If a lens is placed in water instead of air, its focal length :

1. increases
2. decreases
3. remains the same
4. can both increase or decrease

If half part of a convex lens is covered, the focal length …………… but the intensity of the image ……………

1. increases, decreases
2. decreases, increases
3. does not change, increases
4. does not change, decreases

The radius of the sphere whose part is the lens surface is called the …………… of that surface.

1. centre of curvature
2. focal length
3. pole
4. radius of curvature

Identify the incorrect statement :

1. if the medium on both sides of a lens is same, its first and second focal lengths are equal, i.e. f₁ = f₂ (numerically).
2. when we say focal length of a lens, we mean the first focal length of the lens.
3. a convex lens has a real focus.
4. a concave lens has a virtual focus.