What is the origin of geometrical isomerism in alkenes ?

Explanation

**Step-by-Step Solution:**

1. **Understanding Alkenes**: Alkenes are hydrocarbons that contain at least one carbon-carbon double bond (C=C). The presence of this double bond is crucial for the concept of geometrical isomerism.

2. **Nature of the Double Bond**: The double bond in alkenes consists of one sigma (σ) bond and one pi (π) bond. The σ bond allows for free rotation around the bond axis, while the π bond does not.

3. **Restricted Rotation**: The pi bond creates a restriction on rotation. If we attempt to rotate the carbon atoms around the double bond, the π bond would break, which is not feasible under normal conditions. This restriction leads to fixed spatial arrangements of the groups attached to the carbon atoms involved in the double bond.

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