State two precautions to ensure accurate result. In why light suffer deviation when it passes one medium into another of different density.

Here are the answers to the questions from the image:

1. State two precautions to ensure accurate results.

• Ensure the optical pins are fixed vertically to avoid parallax error when viewing their images.
• Use a sharp pencil to draw the outline of the prism and the ray paths accurately.

2. Explain why light suffers deviation when it passes one medium into another of different density.
Light suffers deviation (refraction) when it passes from one medium to another of different optical density because its speed changes. If the light ray enters the new medium at an angle other than $90^\circ$ to the surface, one side of the wavefront changes speed before the other, causing the ray to bend or change direction.