Refraction Through Lens

Question 1

define lens

Answer 1

a lens is a transparent refracting medium bounded by either two spherical surfaces, or one spherical surface and the other plane

Question 2

convex/converging lens

Answer 2

thick in the middle and thin at periphery

Question 3

concave/diverging lens

Answer 3

thick at periphery, and thin at the middle

Question 4

define centre of curvature

Answer 4

the centre of the sphere, whose part is the lens surface, is called the centre of curvature of that surface of the lens

Question 5

define radius of curvature

Answer 5

the radius of sphere, whose part is the lens surface, is called the radius of curvature of the lens

Question 6

define principal axis

Answer 6

line joining the centres of curvatures of the two surfaces of the lens

Question 7

define optical centre of thick lens

Answer 7

the point which lies on the principal axis through which the rays of light pass without any deflection.

Question 8

define optical centre of thin lens

Question 9

first focal point of convex lens

Answer 9

a point F1 on the principal axis of the lens such that the rays of light coming from it, after refraction through lens lens, becomes parallel to principal axis

Question 10

first focal point of concave lens

Answer 10

point on the principal axis of the lens such that the incident rays of light appearing to meet at it, after refraction from the lens, becomes parallel to principal axis

Question 11

second focal point of convex lens

Answer 11

point on the principal axis of the lens such that the rays of light incident parallel to principal axis, after refraction from lens, pass through it

Question 12

second focal point of concave lens

Answer 12

a point on principal axis such that rays of light incident parallel to the principal axis, after refraction from the lens, appear to be diverging from this point

Question 13

focal plane

Answer 13

a plane normal to principal axis, passing through F1 and F2

Question 14

define focal length

Answer 14

distance between focus and optical centre

Question 15

focal length of a lens depends on what

Answer 15

1. refractive index of material of lens relative to its surrounding medium
2. radii of curvature of two surfaces of the lens (thick lens has less focal length)

Question 16

principal rays for ray diagrams

Answer 16

1. ray of light incident at optical centre O of the lens passes un-deviated through lens
2. a ray of light incident parallel to principal axis of lens, after refraction, passes through second focus/ appears to come from F2
3. A ray of light passing through the first focus / directed towards F1, emerges parallel to principal axis after refraction

Question 17

why can the eye see images formed by concave lens in convex

Answer 17

the eye, being convex, when kept in between diverging rays, can see the image because the rays are converged to form an image on the retina of eye

Question 18

when is image the same size as object in convex

Answer 18

when object is kept at (2)F1

Question 19

define power of a lens

Answer 19

the deviation of the incident light rays produced by a lens on refraction through it, is a measure of its power

Question 20

dependence of focal length on power and deviation

Answer 20

a thick lens has short focal length, and deviates rays more
a thin lens has long focal length, and deviates rays less

Question 21

formula for power of lens

Answer 21

reciprocal of focal length (in metre)

Question 22

unit of power

Answer 22

dioptre (D)

Question 23

power is positive when…

Answer 23

lens deviates ray towards its centre (convex)

Question 24

power is negative when…

Answer 24

lens deviates ray away from the centre (concave)

Question 25

when two thin lenses are placed in close contact..

Answer 25

combination has a power equal to the algebraic sum of powers of each lens

Question 26

the size of image on the retina is determined by?

Answer 26

angle subtended by the object at the eye. smaller the angle subtended, smaller the image.

Question 27

construction of magnifying glass

Answer 27

a simple microscope is a convex lens of short focal length mounted in a lens holder. the object is placed such that it lies within focal length. image formed behind the object.

Question 28

magnifying power of a microscope

Answer 28

1 + (least distance of distinct vision D/ focal length f)

Question 29

application of lenses

Answer 29

1. objective lens of telescope, camera, slide projector
2. eye lens is a convex lens which forms real, inverted image on the retina
3. making spectacles for hypermetropia (convex) and myopia (concave)
4. magnifying glass(convex)
5. collimator of spectroscope (convex)

Question 30

how to differentiate lenses by seeing image

Answer 30

1. if letter appear magnified, lens is convex. if letters are diminished, lens is concave
2. on seeing distant object, if image is inverted, lens is convex. if image is upright, lens is concave