Sponsor Area
An object is placed at a distance of 30 cm in front of a convex mirror of focal length 15 cm. Write four characteristics of the image formed by the mirror.
Four properties of the image formed by the given convex mirror are:
(i) Virtual
(ii) Always forms behind the mirror between focus and pole.
(iii) Image is always erect
(iv) Small in size
Sponsor Area
Given
h1 =+4
f = - 10 cm
u = -15 cm
v = ?
h2 = ?
Using the mirror equation,
Thus, to obtain a sharp image of the object the screen should be placed in front of the mirror at a distance of 30 cm from the mirror.
Magnification:
Analyse the following observation table showing a variation of image distance (v) with object distance (u) in the case of a convex lens and answer the questions that follow, without doing any calculations:
| S. No. |
Object distance |
Image distance |
| 1 | -90 | +18 |
| 2 | -60 | +20 |
| 3 | -30 | +30 |
| 4 | -20 | +60 |
| 5 | -18 | 90 |
| 6 | -10 | 100 |
(a) What is the focal length of the convex lens ? Give reason in support of your answer.
(b) Write the serial number of that observation which is not correct.How did you arrive at this conclusion?
(c) Take an appropriate scale to draw a ray diagram for the observation at S. No. 4 and find the approximate value of magnification.
(a) From serial number 3 we can say that the radius of curvature of the lens is 30 cm because when an object is placed at the centre of curvature of a convex lens its image is formed on the other side of the lens at the same distance from the lens. And, we also know that focal length is half of the radius of curvature. Thus, the focal length of the lens is + 15 cm.
(b) Serial number 6 is not correct as the object distance is between focus and pole so for such cases the image formed is always virtual but in this case, a real image is forming as the image distance is positive.
(c) Approximate value magnification for object distance - 20 cm and image distance + 60 cm is 3. 
To construct a ray diagram we use two rays which are so chosen that it is easy to know their directions after reflection from the mirror. List two such rays and state the path of these rays after reflection in case of concave mirrors. Use these two rays and draw ray diagram to locate the image of an object placed between pole and focus of a concave mirror.
Two light rays whose path of reflection are priorly know are:
(i) The incident ray passes through the centre of curvature: In this case, light ray after reflecting from the concave mirror moves back in the same path. This happens because the light ray is incident perpendicularly on the mirror surface.
(ii) The ray incident obliquely to the principal axis: In this case, the incident ray will be reflected back by the reflecting surface of the concave mirror obliquely and making equal angles with the principal axis.
Let an object "a candle" is placed between the focus and pole of the concave mirror. Then using above two rays, image of the candle can be located as shown below:
Sponsor Area
Mock Test Series
