Advertisements
Advertisements
प्रश्न
A lens forms a real image 3 cm high of an object 1 cm high. If the separation of object and image is 15 cm, find the focal length of the lens.
Advertisements
उत्तर
Given,
Height of image (h') = - 3 cm (negative because image is real)
Height of object = 1 cm
Let us first define the sign convention:
Since the object distance is always negative, let it be -u.
Since the focal length is positive for a convex lens, let it be +f .
Magnification =`h/h=v/u`
`v/u=-3`
(i) Since object distance (u) is always negative, and the ratio `v/u` here is negative, v must be positive, i.e., it is on the right side of the lens.
Now, we have object distance + image distance = 15
`v+(-u)=15 (using sign convention)`
`-3u-u=15`
`-4u=15`
`u=(-15)/4`
Applying the lens formula:
`1/f=1/v-1/u`
`1/f=1/(-3u)-1/u`
`1/f=(-4)/(3u)`
`f=(3u)/-4=(3xx-15/4)/4=45/16=2.81`cm
Hence, the focal length of the lens is +2.81 cm.
APPEARS IN
संबंधित प्रश्न
Explain the following term related to spherical lenses: focal length
A ray of light is incident normally on a plane mirror. What will be the
angle of reflection?
Name the spherical mirror which has:
(a) virtual principal focus.
(b) real principal focus.
Name the kind of mirror used to obtain
(a) a real and enlarged image
(b) a virtual and enlarged image
(c) a real and diminished image, and
(d) a virtual and diminished image.
Mention the types of mirrors used as (i) rear view mirrors, (ii) shaving mirrors. List two reasons to justify your answers in each case.
Figure shows a concave mirror with its pole at P, focus F and centre of curvature C. Draw ray diagram to show the formation of image of an object OA.
Draw a ray diagram to show the formation of image of an object kept in front of a convex mirror. State three characteristics of the image.
The erect image formed by a concave mirror is of size double the size of object. How are u and v related?
When an object of height 1 cm is kept at a distance 4 cm from a concave mirror, its erect image of height 1.5 cm is formed at a distance 6 cm behind the mirror. Find the focal length of mirror, by drawing.
Define the term Principle focus.
The following Figure shows a concave mirror MM' on which a ray of light incident from a point P gets reflected to meet the principle axis at O.
(a) Find, by construction, the position of the centre of curvature of the concave mirror.
(b) Write down the value for the radius of curvature of the mirror.
(c) Calculate the focal length of the mirror.
(d ) Which relation is used in deducing the focal length from the radius of curvature?
The diagram below shows the parallel rays incident on a convex mirror. C is the centre of the curvature of the mirror. By drawing the paths of the reflected rays, label the focus F and hence find the focal length of the mirror.
Complete the following diagrams shown in the following figures by drawing the reflected rays.
What is the difference between virtual images of an object formed by a concave mirror and a convex mirror?
For a spherical mirror, ______ is true.
What is the focal length (f) of a mirror?
