An Object 4.0 Cm in Size, is Placed 25.0 Cm in Front of a Concave Mirror of Focal Length 15.0 Cm. (I) at What Distance from the Mirror Should a Screen Be Placed in Order to Obtain a Sharp Image? - Science


An object 4.0 cm in size, is placed 25.0 cm in front of a concave mirror of focal length 15.0 cm.

(i) At what distance from the mirror should a screen be placed in order to obtain a sharp image?
(ii) Find the size of the image.
(iii) Draw a ray diagram to show the formation of the image in this case.




Height of object `("h"_"o")` = 4 cm
Object distance `("u")` = -25cm
Focal length `("f")` = -15cm

Applying mirror formula and substituting the values,

`1/"v" + 1/"u"=1/"f"`

`1/"v" = 1/(-15) - 1/(-25)`

`1/"v" = (-5 + 3)/75`

`"v" = (-75)/2 = -37.5` cm

Negative sign indicates that the image is in front of the mirror.
Therefore, the screen must be placed in front of the mirror at a distance of 37.5 cm.

Applying magnification formula and substituting the values,

`"m" = (-"v")/"u" = "h"_"i"/"h"_"o"`

`(-((-75)/2))/-25 = "h"_"i"/4`

`"h"_"i" = (-75)/(2xx25) xx 4`

`"h"_"i" = -6  "cm"`

Negative sign indicates that the image is below the principal axis.
Therefore, the size of the image is 6 cm.

The ray diagram showing the formation of image in this case is,

Concept: Convex Lens
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2019-2020 (February) Delhi (Set 1)

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To determine the approximate focal length of the given convex lens by focussing a distant object (say, a sign board), you try to focus the image of the object on a screen. The image you obtain on the screen is always

(a) erect and laterally inverted

(b) erect and diminished

(c) inverted and diminished

(d) virtual, inverted and diminished

An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.

An object is placed at a distance of 12 cm from a convex lens of focal length 8 cm. Find :

1) the position of the image

2) nature of the image

Complete the following table:

Instrument Number of Convex
Simple Microscope .............. ..............
Compound Microscope .............. ..............
Telescope .............. ..............

A beam of light travelling in air is incident of water. Draw a ray-diagram indicating the change in its path in water.

A ray of light travelling in water emerges into air. Draw a ray-diagram indicating the change in its path.

If an object is at a considerable distance (or infinity) in front of a convex lens, where is the image formed?

Describe the nature of image formed when an object is placed at a distance of 30 cm from a convex lens of focal length 15 cm.

A student did an experiment with a convex lens. He put an object at different distances 25 cm, 30 cm, 40 cm, 60 cm and 120 cm from the lens. In each case he measured the distance of the image from the lens. His results were 100 cm, 24 cm, 60 cm, 30 cm and 40 cm, respectively. Unfortunately his results are written in wrong order. 

 Which of the object distances gives the biggest image?

In figure , name the ray which represents the correct path of light while emerging out through
a glass block. 


The diagrams (a) and (b) in Figure below show the refraction of a monochromatic ray of light through a parallel sided glass block and a prism respectively. In each diagram, label the incident, refracted emergent rays and the angle of deviation.


Study the diagram given below.

Name the lens LL’

Study the following diagram :

state the three characteristics of the image.

Study the diagram below.

Complete the diagram to form the image of the object AB.

Study the diagram below.

State three characteristics of the image

Analyse the following observation table showing variation of image-distance (v) with object-distance (u) in case of a convex lens and answer the questions that follow without doing any calculations:

S. No. Object-Distance
u (cm)
v (cm)
1 –60 +12
2 –30 +15
3 –20 +20
4 –15 +30
5 –12 +60
6 –9 +90

(a) What is the focal length of the convex lens? State reason for your answer.
(b) For what object-distance (u) is the corresponding image-distance (v) not correct? How did you arrive at this conclusion?
(c) Choose an appropriate scale to draw a ray diagram for the observation at S. No. 4 and find the approximate value of magnification.

Distinguish between:

Concave lens and Convex lens

i. Which type of microscope has the arrangement of lenses shown in the  adjoining figure?
ii. Label the figure correctly.
iii. Write the working of this microscope.
iv. Where does this microscope used?
v. Suggest a way to increase the efficiency of this microscope.

The image obtained while finding the focal length of convex lens is ....................

Observe the following figure and answer the questions.

a) Which optical instrument shows arrangement of lenses as shown in the figure?
b) Write in brief the working of this optical instrument.
c) How can we get different magnifications in this optical instrument?
d) Draw the figure again and labelled it properly

Yesh find out F1 and F2 of symmetric convex lens experimentally then which conclusion is true.

Observe the following figure and complete the table:

Points Answer
(i) Position of the object  
(ii) Position of the image  
(iii) Size of the image   
(iv) Nature of the image  

A lens forms the image of an object placed at a distance 15 cm from it, at a distance 60 cm in front of it. Find :  the magnification.

If an object is placed in front of a convex lens beyond 2F1, then what will be the position, relative size, and nature of an image which is formed? Explain with a ray diagram.

Rewrite the following statement by selecting the correct option:
The power of a convex lens is 2.5 dioptres. Its focal length is ______________.

How will you decide whether a given piece of glass is a concave lens, convex lens, or a plane glass plate?

What happens to the image formed by a convex lens if its lower part is blackened?

Why do we say that the ‘2F’ and ‘F’ points of a convex lens can be regarded as a sort of ‘turning points’ as far as the nature of the image formed by it is concerned?

Where should an object be placed in front of a convex lens so as to form an inverted and enlarged image? Will the image be real or virtual? Draw a ray diagram to illustrate your answer.

_______ times larger images can be obtained by using a simple microscope.

_______ is a combination of two convex lenses with small focal length.

Object at 2F1 of a convex lens : Image at 2F2 : : Object at F1 : _______

Simple microscope : Number of convex lens one : : compound microscope : _______

: Object near the lens : : ______ :

Find the odd one out and give its explanation.

Find the odd one out and give its explanation.

Write the name.

The lens used in simple microscope.

Write scientific reason.

Adults need bifocal lens spectacle.

Observe the given figure and answer the following questions.

  1. Where is the above type of lens construction used?
  2. What type of image is formed by an objective lens?
  3. What happens instead of placing at Fo if the object is placed in between O and Fo?

Differentiate convex lens and concave lens.

  1. In which type of microscope do you find the lens arrangement as shown in the following diagram?
  2. Write about the working and the use of this microscope.

Distinguish between Concave lens and Convex lens.

Distinguish between:

Concave lens and Convex Lens

Distinguish between Concave lens and Convex Lens.

Distinguish between:

Concave lens and Convex Lens

Distinguish between:

Concave lens and Convex Lens


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