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Question
The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between lens and image is 80 cm, at what distance should the candle be placed from the lens? What is the nature of the image at a distance of 80 cm and the lens?
Solution
Given :
Height of image (hi) = 3 × Height of object (ho)
Image distance (υ) = 80 cm
Using the magnification formula,
`"h"_"i"/"h"_"o" = upsilon/"u"`
The image is formed on the other side of the lens, so it must be real and inverted, substituting the values in the above equation,
`(-3 xx "h"_"o")/"h"_"o" = 80/"u"`
`"u" = (-80)/3`
The negative sign shows that the object is in front of the lens and the unit is in cm.
The image is formed on the other side of the lens, so the image must be real and inverted. The image is formed on the other side of the lens, it is possible only in the case of convex lens.
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A student was asked by his teacher to find the image distance for various object distance in case of a given convex lens. He performed the experiment with all precautions and noted down his observations in the following table:
| S. No. |
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Image distance (cm) |
| 1 | 60 | 15 |
| 2 | 48 | 16 |
| 3 | 36 | 21 |
| 4 | 24 | 24 |
| 5 | 18 | 36 |
| 6 | 16 | 48 |
After checking the observations table the teacher pointed out that there is a mistake in recording the image distance in one of the observations. Find the serial number of the observations having faulty image distance.
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