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This mock test of Test: Ray Optics And Optical (CBSE Level With Solutions) for Class 12 helps you for every Class 12 entrance exam.
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QUESTION: 1

Light rays from a point object

Solution:

Light from each point on a luminous object travels outward in all directions in straight lines. Light travels at very high speeds, but is not instantaneously everywhere. Light from a point object travels indefinitely until it collides with matter in its path to be partially absorbed and reflected.

QUESTION: 2

The graph drawn with object distance along abscissa & image as ordinate for a convex lens is

Solution:

QUESTION: 3

A lens of power + 2.0 D is placed in contact with another lens of power – 1.0 D. The combination will behave like

Solution:

P=P1+P2=+2−1=+1 dioptre, lens behaves as convergent

F=1/P=1/1=1m=100cm

QUESTION: 4

A short pulse of white light is incident from air to a glass slab at normal incidence. After travelling through the slab, the first colour to emerge is

Solution:

QUESTION: 5

According to Cartesian sign convention.

Solution:

QUESTION: 6

At the minimum deviation Dm, the refracted ray inside the prism

Solution:

QUESTION: 7

An observer looks at a tree of height 15 metre with a telescope of magnifying power 10. To him, the tree appears

Solution:

Magnification of a telescope is given by:

M= θ_{i}/θ_{o}

10= (h/d_{i})/(h/d_{o})

10= d_{o}/d_{i}

Distance of image, d_{i}=d_{o}/10

QUESTION: 8

Magnifying power of a compound microscope is high if

Solution:

Magnifying power of a compound- microscope is given by m=− (lD/f_{0}f_{e})

where, l is length of tube

D is least distance of clear vision

f_{0} is focal length of objective

f_{e} is focal length of eyepiece

So, clearly, it can be seen that focal length of objective and eyepiece needs to be decreased so that magnifying power increases.

QUESTION: 9

According to Cartesian sign convention the heights measured

Solution:

QUESTION: 10

The bluish color predominates in a clear sky,

Solution:

QUESTION: 11

The magnifying power of the telescope can be increased by

Solution:

QUESTION: 12

A lamp and a screen are set up 100 cm apart and a convex lens is placed between them. The two positions of the lens forming real images on the screen are 40 cm apart. What is the focal length of the lens ?

Solution:

L_{1}L_{2}=40cm and O I=100cm

Therefore, x+40+x=100 or x=30cm

For lens at L1, we have

U=-30cm and v=+70

Thus,

(1/v)-(1/u)=1/f

Or, 1/f=(1/70) - (1/-30)

Or, f=+21cm

QUESTION: 13

The magnifying power of telescope is high if

Solution:

QUESTION: 14

Refraction is

Solution:

QUESTION: 15

Accommodation of the human eye is

Solution:

QUESTION: 16

The largest telescope in the world has a reflector with an aperture of 200 inches in order to achieve

Solution:

QUESTION: 17

A convex lens forms a real image of an object on a screen; the magnification of the image being 3/2. The object and the screen are kept fixed and the lens is moved through a distance of 16 cm when a sharp image is again formed on the screen; the magnification now being 2/3. What is the focal length of the lens?

Solution:

QUESTION: 18

A plano-convex lens, when silvered on the plane side, behaves like a concave mirror of focal length 30 cm. When it is silvered on the convex side, it behaves like a concave mirror of focal length 10 cm. The refractive index of the material of the lens is

Solution:

Given: A plano-convex lens when silvered on the plane side behaves like a concave mirror of focal length 60cm. However, when silvered on the convex side, it behaves like a concave mirror of focal length 20cm.

To find the refractive index of the lens

We know,

1/F=(2/f)+(1/f_{m})............(i), where

When plane surface is silvered it becomes concave mirror of focal length 60cm,

and f_{m}=∞

So the eqn(i) becomes,

1/60=(2/f)+(1/∞)

⟹f=120cm

When convex side is silvered it becomes concave mirror of focal length 10cm,

and f_{m}=R/2

So the eqn(i) becomes,

1/20=2/f+2/R

⟹2/R=(1/20)−(2/f)

⟹2/R=(1/20)−(2/120)

⟹2/R=(6−2/120)

⟹R=60cm

Now using the focal length formula of the plano-convex lens, we get

1/f=(μ−1)×1/R

⟹1/120=(μ−1)×1/60

⟹μ−1=(60/120)

⟹μ=(1/2)+1

⟹μ=1.5

is the refractive index of the lens.

QUESTION: 19

For a rectangular slab, refraction takes place at

Solution:

QUESTION: 20

Farsighted or hypermetropic eye can be corrected by

Solution:

QUESTION: 21

A Plano convex lens of curvature of 30 cm & R.I. 1.5, produces a real image of an object kept 90cm from it. The magnification is

Solution:

QUESTION: 22

How will the image formed by a convex lens be affected if the central portion of the lens is wrapped in a black paper

Solution:

QUESTION: 23

An object is placed at a distance of 10 cm from a co-axial combination of two lenses A and B in contact. The combination forms a real image three times the size of the object. If lens B is concave with a focal length of 30 cm, what is the nature and focal length of lens A ?

Solution:

QUESTION: 24

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

Solution:

The only difference in covering any part of a lens is that the intensity of the image will reduce. Since the focal length is a function of curvature of the surfaces of the lens and covering a part does not change it, the image will be formed as usual.

QUESTION: 25

Total internal reflection can take place only if light is travelling from

Solution:

QUESTION: 26

An equi-convex thin lens (μ =1.5) of focal length in air as 30cm is sealed into an opening in one end of tank filled with water ((μ =1.33). At the end of the tank opposite the lens is a plane mirror, 80 cm distant from the lens. The position of the image formed by the lens-water-mirror system of a small object outside the tank on the lens axis and 90cm to the left of the lens will be

Solution:

QUESTION: 27

Optical fibre communication uses the principle of

Solution:

QUESTION: 28

According to Cartesian sign convention.

Solution:

QUESTION: 29

When an astronomical telescope is focused on a distant star, the distance of the eye piece from the objective is 60 cm. When focused on a distant flag post the eye piece must be drawn at 0.1cm. If the focal length of the eye piece is 5cm; what will be the distance of the pole from the objective? Assume that the eye is focused for infinity?

Solution:

QUESTION: 30

The objective of a telescope has a focal length of 1.2 m. It is used to view a 10.0 m tall tower 2 km away. What is the height of the image of the tower formed by the objective?

Solution:

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