A student focuses the image of a candle flame, placed at about 2 m from a convex lens of focal length 10 cm, on a screen. After that he gradually moves the flame towards the lens and each time focuses its image on the screen.
(a) In which direction does he move the lens to focus the flame on the screen?
(i) away from screen
(ii) towards the screen
(iii) should not move
(iv) away from flame
Answer
In a convex lens ,
As the object and lens come closer (‘u’ decreases) , the image distance (‘v’) increases.
Let us check
- Moving lens away from screen
- Choosing a point much closer to the lens, let’s say 30 cm (beyond 2F), we see the image move closer to the screen, but still not properly focused.
- Now, we move the lens further away from the screen (close to the objects), and away from the screen (let’s say at 15 cm, b/w C and F), we can see the proper image forming on the screen (beyond 2F)
- To move the object closer to the lens, the lens must be moved away from the screen so the image can be focused.
- Moving towards the screen, the image to form closer to the lens, resulting in blurring
- If we do not move , we will see no change in the image.
- Moving the lens away from the flame takes us closer to the screen (same as option (ii))
Thus, the correct answer is (i) away from screen
(b) What happens to the size of the image of the flame formed on the screen (as the candle moves)?
(i) size of image will decrease
(ii) size of image will increase
(iii) remains unchanged
(iv) size will become too small
Ans.
In a convex lens
- As the object moves towards the lens, the size of the image increases
- until the object crosses the focus.
- Between F and P, the image becomes virtual.
Thus, the correct answer is (ii) size of image will increase
(c) What difference is seen in the intensity (brightness) of the image of the flame on the screen?
(i) intensity of flame increases
(ii) intensity of flame remains same
(iii) intensity of flame reduces
(iv) the flame disappears
Ans.
Intensity is the amount of energy per unit area per unit time .
Thus
- Intensity is inversely proportional to area .
- As we move closer, the size of the image increases ,
- As size increases, area increases
- Thus, intensity decreases
Thus, the correct answer is (iii) intensity of flame reduces
(d) What is seen on the screen when the flame is very close (at about 5 cm) to the lens?
(i) a bright image
(ii) a magnified image
(iii) diminished image
(iv)no image
Ans.
Now,
- Given focal length of convex lens is 10 cm
- Thus, 5 cm means that object is between F and P.
- Convex lens produces a virtual image when object is between F and P .
- Hence, this image cannot be seen on screen.
Thus the correct answer is (iv) no image
