Lens

Question 1

What is a lens

Answer 1

A lens is a transparent refracting medium bound by either two refracting surfaces or one refracting surface and one plane surface

Question 2

Ways to identify convex lens

Answer 2

Thick in middle, thin at periphery. Lens bulges out in the middle and light gets converged

Question 3

Three types of convex lens

Answer 3

Bi convex
Plano Convex
Concave convex

Question 4

Ways to identify concave lens

Answer 4

Thick at periphery, thin in middle, bent inwards, light diverges or appears to diverge

Question 5

Types of concave lens

Answer 5

Bi concave
plano concave
convexo concave

Question 6

Define centre of curvature

Answer 6

Centre of the sphere whose part is in the lens surface

Question 7

Define radius of curvature

Answer 7

Radius of the sphere whose part is the lens surface

Question 8

What is principal axis

Answer 8

It is the line joining the centre of curvature of two surfaces of the lens

Question 8

What is principal axis

Answer 8

It is the line joining the centre of curvature of two surfaces of the lens

Question 9

Define optical centre

Answer 9

It is a point on the principal axis such that a ray of light directed towards this point emerges parallel to its direction of incidence

Question 10

What is the first focal point for a convex lens?

Answer 10

Point on principal axis such that a light emerging from that point, after refraction thru the lens become parallel to principal axis

Question 11

What is the first focal point for a concave lens

Answer 11

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

Question 12

What is the second focal point of a convex lens

Answer 12

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

Question 13

What is the second focal point of a concave lens

Answer 13

Point on the principal axis such that rays of light parallel to principal axis, after refraction appear to diverge from this point

Question 14

Define : Focal plane
first focal plane
second focal plain

Answer 14

Focal plane is a plane normal to the principal axis passing through the focus

First focal plane is the plane passing through the first focal point and normal to the principal axis

Second focal plane is the plane passing through the second focal point and is normal to the principal axis

Question 15

Define focal length
first focal length
second focal length

Answer 15

distance of focus from optical centre is focal length

distance from optical centre to its first focal point is first focal length

distance from optical centre to its second focal point is second focal length

Question 16

What 2 factors does focal length depend on and how is it affected by them?

Answer 16

1. R.I of lens w respect to surrounding medium. Inc in R.I = Inc in Focal length
2. Radii of curvature. Thicker the lens less the focal length

Question 17

when obj is at infinity

Answer 17

Real, inverted, point sized, image formed at f2
Used in burning glass

Question 18

What lens is used in cameras ? State its nature and position

Answer 18

When the lens is beyond 2F1
image is real inverted and highly diminished

Question 19

What distance is the object at from the lens of a terrestrial telescope? State the nature of the image

Answer 19

Object is at 2F1
real inverted same size as obj

Question 20

What distance is the object at from the lens of a slide projector? State the nature of the image

Answer 20

Between 2F1 and F1
real, inverted and magnified

Question 21

What distance is the object at from the lens of a collimator of a spectrometer? State the nature of the image

Answer 21

Object is at F1
real, inverted , highly magnified

Question 22

What distance is the object at from the lens of a reading lens

Answer 22

Between F1 and Optical centre
virtual, erect, magnified

Question 23

What distance is the object at from the lens of a galilean telescope? State the nature of the image

Answer 23

Obj at infinity
virtual, erect, point sized

Question 24

What distance is the object at from the lens of spectacles for myopic eye? State the nature of the image

Answer 24

Obj at finite distance from concave lens

virtual, erect, diminished