### Refraction of Light at Curved Surfaces Problems:

**Ques:** The dispersive power of the material of lens of focal length 20 *cm* is 0.08. The longitudinal chromatic aberration of the lens is

(a) 0.08 *cm*

(b) 0.08/20 *cm*

(c) 1.6 *cm*

(d) 0.16 *cm
*Ans. (c)

**Ques:** The focal lengths of convex lens for red and blue light are 100 *cm* and 96.8 *cm* respectively. The dispersive power of material of lens is

(a) 0.325

(b) 0.0325

(c) 0.98

(d) 0.968

Ans. (b)

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**Ques:** The plane faces of two identical plano-convex lenses each having focal length of 40 *cm* are pressed against each other to form a usual convex lens. The distance from this lens, at which an object must be placed to obtain a real, inverted image with magnification one is

(a) 80 *cm*

(b) 40 *cm
*(c) 20

*cm*

(d) 162

*cm*

Ans. (b)

**Ques:** A bi-convex lens made of glass (refractive index 1.5) is put in a liquid of refractive index 1.7. Its focal length will

(a) Decrease and change sign

(b) Increase and change sign

(c) Decrease and remain of the same sign

(d) Increase and remain of the same sign

Ans. (b)

**Ques:** At what distance from a convex lens of focal length 30 *cm*, an object should be placed so that the size of the image be 1/2 of the object

(a) 30 *cm*

(b) 60 *cm
*(c) 15

*cm*

(d) 90

*cm*

Ans. (d)

**Ques:** A thin plano-convex lens acts like a concave mirror of focal length 0.2 *m* when silvered from its plane surface. The refractive index of the material of the lens is 1.5. The radius of curvature of the convex surface of the lens will be

(a) 0.4 *m*

(b) 0.2 *m
*(c) 0.1

*m*

(d) 0.75

*m*

Ans. (b)

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**Ques:** The chromatic Aberration in lenses becomes due to

(a) Disimilarity of main axis of rays

(b) Disimilarity of radii of curvature

(c) Variation of focal length of lenses with wavelength

(d) None of these

Ans. (c)

**Ques:** If convex lens of focal length 80*cm* and a concave lens of focal length 50 *cm* are combined together, what will be their resulting power

(a) + 6.5* D*

(b) – 6.5 *D
*(c) + 7.5

*D*

(d) – 0.75

*D*

Ans. (d)

**Ques:** A convex lens forms a real image of an object for its two different positions on a screen. If height of the image in both the cases be 8 *cm* and 2 *cm*, then height of the object is

(a) 16 *cm*

(b) 8* cm
*(c) 4

*cm*

(d) 2

*cm*

Ans. (c)

**Ques:** In a plano-convex lens the radius of curvature of the convex lens is 10 *cm*. If the plane side is polished, then the focal length will be (Refractive index = 1.5)

(a) 10.5 *cm*

(b) 10* cm
*(c) 5.5

*cm*

(d) 5

*cm*

Ans. (b)

**Ques:** A lens of focal power 0.5 *D *is

(a) A convex lens of focal length 0.5 *m*

(b) A concave lens of focal length 0.5 *m
*(c) A convex lens of focal length 2

*m*

(d) A concave lens of focal length 2

*m*

Ans. (c)

**Ques:** Two thin lenses whose powers are +2*D* and –4*D* respectively combine, then the power of combination is

(a) – 2*D
*(b) + 2

*D*

(c) – 4

*D*

(d) + 4

*D*

Ans. (a)

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**Ques:** A convex lens

(a) Converges light rays

(b) Diverges light rays

(c) Form real images always

(d) Always forms virtual images

Ans. (a)

**Ques:** The image distance of an object placed 10 *cm* in front of a thin lens of focal length + 5 *cm* is

(a) 6.5 *cm*

(b) 8.0 *cm
*(c) 9.5

*cm*

(d) 10.0

*cm*

Ans. (d)

**Ques:** If a lens is cut into two pieces perpendicular to the principal axis and only one part is used, the intensity of the image

(a) Remains same

(b) ½ times

(c) 2 times

(d) Infinite

Ans. (a)

**Ques:** A convex lens of focal length 0.5 *m* and concave lens of focal length 1 *m* are combined. The power of the resulting lens will be

(a) 1 *D
*(b) – 1

*D*

(c) 0.5

*D*

(d) – 0.5

*D*

Ans. (a)

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**Ques:** Focal length of a convex lens of refractive index 1.5 is 2 *cm*. Focal length of lens when immersed in a liquid of refractive index of 1.25 will be

(a) 10 *cm
*(b) 2.5

*cm*

(c) 5

*cm*

(d) 7.5

*cm*

Ans. (c)

**Ques:** A plano convex lens is made of glass of refractive index 1.5. The radius of curvature of its convex surface is *R*. Its focal length is

(a) *R*/ 2

(b) *R
*(c) 2

*R*

(d) 1.5

*R*

Ans. (c)

**Ques:** An equiconvex lens of glass of focal length 0.1 metre is cut along a plane perpendicular to principle axis into two equal parts. The ratio of focal length of new lenses formed is

(a) 1 : 1

(b) 1 : 2

(c) 2 : 1

(d) 2 : ½

Ans. (a)

**Ques:** A convex lens forms a real image of a point object placed on its principal axis. If the upper half of the lens is painted black, the image will

(a) Be shifted downwards

(b) Be shifted upwards

(c) Not be shifted

(d) Shift on the principal axis

Ans. (c)

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**Ques:** Focal length of a convex lens will be maximum for

(a) Blue light

(b) Yellow light

(c) Green light

(d) Red light

Ans. (d)

**Ques:** An achromatic combination of lenses is formed by joining

(a) 2 convex lenses

(b) 2 concave lenses

(c) 1 convex lens and 1 concave lens

(d) Convex lens and plane mirror

Ans. (c)

**Ques:** Two thin lenses, one of focal length + 60 *cm* and the other of focal length – 20 *cm* are put in contact. The combined focal length is

(a) + 15 *cm*

(b) – 15* cm
*(c) + 30

*cm*

(d) –30

*cm*

Ans. (d)

**Ques:** A convex lens of focal length 84 *cm* is in contact with a concave lens of focal length 12 *cm*. The power of combination (in diopters) is

(a) 25/24

(b) 25/18

(c) – 50/7

(d) + 50/7

Ans. (c)

**Ques:** A thin convex lens of refractive index 1.5 has a focal length of 15 *cm* in air. When the lens is placed in liquid of refractive index 4/3 , its focal length will be

(a) 15 *cm*

(b) 10 *cm
*(c) 30

*cm*

(d) 60

*cm*

Ans. (d)

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**Ques:** Two lenses of power +12 and – 2 diopters are placed in contact. What will the focal length of combination

(a) 10 *cm*

(b) 12.5 *cm
*(c) 16.6

*cm*

(d) 8.33

*cm*

Ans. (a)

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