Light Test

Result

Light Test - 5

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Weekly Quiz Competition

Question 1
1 / -0

Convex lens of power 4 D and a concave lens of power 3D are placed in contact. What is the equivalent power of the combination?

A
7 D

B
D

C
1 D

D
D
Question 2
1 / -0

A convex lens of focal length \(10 {~cm}\) is placed in contact with a concave lens of focal length \(20 {~cm}\). The focal length of this combination of lenses will be:

A
\(+10 {~cm}\)

B
\(+20 {~cm}\)

C
\(-10 {~cm}\)

D
\(-20 {~cm}\)

Solution

Let the focal length of the convex lens be \(f_{1}\), and that of the concave lens be \(f_{2}\).
According to the question, we have the focal length of the convex lens equal to \(10 {~cm}\), and that of the concave lens equal to \(20 {~cm}\).
We know from the Cartesian sign convention that the focal length of a convex lens is positive while that of a concave lens is negative.
So the values of the focal length with proper sign convention are given as:
\( f_{1}=+10 {~cm}\quad\ldots(1)\)
\(f_{2}=-20 {~cm}\quad\ldots(2)\)
Now, we know that the equivalent focal length of the combination of thin lenses in contact is given by the formula,
\(\frac{1}{f}=\frac{1}{f_{1}}+\frac{1}{f_{2}}+\frac{1}{f_{3}}+\ldots \ldots+\frac{1}{f_{n}}\)
As there are two lenses of focal lengths \(f_{1}\) and \(f_{2}\), so the equivalent focal length of the given combination is given by,
\(\frac{1}{f}=\frac{1}{f_{1}}+\frac{1}{f_{2}}\)
Substituting equation (1) and (2) we have:
\(\frac{1}{f}=\frac{1}{+10}+\frac{1}{-20}\)
\(\Rightarrow \frac{1}{f}=\frac{2-1}{20}\)
\(\Rightarrow \frac{1}{f}=\frac{1}{20}\)
Taking the reciprocal we get,
\(f=+20 {~cm}\)
Thus, the focal length of the given combination of the lenses is equal to \(+20 {~cm}\).
Hence, the correct option is (B).
Question 3
1 / -0

How does the magnification (m) of the real image formed by a lens vary with the distance (X) of the object from the focus of a concave mirror?

A
m x

B
m

C
m x²

D
m
Question 4
1 / -0

A ray of light passes through a plane glass slab of thickness 't' and refractive index u = 1.5. The angle between the incident ray and emergent ray will be

A
0°

B
30°

C
45°

D
60°
Question 5
1 / -0

A convex lens forms a real image of the object when the lens is at a distance of 1 m from the object. The size of the image is 3 times that of the object. The position of the object and the screen are kept fixed but the distance of lens from the object is doubled. The image is now one half of the size of the object. What is the focal length of the lens?

A
1.6 m

B
0.80 m

C
0.40 m

D
0.20 m
Question 6
1 / -0

A liquid is placed in a hollow prism of angle 60°. If the angle of minimum deviation is 30°, what is the refractive index of the liquid?

A
1.41

B
1.50

C
1.65

D
1.95
Question 7
1 / -0

Two convex lenses separated by a distance are brought in contact. What happens to the focal length of the combination?

A
Decreases

B
Increases

C
Decreases only when d is smaller than the focal length

D
Increases only when d is smaller than the focal length
Question 8
1 / -0

What is the minimum distance between an object and its real image formed by a convex lens?

A
0

B
f

C
2f

D
4f
Question 9
1 / -0

A plane convex lens is made of material having refractive index 1.5. The radius of curvature of the curved surface is 20 cm. How far away from its optical centre will the formed image be, if the object is at infinity?

A
15 cm

B
30 cm

C
45 cm

D
None of these
Question 10
1 / -0

A convergent beam is incident on a convex lens. What will be the nature of formed image?

A
Real, erect and magnified

B
Real, inverted and magnified

C
Real, erect and diminished

D
Virtual, inverted and diminished
Question 11
1 / -0

If R₁ and R₂ are radii of curvature of a double convex lens, which of the following will have the largest power?

A
R₁ = , R₂ = 10 cm

B
R₁ = 10 cm, R₂ =

C
R₁ = R₂ = 10 cm

D
R₁ = R₂ = 15 cm
Question 12
1 / -0

A convex lens of focal length 16 cm forms a real image, which is double the size of the object. What is the distance of the object from the lens?

A
8 cm

B
16 cm

C
24 cm

D
32 cm
Question 13
1 / -0

An object is placed at a distance 24 cm from the lens and then at a distance of 16 cm from the lens. The magnification of the image formed in both the cases is the same. What is the focal length of the lens?

A
22 cm

B
20 cm

C
18 cm

D
None of these
Question 14
1 / -0

A convex lens of focal length 25 cm is placed at a distance X from a concave lens of focal length 7 cm.
When parallel beam is incident on the convex lens, it emerges as a parallel beam out of the concave lens.
What is the value of X?

A
20 cm

B
18 cm

C
16 cm

D
14 cm
Question 15
1 / -0

The greatest thickness of a plano convex glass lens appears to be 2 cm when observed normally through the plane surface, and appears 20/9 when observed through the curved face. If the real thickness is 3 cm, what is the refractive index of the lens?

A
20/9

B
2.0

C
1.5

D
1.3
Question 16
1 / -0

A lens behaves as diverging lens in air ( = 1) and converging lens in water ( = 1.3). The refractive index of the material of the lens is

A
1.0 < < 1.3

B
> 1.3

C
< 1.0

D
=