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A convex lens is put 10 cm from a light source and it makes a sharp image on a screen, kept 10 cm from the lens. Now a glass block (refractive index 1.5) of 1.5 cm thickness is placed in contact with the light source. To get the sharp image again, the screen is shifted by a distance d. Then d is:

1. 1.1 cm away from the lens
2. 0
3. 0.55 cm towards the lens
4. 0.55 cm away from the lens

Correct answer: 0.55 cm away from the lens

Solution

When the glass block is placed in front of the light source, it effectively increases the optical path length due to its refractive index. This causes the light rays to converge at a point further away from the lens, necessitating a shift of the screen by 0.55 cm to maintain a sharp image.

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