Optics(12th Grade > Physics ) Questions and answers for exam Preparation

12th Grade > Physics

OPTICS MCQs

Wave Optics, Ray Optics Refraction, Ray Optics Fundamentals, Ray Optics Curved Surface Refraction, Ray Optics Curved Mirrors

Total Questions : 111 | Page 9 of 12 pages

Question 81. A convex lens of crown glass ( =1.525) will behave as a divergent lens if immersed in

Water (n =1.33)
In a medium of n = 1.525
Carbon disulphide n =1.66
It cannot act as a divergent lens

Discuss Question

Answer: Option C. -> Carbon disulphide n =1.66
:
C
A lens shows opposite behaviour if

μ_{m e d i u m} > μ_{l e n s}

.e., a convergent lens becomes divergent in nature

Question 82. A converging lens is used to form an image on a screen. When upper half of the lens is covered by an opaque screen

Half the image will disappear
Complete image will be formed of same intensity
Half image will be formed of same intensity
Complete image will be formed of decreased intensity

Discuss Question

Answer: Option D. -> Complete image will be formed of decreased intensity
:
D
To form the complete image only two rays are to be passed through the lens. Moreoverthe total amount of light released by the object is not passing through the lens because of the covering. Therefore the intensity of the image will be reduced and theimage formed will befaint, not sharp.

Question 83. A transparent cylinder has its right half polished so as to act as a mirror. A paraxial light ray is incident from left, that is parallel to principal axis, exits parallel to the incident ray as shown. The refractive index of the material of the cylinder is:

Discuss Question

Answer: Option D. -> 2.0
:
D
For spherical surface using

\frac{n_{2}}{v} - \frac{n_{1}}{u} = \frac{n_{2} - n_{1}}{R}

\frac{n}{2 R} - \frac{1}{\infty} = \frac{n - 1}{R}

;

n = 2 n - 2 \Rightarrow n = 2

Question 84. A hollow double concave lens is made of very thin transparent material. It can be filled with air or either of two liquids

L_{1}

and

L_{2}

having refractive indices

n_{1}

and

n_{2}

respectively

(n_{2} > n_{1} > 1)

. The lens will diverge a parallel beam of light if it is filled with

Air and placed in air
Air and immersed in Li
L1 and immersed in L2
L2 and immersed in L1

Discuss Question

Answer: Option D. -> L2 and immersed in L1
:
D

\frac{1}{f} = (\frac{n_{2}}{n_{1}} - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})

where

n_{2}

and

n_{1}

are the refractive indices of the material of the lens and of the surroundings respectively. For a double concave lens,

(\frac{1}{R_{1}} - \frac{1}{R_{2}})

is always negative.

A Hollow Double Concave Lens Is Made Of Very Thin Transparen...

Hence fis negative only when

n_{2} > n_{1}

Question 85. A glass hemisphere of radius 0.04 m and R.I. of the material 1.6 is placed centrally over a cross mark on a paper (i) with the flat face; (ii) with the curved face in contact with the paper. In each case the cross mark is viewed directly from above. The position of the images will be
[ISM Dhanbad 1994]

(i) 0.04 m from the flat face; (ii) 0.025 m from the flat face
(i) At the same position of the cross mark; (ii) 0.025 m below the flat face
(i) 0.025 m from the flat face; (ii) 0.04 m from the flat face
For both (i) and (ii) 0.025 m from the highest point of the hemisphere

Discuss Question

Answer: Option B. -> (i) At the same position of the cross mark; (ii) 0.025 m below the flat face
:
B

A Glass Hemisphere Of Radius 0.04 M And R.I. Of The Material...

