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

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

  1. $+ 6.67 D$
  2. $- 6.5 D$
  3. $- 1.5 D$
  4. $+ 6.5 D$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Given,


$f _1=+40cm$


$f _2=-25cm$

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

The power of combination,

$P=\dfrac{1}{f _1}+\dfrac{1}{f _2}$

$P=\dfrac{100}{+40cm}+\dfrac{100}{-25cm}$

$P=-1.5D$

The correct option is C.

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

If the magnitude of dispersive power of two lenses are $0.024$ and $0.036$. There focal length will be for abberation free combination.

  1. $30\ cm,\ -40\ cm$
  2. $30\ cm,\ -45\ cm$
  3. $10\ cm,\ 30\ cm$
  4. $20\ cm,\ -35\ cm$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

For an aberration-free combination of two thin lenses, the condition is w1/f1 + w2/f2 = 0, where w is the dispersive power and f is the focal length. Given w1=0.024 and w2=0.036, the ratio f1/f2 = -w1/w2 = -0.024/0.036 = -2/3. Option B provides f1=30 cm and f2=-45 cm, which satisfies 30/-45 = -2/3.

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

The refractive index of the material of a double convex lens is $1.5$ and its focal lengths in $5cm$. If the radii of curvature are equal, the value of the radius of curvature is

  1. 5.0

  2. 6.5

  3. 8.0

  4. 9.5
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Using the lens maker formula 1/f = (n-1)(1/R1 - 1/R2). For a double convex lens, R1=R and R2=-R. So 1/5 = (1.5-1)(1/R + 1/R) = 0.5 * (2/R) = 1/R. Thus R = 5 cm.

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

A lens made from a material of absolute refractive index $\mathrm { n } _ { 1 }$  and it is placed in a medium of absolute refractive index $\mathrm { n } _ { 2 }$   The focal length of the lens is related to $\mathrm { n } _ { 1 } \text { and } \mathrm { n } _ { 2 }$ as:

  1. $f \alpha \left( n _ { 1 } - n _ { 2 } \right)$
  2. $f \alpha \frac { 1 } { \left( n _ { 1 } - n _ { 2 } \right) }$
  3. $f \alpha \left( n _ { 1 } + n _ { 2 } \right)$
  4. $f \alpha \frac { 1 } { \left( n _ { 1 } + n _ { 2 } \right) }$
Reveal answer Fill a bubble to check yourself
A Correct answer
Multiple choice physics ray optics and optical instruments power of the lens thin lens combination of lenses

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

  1. $48\ cm$
  2. $72\ cm$
  3. $12\ cm$
  4. $28\ cm$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The system consists of two plano-convex lenses and a liquid lens. P_total = P1 + P2 + P_liquid. P1 = (1.5-1)/0.2 = 2.5D. P2 = (1.5-1)/0.3 = 1.67D. P_liquid = -2*(n_liq-1)/R_avg? No, the liquid lens is biconcave with radii 20 and 30. P_liq = (4/3 - 1) * (-1/20 - 1/30) = (1/3) * (-5/60) = -5/180 = -1/36. P_total = 1/40 + 1/60 - 1/36 = (9+6-10)/360 = 5/360 = 1/72. So f = 72 cm.

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

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?

  1. -15, 10

  2. -10, 15

  3. 75, 50

  4. -75, 50
Reveal answer Fill a bubble to check yourself
A Correct answer
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