Question

1.1) A convex lens has focal length 50cm. An object is 60cm from the lens. Compute the location of the image.

(a) 0.003cm (b) 27cm (c) −27cm (d) −300cm (e) 300cm

1.2) As part of an hon- ors project, a student wishes to construct a series of lenses with different focal lengths. Not having glass grinding equipment, the student makes a series of lenses by partially filling cylindrical con- tainers with water (n = 4/3). For the container to the right with radius 4.0cm, what is the focal length of the lens? Ignore the glass of the container and assume a thin lens. You do not need to know the thickness of the water to complete the problem.

(a) 0cm. (b) 2cm. (c) 8cm. (d) 12cm.

1.3)

You want to build a search light. The search light is to have an outgoing parallel beam. You have a small, very bright light (a point light source) and a concave mirror with radius of curvature 2m. How far from the mirror should the point light source be placed so the mirror produces an outgoing parallel beam?

(a) The point light source should be placed infinitely far from the mirror.

(b) 1m

(c) 2m
(d) 4m

Answer 1

Here ,

1.1)

focal length , f = 50 cm

object distance , do = 60 cm

Using lens formula

1/f = 1/di + 1/do

1/50 = 1/60 + 1/di

di = 300 cm

the image is e) 300 cm from the lens

1.2)

radius , r = 4 cm

n = 4/3

focal length = (n-1) * (1/R1)

focal length = (4/3 - 1) * (1/4 )

focal length = 12 cm

the focal length is 12 cm

1.3)

as fr parallel beam , the object should be placed at the focus

object distance = focal length

object distance = radius/2

object distance =2/2

object distance = 1 m

the object distance should be b) 1 m