The focal length of a magnifying glass is the distance between the focal point and optical centre of the magnifying glass.
<h3>Focal length</h3>
The focal length, f is the distance from a lens or mirror to the focal point, F.
This is the distance from a lens or mirror at which parallel light rays will meet for a converging lens or mirror or appear to diverge from for a diverging lens or mirror.
A magnifying glass is a converging lens which produces a enlarged, erect and virtual image when an object is placed between the focal point and optical centre.
A magnifying glass will bring to focus at a point sun rays which can cause the paper to catch fire if it is held in place for long.
This point at which the most concentrated ray of light is shining on the paper, is the focal point for that magnifying glass.
Therefore, the focal length of a magnifying glass is the distance between the focal point and optical centre of the magnifying glass.
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The speed of the 0.8 kg ball immediately after collision is 0.625 m/s in opposite direction to the stationary ball.
The given parameters;
- mass of the ball, m₁ = 0.8 kg
- speed of the ball, u₁ = 2.5 m/s
- mass of the object at rest, m₂ = 2.5 kg
- final velocity of the object at rest, v₂ = 1 m/s
Let the final velocity of the 0.8 kg ball immediately after collision = v₁
Apply the principle of conservation of linear momentum;
m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
(0.8 x 2.5) + (2.5 x 0) = (0.8)v₁ + 2.5(1)
2 = 2.5 + (0.8)v₁
-0.5 = (0.8)v₁

Thus, the speed of the 0.8 kg ball immediately after collision is 0.625 m/s in opposite direction to the stationary ball.
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To solve this problem it is necessary to apply the equations related to the conservation of momentum. Mathematically this can be expressed as

Where,
= Mass of each object
= Initial velocity of each object
= Final Velocity
Since the receiver's body is static for the initial velocity we have that the equation would become



Therefore the velocity right after catching the ball is 0.0975m/s