Physics

Optics and Lenses

206 Questions

Enhance your physics knowledge by practicing questions on optics and the functioning of lenses. The set covers calculating combined focal lengths, lens power, and correcting vision defects like myopia and hypermetropia. These physics fundamentals are crucial for medical entrance and state board exams.

Combined focal lengthCorrecting vision defectsLens power calculationConvex and concave lensesTelescopesHuygens principle

Optics and Lenses Questions

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 structure of human eye human eye and colourful world

State whether true or false:

A concave lens is used to correct short-sightedness in eye.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Answer is A.

A person who is short sighted can focus clearly on near objects but cannot focus on distant objects. This is because the eyeball is too long. Light from distant objects is focused at a point in front of the retina resulting in a blurred image.
This defect can be corrected by wearing a concave (diverging) spectacle lens. The rays of light from a near object are diverged before entering the eye so that the cornea and eye lens can direct the focal point onto the retina.
Hence, the statement is true.

Multiple choice structure of human eye human eye and colourful world

The hyperopic eye can be corrected using --- lens.

  1. concave

  2. convex

  3. plano convex

  4. plano concave
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Hyperopia or Far-sightedness is the condition where person can see the far objects clearly but cannot focus on near objects. This occurs since the image of nearby objects is formed behind retina. This corrected by using a convex lens which increases the power of the eye, and image is obtained on the retina.

Multiple choice structure of human eye human eye and colourful world

Which type of lens is used to treat myopia:

  1. Concave lens

  2. Convex lens

  3. Cylindrical lens

  4. Bifocal lens
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Near sightedness, or myopia, is the most common refractive error of the eye. Myopia occurs when the eyeball is too long, relative to the focusing power of the cornea and lens of the eye. This causes light rays to focus at a point in front of the retina, rather than directly on its surface. So, to treat myopia the glasses will be preceded by a minus sign ().
Hence, to treat myopia concave lens should be used.

Multiple choice structure of human eye human eye and colourful world

A man cannot see clearly the objects beyond a distance of 20 cm from his eyes. To see distant objects clearly the kind of lenses and its focal length must be 

  1. 100 cm convex

  2. 100 cm concave

  3. 20 cm convex

  4. 20 cm concave
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$u= +20cm$

$v= \infty $

$\dfrac{1}{v}-\dfrac{1}{u}= \dfrac{1}{f}$

$\dfrac{1}{\infty }-\dfrac{1}{+20}= \dfrac{1}{f}$

$f=-20\ cm$

Multiple choice structure of human eye human eye and colourful world

A long sighted person has a least distance of distinct vision of 50 cm. He wants to reduce it to 25 cm. He should use a :

  1. Concave lens of focal length 50 cm

  2. Convex of focal length 25 cm

  3. Convex lens of focal length 50 cm

  4. Concave lens of focal length 25 cm
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Given, $u= \infty $


Image distance, $v= 50cm$

From lens formula, $\dfrac{1}{v}-\dfrac{1}{u}= \dfrac{1}{f}$

$\dfrac{1}{50}-\dfrac{1}{\infty }= \dfrac{1}{f}$

$f= +50\ cm$

He should use a convex lens of focal length 50 cm.

Multiple choice structure of human eye human eye and colourful world

A person suffering from myopia cannot see beyond 1m. What should be the focal length of the concave lens that will correct his vision?

  1. -1 m

  2. -2 m

  3. 0.5 m

  4. 10 m
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The focal length of the concave lens should be -1 m as object at infinity forms image at focus.

Multiple choice structure of human eye human eye and colourful world

Myopia can be corrected by using spectacle having ________.

  1. Concave lens

  2. Convex lens

  3. Biconvex lens

  4. Biconcave lens
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Myopia (nearsightedness) occurs when light focuses in front of the retina. A concave lens is used to diverge the light before it enters the eye, pushing the focal point back onto the retina.

Multiple choice structure of human eye human eye and colourful world

A person can see clearly between 1 m and 2 m his corrective lenses should be 

  1. Bifocals with power -0.5 D and additional +3.5 D

  2. bifocals with power -1.0 D and additional +3 D

  3. Concave with power 1.0 D

  4. convex with power 0.5 D
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

To see at 1 m (near point correction), the lens needs P = 1/0.25 - 1/1 = 3D. To see at infinity (far point correction), the lens needs P = 1/infinity - 1/2 = -0.5D. Bifocals are needed.

Multiple choice structure of human eye human eye and colourful world

Which of the following lenses of appropriate focal length is used to correct shortsightedness ?

  1. Concave

  2. Convex

  3. Both A & B

  4. None of the above
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A convex lens of appropriate focal length is used to correct farsightedness.
Farsightedness is the inability to see nearby objects clearly due to the formation of image by convergence of rays at a point behind the retina. A convex or converging lens with the correct focal length allows the rays to converge at a point on the retina. The convex lens first converge the rays before it reaches the eye and thus, reducing the image distance. The eye-lens will now be able to converge the refracted rays ton the retina.