When a colloidal sol is observed under a microscope we can see,

Options

A. Light scattered by colloidal particles only
B. Size of the particle
C. Shape of the particle
D. All of these

Answer

**Step-by-Step Solution:**

1. **Understanding Colloidal Solutions:**
- A colloidal solution consists of tiny particles (1 nm to 500 nm) dispersed in a medium. These particles are larger than those in true solutions but smaller than those in coarse suspensions.

2. **Observing Colloidal Solutions Under a Microscope:**
- When observing a colloidal solution under a microscope, one typically looks for specific characteristics of the particles.

3. **Tyndall Effect:**
- One of the key phenomena associated with colloidal solutions is the Tyndall effect. This occurs when light is scattered by the colloidal particles, making the path of the light beam visible.

4. **Direction of Light Observation:**
- If you observe a colloidal solution in the direction of the light beam, it appears clear. However, when viewed at a right angle to the light beam, the solution appears dark, highlighting the scattering effect.

5. **Conclusion:**
- Therefore, when a colloidal sol is observed under a microscope, we can see light scattered by colloidal particles, which is a manifestation of the Tyndall effect. Other options regarding the size or shape of the particles are not observable directly under a microscope.

**Final Answer:**
When a colloidal sol is observed under a microscope, we can see light scattered by colloidal particles.

---