JEE MAIN - Physics (2023 - 12th April Morning Shift - No. 22)
Two convex lenses of focal length $$20 \mathrm{~cm}$$ each are placed coaxially with a separation of $$60 \mathrm{~cm}$$ between them. The image of the distant object formed by the combination is at _____________ $$\mathrm{cm}$$ from the first lens.
Answer
100
Explanation
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1. First refraction in L1 (lens 1):
When considering the first lens (L1), the object is at infinity, so the image I1 formed by this lens is at its focal point. Using the lensmaker's equation:
$$\frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i}$$
Given that the object is at infinity, $$d_o = \infty$$, and the focal length of the first lens is $$f = 20 \mathrm{~cm}$$. Plugging in these values, we get:
$$\frac{1}{20} = \frac{1}{\infty} + \frac{1}{d_i}$$
As $$\frac{1}{\infty}$$ is essentially 0, we have:
$$\frac{1}{20} = \frac{1}{d_i}$$
This implies that $$d_i = 20 \mathrm{~cm}$$, meaning that the image I1 is formed 20 cm from the first lens L1.
2. Second refraction in L2 (lens 2):
Now, the image I1 formed by L1 becomes the object for L2. The distance between the lenses is 60 cm, so the object distance for L2 (u) is -40 cm, because the object is to the left of the lens (u is negative). The focal length of L2 (f) is 20 cm. We can use the lensmaker's equation to find the image distance (v) for L2:
$$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$$
Plugging in the values:
$$\frac{1}{v} - \frac{1}{(-40)} = \frac{1}{20}$$
$$\frac{1}{v} = \frac{1}{20} - \frac{1}{40} = \frac{2 - 1}{40}$$
$$\frac{1}{v} = \frac{1}{40}$$
Therefore, $$v = 40 \mathrm{~cm}$$
Since the image distance (v) is positive, the image I2 is formed on the right side of L2 at a distance of 40 cm. To find the distance of the final image from L1, we add the distance between the lenses (60 cm) and the image distance (v) from L2.
Final image distance from L1 = 60 cm + 40 cm = 100 cm
Thus, the correct answer is 100 cm.
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