Physics

Optics and Lenses

251 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

A biconvex lens of focal length $f$ is cut into two plano convex lenses. If these two plano convex lenses are joined such that their convex surfaces are in contact, then the focal length of the combination is :

  1. $\dfrac{f}{2}$
  2. $f/4$
  3. $2 f$
  4. $\text{infinite}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$f _{1}=f/2  $       $f _{2}=f/2$

$\dfrac{1}{f^{'}}=\dfrac{1}{f _{1}}+\dfrac{1}{f _{2}}$


$\dfrac{1}{f^{'}}=\dfrac{2}{f}+\dfrac{2}{f}$

$f'  = f/4$

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 luminous intensity measurements physics

The luminous efficiency of a lamp is $8.8$ lumen/watt and its luminous intensity is $700\ Cd$. The power of the lamp will be 

  1. $10^{1}$ $W$
  2. $10^{2}$ $W$
  3. $10^{3}$ $W$
  4. $10^{4}$ $W$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
$Luminous \ flux = Luminous \ intensity * solid \ angle $

Total solid angle = 4 $\pi$

So, Luminous flux = 700 x 4 $\pi$

Luminous flux = Luminous efficiency x power
=> 700 x 4 $\pi$ = 8.8 x power
=> Power of lamp = 1000 W

Answer. C) 10$^3$ W
Multiple choice luminous intensity measurements physics

The luminous efficiency of a lamp is $5$ lm $W^{-1}$ and its luminous intensity is $30$ candela. The power of the lamp will be 

  1. $6\pi$ $W$
  2. $12\pi$ $W$
  3. $24\pi$ $W$
  4. $48\pi$ $W$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$Luminous \ flux = luminous \ intensity * 4 \pi$

$\longrightarrow$ F = $30 * 4 \pi   lm$

Luminous efficiency = $ 5  lm / W$

Power = $ F / Power$
            = $ 30 * 4 \pi / 5 = 24 \pi$

Answer. C) $24 \pi  W$

Multiple choice luminous intensity measurements physics

A lamp of $250$ candle power is hanging at a distance of $6$m from a wall. The illuminace at a point on the wall at a minimum distance from the lamp will be 

  1. $9.64$ lux
  2. $4.69$ lux
  3. $6.94$ lux

  4. none of these
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

1 Candle power = 0.981 Candela

Luminous intensity = 250*0.981 Candela

Minimum distance of lamp from wall = 6 m

Illuminance = $L/d^{2}$
= $250*0.981/6^{2}$
= 6.8125 lux

Answer. D) none of these

Multiple choice luminous intensity measurements physics

The lumen efficiency, if an electric bulb emit $68.5\dfrac{lumen}{watt}$ is:

  1. $2.5\%$
  2. $5\%$
  3. $10\%$
  4. $20\%$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Luminous efficiency is the ratio of luminous flux to power. A standard 100% efficient source (monochromatic green light at 555 nm) produces 685 lumens/watt. Efficiency = (68.5 / 685) * 100% = 10%.

Multiple choice luminous intensity measurements physics

The luminous efficiency of the bulb in lumen/watt, if luminous intensity of a $100$ watt unidirectional bulb is $100$ candela, is

  1. $12$
  2. $12.56$
  3. $13$
  4. $15$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$F=4\pi l$
$F=(4\times 3.14)\times 100=1256$ lumen
$\therefore$ Luminous efficiency $=\dfrac{luminous\ flux}{electric\ power}=\dfrac{1256}{100}=12.56$

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.