Focal Length Depends on Surrounding Medium

Q. A convex lens is made-up of two different material. How many image will it form when object is kept in front of it?

Q. While calculating the focal length of a lens formula; what is different when the lens is immersed in or surrounded by some other medium than that of air?

1. ${\mu }_1$
2. ${\mu }_2$
3. $R_1\&R_2$
4. $Noneofthese$

Q. A concave lens and a convex lens of same focal length 10 cm are separated by 24 cm. A point size object is placed in front of concave lens at 15 cm. Distance between object and final image is

समान फोकस दूरी 10 cm के एक अवतल लेन्स तथा एक उत्तल लेंस के मध्य दूरी 24 cm है। बिंदु के आकार की एक वस्तु को अवतल लेंस के सामने 15 cm की दूरी पर रखा जाता है। वस्तु तथा अंतिम प्रतिबिम्ब के मध्य दूरी है

1. 45 cm
2. 50 cm
3. 35 cm
4. 54 cm

Q. A convex lens is made-up of two different material. How many image will it form when object is kept in front of it?

Q. The focal length of a lens is $10cm$ when kept in air. Now if we keep the lens into the water, the focal length ___________.

1. will be equal to 10 cm.
2. will be greater than 10 cm
3. cannot be predicted
4. will be lesser than 10 cm

Q. The focal length of a biconvex lens is $f$. If we cut the lens into two equal pieces as shown in the figure, the focal length of each piece will be ___.

1. $\frac{f}{2}$
2. $f$
3. $2f$
4. $0$

Q. The refractive index of diamond is 2. The velocity of light (in cm/sec) in diamond will be:

1. $2\times {10}^{13}$
2. $6\times {10}^{10}$
3. $1.5\times {10}^{10}$
4. $3\times {10}^{10}$

Q. Refractive index of diamond for light of wavelength 589 nm is 2.419. What is the velocity of light in diamond. Velocity of light in vacuum is $3\times {10}^8$ m/s.

Q. A biconcave symmetric lens is placed inside a medium whose refractive index is $n_{medium}$ with respect to the air. The lens acts as a converging lens (convex lens) in this medium. So what can you say about the refractive index of the lens ($n_{lens}$) with respect to the air?

1. $n_{lens}$ > $n_{medium}$
2. $n_{lens}$ < $n_{medium}$
3. $n_{lens}$ = $n_{medium}$
4. cannot be predicted

Q.

The centre of curvature of a convex lens is at $40cm$ and its central portion is wrapped in black paper as shown below. If an object is placed at $90cm$, find the nature of image formed.

1. The intensity of the image formed will increase.

2. The Image formed will be diminished.

3. The intensity of the image formed will decrease.

4. Image will not be formed.

Q. An equiconvex lens of refractive index $\frac{3}{2}$ and focal length 10 cm is held with its axis vertical and its lower surface immersed in water($\mu$=$\frac{4}{3}$, the upper surface being in air.At wat distance from the lens , will a vertical beam of parallet light incident on the lens be focused?

1. 20 cm
2. 30 cm
3. 10 cm
4. 5 cm

Q. The distance between optical centre and is known as focal length of the lens.

1. principal focus
2. centre of curvature
3. aperture

Q. Complete the following statement. The index of refraction compares the speed of light in a certain medium with the speed of light in:

1. vacuum
2. air
3. a prism
4. glass
5. water

Q. A biconcave symmetric lens is placed inside a medium whose refractive index is $n_{medium}$ with respect to the air. The lens acts as a converging lens (convex lens) in this medium. So what can you say about the refractive index of the lens ($n_{lens}$) with respect to the air?

1. $n_{lens}$ > $n_{medium}$
2. $n_{lens}$ < $n_{medium}$
3. $n_{lens}$ = $n_{medium}$
4. cannot be predicted

Q. A double convex lens, lens made of a material of refractive index ${\mu }_1$ is placed inside two liquids or refractive indices ${\mu }_2$ and ${\mu }_3$, as shown ${\mu }_2>{\mu }_1>{\mu }_3$. A wide, parallel beam of light is incident on the lens from the left.

1. A convergent and a divergent beam
2. Two different divergent beams
3. A single convergent beam
4. Two different convergent beams

Q. An object is placed at a distance of $30cm$ from the lens. Its image is formed at a distance of $10cm$ on the same side of the lens. Find the focal length of the lens used.

1. $f=10cm$
2. $f=-10cm$
3. $f=+15cm$
4. $f=-15cm$

Q.

If there is a convex lens which is made of material whose optical density is less than that of air, then it would be a converging lens.

1. True

2. False

Q. A biconcave symmetric lens is placed inside a medium whose refractive index is $n_{medium}$ with respect to the air. The lens acts as a converging lens (convex lens) in this medium. So what can you say about the refractive index of the lens ($n_{lens}$) with respect to the air?

1. $n_{lens}$ < $n_{medium}$
2. $n_{lens}$ = $n_{medium}$
3. cannot be predicted
4. $n_{lens}$ > $n_{medium}$

Q.

In the given figure, where do the diverging rays seem to meet?
($F_1$ , $F_2$ are the primary focus of the lens)

1. $2F_1$

2. $F_2$

3. $F_1$

4. $2F_2$