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A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length will
  • a)
    become zero
  • b)
    become infinite
  • c)
    become small, but non-zero
  • d)
    remain unchanged
Correct answer is option 'B'. Can you explain this answer?
Most Upvoted Answer
A convex lens is dipped in a liquid whose refractive index is equal to...
Sure. The answer is b). The focal length of a lens is inversely proportional to the refractive index of the medium in which it is placed. When the refractive index of the lens is equal to the refractive index of the liquid, the lens will act like a plane parallel slab, which has an infinite focal length.

Here's an explanation:

The focal length of a lens is determined by the following formula:

f = (n - 1)r / (n + 1)
where:

f is the focal length of the lens
n is the refractive index of the lens material
r is the radius of curvature of the lens
When the refractive index of the lens is equal to the refractive index of the liquid, the term (n - 1) becomes 0. This means that the focal length of the lens becomes infinite.

In other words, when a convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens, it will no longer be able to focus light rays. Instead, the light rays will pass through the lens without being refracted.

This is because the lens is no longer able to bend the light rays, as the refractive index of the lens and the liquid are the same. As a result, the focal length of the lens becomes infinite.
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Community Answer
A convex lens is dipped in a liquid whose refractive index is equal to...
Explanation:

When a convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens, the focal length of the lens will become infinite.

Refraction at a convex lens:

When light passes through a convex lens, it undergoes refraction. The lens bends the light rays towards its optical axis, causing them to converge at a point called the focal point. The distance between the lens and the focal point is known as the focal length (f) of the lens.

Refraction at the interface of two media:

When light travels from one medium to another, it undergoes refraction at the interface between the two media. The amount of bending of the light rays depends on the refractive indices of the two media.

Refractive index:

The refractive index (n) of a medium is a measure of how much the medium can bend light. It is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in the medium (v):

n = c/v

Equal refractive indices:

When the refractive index of the liquid is equal to the refractive index of the lens, it means that the speed of light in the liquid is equal to the speed of light in the lens.

Effect on focal length:

When the refractive index of the liquid is equal to the refractive index of the lens, there is no change in the speed of light as it passes from the lens to the liquid. This means that there is no bending of the light rays at the interface between the lens and the liquid.

Since the bending of light at the interface is what causes the lens to focus light, when there is no bending of light, the focal length of the lens becomes infinite. This means that the lens does not converge the light rays to a focal point anymore.

Therefore, the correct answer is option 'B' - the focal length becomes infinite.
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A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length willa)become zerob)become infinitec)become small, but non-zerod)remain unchangedCorrect answer is option 'B'. Can you explain this answer?
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