# Power and Thin Lenses in Contact

## Trending Questions

**Q.**Match the corresponding entries of column 1 with column 2 where m is the magnification produced by the mirror.

Column 1 Column 2(A) m=−2(p) Convex mirror(B) m=−12(q) Concave mirror(C) m=+2(r) Real image(D) m=+12(s) Virtual image

- A→r, s; B→q, s; C→q, r; D→p, s
- A→q, r; B→q, r; C→q, s; D→p, s
- A→p, r; B→p, s; C→p, q; D→r, s
- A→p, s; B→q, r; C→q, s; D→q, r

**Q.**Two identical glass (μg=3/2) equiconvex lenses of focal length f are kept in contact. The space between the two lenses is filled with water (μw=4/3). The focal length of the combination is

- f
- f2
- 4f3
- 3f4

**Q.**A car is fitted with a convex side view mirror of focal length 20 cm. A second car 2.8 m behind the first car is overtaking the first car at a relative speed of 15 m/s. The speed of the image of the second car as seen in the mirror of the first car will be :

- 115 m/s
- 120 m/s
- 125 m/s
- 130 m/s

**Q.**

The minimum distance between an object and its real image formed by a convex lens is

**Q.**

SI unit of power of lens is

**Q.**

Is the power of a concave mirror positive or negative?

**Q.**A point object at 15 cm from a concave mirror of radius of curvature 20 cm is made to oscillate along the principal axis with amplitude 2 mm. The amplitude of its image will be

- 2 mm
- 4 mm
- 8 mm
- 16 mm

**Q.**In the figure shown , the radius of curvature of the left & right surfaces of the concave lens are 10 cm & 15 cm respectively. The radius of curvature of the mirror is 15 cm. Equivalent focal length of the combination is:

- The system behaves like a convex mirror of focal length 18 cm
- The system behaves like a concave mirror of focal length 18 cm
- The system behaves like a convex mirror of focal length 36 cm
- The system behaves like a concave mirror of focal length 36 cm

**Q.**A person can see clearly only up to a distance of 30 cm. He wants to read a book placed at a distane of 50 cm from his eyes. What is the power of the lens he requires for his spectacles?

- −1.67 D
- −2.0 D
- −1.33 D
- −1.0 D

**Q.**

A person cannot see the object clearly placed at a distance more than $40cm$. He is advised to use the lens of power.

$-2.5D$

$+2.5D$

$-1.5D$

$+1.5D$

**Q.**A biconvex lens is formed with two thin plano-convex lenses as shown in figure. Refractive index of left lens is 1.5 and that of right lens is 1.2. Both the curved surfaces are of same radius of curvature R=14 cm. For this bi-convex lens, for an object distance of 40 cm, the image distance (in cm) will be –

**Q.**

The radii of curvature of the surfaces of a double convex lens are $20\mathrm{cm}$ and $40\mathrm{cm}$ respectively, and its focal length is $20\text{cm}$. What is the refractive index of the material of the lens?

**Q.**

In glass which colour has greater refractive index, violet or red?

**Q.**A glass hemisphere of radius R and material having refractive index 1.5 is silvered on its flat face as shown in the figure. A small object of height h is located at a distance 2R from the surface of the hemisphere. The final image will be

- on the hemispherical surface.
- inside the hemisphere.
- at a distance of 2R from the silvered surface on the left side.
- at a distance of 3R from the silvered surface on the left side.

**Q.**

What do you mean by power of a lens?

**Q.**A convex lens is placed in contact with a mirror as shown. If the space between them is filled with water, its power will

- decrease
- increase
- remain unchanged
- increase or decrease depending on the focal length.

**Q.**

A double convex thin lens made of glass (refractive index 1.5) has both radii of curvature of magnitude 20 cm. Incident light rays parallel to the axis of the lens will converge at distance L such that

- L = 20 cm
- L = 10 cm
- L = 40 cm
- L = 20/3 cm

**Q.**A lens of power + 2 diopters is placed in contact with a lens of power – 1 diopter. The combination will behave like

- A divergent lens of focal length 100 cm
- A convergent lens of focal length 100 cm
- A convergent lens of focal length 50 cm
- A convergent lens of focal length 200 cm

**Q.**What is the focal length of a convex lens of focal length 30cm incontact with a concave lens of focal length 20cm? Is the system aconverging or a diverging lens? Ignore thickness of the lenses.

**Q.**A thin equiconvex lens (μ=32) of focal length 10cm is cut and separated and a material of refractive index 3 is filled between them. What is the focal length of the combination?

- −10 cm
- −103 cm
- −104 cm
- None of these

**Q.**

List four precautions that a student should observe while determining the focal length of a given convex lens by obtaining an image of a distant object on a screen.

**Q.**A convex lens of focal length F1 forms a real and inverted image of an object. If the size of the image is the same as that of the object, the object is placed

- at 2F1
- between F1 and 2F1
- between optical centre and F1
- between −∞ and 2F1

**Q.**In a medium of refractive index 1.6 and having a convex surface, a point object is placed at a distance of 12 cm from the pole. The radius of curvature is 6 cm. Locate the image as seen from air.

- A real image at 30 cm
- A virtual image at 30 cm
- A real image at 4.28 cm
- A virtual image at 4.28 cm

**Q.**

If a fish $40cm$ below the surface of a pond, how deep does it appear to an observer directly above? The refractive index of water is $1.33$.

**Q.**For a normal eye, the far point is at infinity and the near point ofdistinct vision is about 25cm in front of the eye. The cornea of theeye provides a converging power of about 40 dioptres, and the leastconverging power of the eye-lens behind the cornea is about 20 dioptres. From this rough data estimate the range of accommodation(i.e., the range of converging power of the eye-lens) of a normal eye.

**Q.**

If R_{1} and R_{2} are the radii of curvature of a double convex lens, which of the following will have the largest power?

**Q.**

In Youngs double-slit experiment, the slit width and the distance of slits from the screen both are doubled. The fringe width

increases

decreases

remains unchanged

none of the above

**Q.**Two thin lenses when in contact have power of 10D . When they are .25m apart the power reduce to 6D The focal lens of lens 1 And lens 2 is

**Q.**A converging and a diverging lens of focal lengths are placed coaxially in contact .find the power and focal length of combination

**Q.**

Define the following terms: (i) Power of a lens (ii) Principal focus of a concave mirror