Question 86. Two identical glass

(μ_{g} = \frac{3}{2})

equiconvex lenses of focal length f are kept in contact. The space between the two lenses is filled with water

(μ_{w} = \frac{4}{3})

. The focal length of the combination is

Discuss Question

Answer: Option D. -> 3f4
:
D
Let R be the radius of curvature of each surface. Then

\frac{1}{f} = (1.5 - 1) (\frac{1}{R} + \frac{1}{R})

∴ R = f

For the water lens

\frac{1}{f} = (\frac{4}{3} - 1) (- \frac{1}{R} - \frac{1}{R}) = \frac{1}{3} (- \frac{2}{R})

\frac{1}{f} = - \frac{2}{3 R}

Now using

\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} + \frac{1}{f_{3}}

we have

\frac{1}{F} = \frac{1}{f} + \frac{1}{f} + \frac{1}{f}

= \frac{2}{f} - \frac{2}{3 f} = \frac{4}{3 f}

∴ F = \frac{3 f}{4}

Question 87. Shown in the figure here is a convergent lens placed inside a cell filled with a liquid. The lens has focal length + 20 cm when in air and its material has refractive index 1.50. If the liquid has refractive index 1.60, the focal length of the system is

+ 80 cm
- 80 cm
-24 cm
-100 cm

Discuss Question

Answer: Option D. -> -100 cm
:
D

\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} + \frac{1}{f_{3}}

\frac{1}{f_{1}} = (1.6 - 1) (\frac{1}{\infty} - \frac{1}{20}) = \frac{0.6}{20} = - \frac{3}{100} . . . (i) \frac{1}{f_{2}} = (1.5 - 1) (\frac{1}{20} - \frac{1}{- 20}) = \frac{1}{20} . . . (i i) \frac{1}{f_{3}} = (1.6 - 1) (\frac{1}{- 20} - \frac{1}{\infty}) = - \frac{3}{100} . . . . (i i i) \Rightarrow \frac{1}{F} = - \frac{3}{100} + \frac{1}{20} - \frac{3}{100} \Rightarrow F = - 100 c m

Question 88. A ray of light falls on the surface of a spherical glass paper weight making an angle

α

with the normal and is refracted in the medium at an angle

β

. The angle of deviation of the emergent ray from the direction of the incident ray is

(α−β)
2(α−β)
(α−β)2
(α+β)

Discuss Question

Answer: Option B. -> 2(α−β)
:
B
From figure it is clear that

Δ O B C

is an isosceles triangle ,
Hence

∠ O C B = β

and emergent angle is

α

Also sum of two in terior angles = exterior angle

∴ δ = (α - β) + (α - β) = 2 (α - β)

A Ray Of Light Falls On The Surface Of A Spherical Glass Pap...

Question 89. Refraction takes place at a convex spherical boundary separating air-glass medium. For the image to be real, the object distance

(μ_{g} = \frac{3}{2})

Should be greater than two times the radius of curvature of the refracting surface
Should be greater than six times the radius of curvature of the refracting surface
Should be greater than the radius of curvature of the refracting surface
Is independent of the radius of curvature of the refracting surface

Discuss Question

Answer: Option A. -> Should be greater than two times the radius of curvature of the refracting surface
:
A
Applying

\frac{μ_{2}}{v} - \frac{μ_{1}}{u} = \frac{μ_{2} - μ_{1}}{R}

\frac{1.5}{v} - \frac{1}{- u} = \frac{(1.5) - 1}{R}

Dividing with

1.5

on both the sides.
or

\frac{1}{v} + \frac{2}{3 u} = \frac{1}{3 R}

For v to be positive

\frac{1}{3 R} > \frac{2}{3 u}

u > 2 R

Question 90. Rays parallel to the principal axis after striking the reflecting side of a convex mirror

intersect in front of the reflecting surface
intersect behind the reflecting surface
appear to intersect in front of the reflecting surface
appear to intersect behind the reflecting surface.

Discuss Question

Answer: Option D. -> appear to intersect behind the reflecting surface.
:
D
Remember, as a rule of thumb, that if parallel rays after falling on any kind of reflecting surface get diverged, they can never actually intersect! And this is what happens in a convex mirror, it is a diverging mirror. So, parallel rays will never meet after getting reflected, but, when extended backwards they will appear to meet behind the mirror, you shall see in later modules that this how they from virtual image.