Terms Related to Spherical Lens

Q.

Why is the central fringe is dark?

Q. Focal length of the plano-convex lens is when its radius of curvature of the surface is
R and n is the refractive index of the lens.

Q. As the radius of curvature for a lens increases, its curvature ___________ and, the focal length ___________. ​

1. decreases, decreases
2. increases​, increases​
3. increases, decreases
4. decreases, increases​

Q. What is the focal length of double concave lens kept in air with two spherical surfaces of radii $R_1=30cm$ and $R_2=60cm$. Take refractive index of lens with respect to surrounding as n = 1.5.

Q. Focal length of a lens is half of the radius of curvature of the lens.

1. True
2. False

Q.

A point on the principle axis of a lens through which light passes undeviated is called__________.

Q. A light ray passing through a convex lens undergoes refraction only once.

1. False
2. True

Q.

Why do we use lenses of very thin aperture?

1. So that the ray passing through optical centre goes un-deviated

2. Thin lens has large focus

3. All of these

4. So that we do not waste too much glass

Q. A double convex lens made of a material of refractive index 1.5 and having a focal length of 10 cm is immersed in liquid of refractive index 3.0. The lens will behave as

1. Diverging lens of focal length 10 cm
2. Diverging lens of focal length $\frac{10}{3}cm$
3. Converging lens of focal length 30 cm
4. Converging lens of focal length $\frac{10}{3}cm$

Q. The distance between the pole and the centre of curvature of a spherical lens is 15 cm. The focal length of the lens will be

1. 30 cm
2. 15 cm
3. 7.5 cm
4. 3.75 cm

Q.

A real image is formed by a lens. What can be said about the magnification produced by the lens?

1. Magnification is positive

2. Magnification is negative

3. Magnification is zero

4. Magnification is infinite

Q. Spherical lenses have two radii of curvature as they have two refracting surfaces.

1. False
2. True

Q.

A light ray passes through the poles of all the four lenses given in the options individually. Through which lens will the deviation of the light ray from its original path be maximum?

1. 

2. 

3. 

4. 

Q.

Choose the most appropriate option.

1. 

2. 

3. 

4. 

Q. How will the magnitude of magnification change if an object kept at infinity is moved towards focus of a convex lens?

1. Decreases
2. Increases
3. Remains constant
4. First decreases, then increases

Q. There is an equiconvex glass lens with radius of each face as R and ${}_a{\mu }_g=\frac{3}{2}and{}_a{\mu }_w=\frac{4}{3}$. If there is water in object space and air in image space, then the focal length is

1. 2R
2. R
3. $\frac{3}{2}$R
4. $R^2$

Q. What is the focal length of double concave lens kept in air with two spherical surfaces of radii $R_1=30cm$ and $R_2=60cm$. Take refractive index of lens with respect to surrounding as n = 1.5.

Q.

If AB = 10 cm & DB = 40 cm (diameter). What is the radius of curvature & focal length of the spherical glass?

1. 40 cm, 10 cm

2. 20 cm, 20 cm

3. 40 cm, 5 cm

4. 20 cm, 10 cm

Q. When a real image is cast by a lens, the magnification produced by it is negative.

1. True
2. False

Q. An object is placed at $90cm$ from the lens with the radius of curvature $40cm$. The central portion of a lens is wrapped in a black paper as shown below. How will the image change after wrapping the black paper?

\( \stackrel{\uparrow}{+\infty} \)

1. Image will not be formed.

2. The intensity of the image formed will increase.

3. The image formed will be diminished.

4. The intensity of the image formed will decrease.

Q.
A person wants a ray of light to pass undeviated from a convex lens shown in figure. Where should the incident ray be directed?

1. C
2. B
3. O
4. A

Q. Trace the correct path of the light ray after passing through the concave lens.

1. 
2. 
3. 
4. 

Q.

A beam of light travelling parallel to the principal axis of a concave lens appears to diverge from a point 25 cm behind the lens after refraction. Calculate the power of the lens.

Q. According to the sign convention, which of the following is true for focal lengths of convex and concave lenses?

1. Convex: Negative, Concave: Positive
2. Convex: Positive, Concave: Negative
3. Convex: Positive, Concave: Positive
4. Convex: Negative, Concave: Negative

Q.

AB is object, A'B' is image formed. Choose the most appropriate opption.

1. 1 - A, 2 - C, 3 - A, 4 - B

2. 1 - D, 2 - C, 3 - A, 4 - B

3. 1 - A, 2 - C, 3 - B, 4 - D

4. 1 - C, 2 - A, 3 - A, 4 - B

Q.

Where will parallel rays meet/appear to meet ?

1. at infinity

2. At 2F₂

3. At F₂

4. At F₁

Q.

Why do we use lenses of very thin aperture?

1. So that we do not waste too much glass

2. So that the ray passing through optical centre goes un-deviated

3. Thin lens has large focus

4. All of these

Q. When a real image is cast by a lens, the magnification produced by it is negative.

1. True
2. False

Q.

If $F_1$ and $F_2$ represents the foci of the lens, then at which point will the light rays meet?

1. $2F_1$

2. $2F_2$

3. $F_1$

4. $F_2$

Q.

Experiments are done to test the optical properties of lenses immersed in media having different indices of refraction.

Experiment 1: A lens made of flint glass index of refraction 1.720 is tested. A beam of parallel light rays is sent into the lens, and the distance from the lens to the point of convergence of the beam is measured. This is the focal length of the lens. This focal length is measured with the lens immersed in media of various indices of refraction.

Medium	Index of refraction	Focal length (cm)
Air	1	8
Folinol	1.24	13
Water	1.33	20
11% Sugar solution	1.5	39
Carbon disulfide	1.62	95

*Rays do not converge at all.

Experiment 2: Another lens is tested. It is made of the same kind of glass as in Experiment 1. but this lens is thicker, more strongly curved.

Medium	Index of refraction	Focal length (cm)
Air	1	5
Folinol	1.24	8
Water	1.33	12
11% Sugar solution	1.5	24
Carbon disulfide	1.62	60
Methylene iodide	1.74	*

Experiment 3: A lens made of new plastic is tested. This lens is identical in size and shape to the glass in Experiment 2.

Medium	Index of refraction	Focal length (cm)
Air	1	13
Folinol	1.24	34
Water	1.33	360
11% Sugar solution	1.5	*
Carbon disulfide	1.62	*
Methylene iodide	1.74	*

In Experiment 3, why do the rays not come to a focus at all when they pass through the lens into certain materials?

1. The index of refraction of each of those materials is greater than that of the lens

2. The curvature of the lens is not great enough

3. Chemical reactions turn some of those materials opaque.

4. In some materials, the light rays are unable to bend as they pass out of the lens.