens behaves as convergent
$F = \displaystyle \frac{1}{P} = \frac{1}{1} = 1m = 100 cm$

The radius of curvature of the convex surface of a plano-convex lens is $10 cm$. What is the focal length of the plano-convex lens? (Here $\displaystyle \mu $ = $1.5$) 

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

  1. $10 cm$

  2. $20 cm$

  3. $15 cm$

  4. $5 cm$

Show answer
Correct Option: B
Explanation:

$\mu=1.5 , R _1=10cm , R _2=0$


As focal length is given by $\dfrac{1}{f}=(\mu-1)(\dfrac{1}{R _1}-\dfrac{1}{R _2})$  (lens maker formula)

$\dfrac{1}{f}=0.5\times (\dfrac{1}{10}-0)$

$f=20cm$

hence, the focal length of the plano-convex lens is $20cm$.