Question

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 moves gradually 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?; B. What happens to the size of the image of the flame formed on the screen?; C. What difference is seen in the intensity (brightness) of the image of the flame on the screen?; D. What is seen on the screen when the flame is very close (at about 5 cm) to the lens?(All India - 2017)

Answer 1

Answer:

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Answer 2

He should move the lens towards the candle. The size of the image of the flame formed increases. Intensity of the image increases as the lens collects more light from the candle when the candle comes closer. Nothing will be seen on the lens when the lens is very close to the candle.

Given the focal length of the lens is f = 10cm. Initial the candle is kept at 2m.

Therefore , the image will be formed beyond f , inverted and small size. Then he gradually moves the flame towards the lens.

• (A) Since the screen is fixed, as candle moves closer, the screen should be farther apart from the lens to focus the image.

• Remember, when object is at infinity, image is at focus and when object is at focus, the image is at infinity.

• Hence, he should move the lens towards the flame.

• (B) As the image moves from focus to infinity, it's size increases.
• At focus its a small point. But as distance between lens and the screen increases, image size also increases.

• (C)  The intensity of image will increase.
• As the lens and object move closer, the amount of light, from the object collected by the lens , increases.

• (D) When the flame is very close to the lens , its impossible to focus the image on it.
• From distance formula we can see that when object distance is less than focal length, the image distance is negative.
• v = uf/u-f  let u = 5cm
• v = 5x10/5-10 = -10cm , same side as the object.
• That means, it is formed  on the same side as the object.