Final answer:
A pencil in a glass half filled with water appears broken or bent due to refraction, as light changes direction passing through different media. Ray diagrams can illustrate the paths of light rays bending at interfaces, showing the cause of the pencil's distorted appearance.
Step-by-step explanation:
The scenario described involves a pencil placed in a glass that is half filled with water, with the bottom half of a mirror covered. When you place a pencil in water, you will observe that the shape of the pencil appears to be broken or bent at the water's surface. This is due to the phenomenon of refraction, where light changes direction as it passes from one medium into another with a different density, in this case, from air into water. Refraction makes the pencil appear to be at a different position than where it actually is.
To understand this, you can draw ray diagrams showing the path of light from the pencil through the air, glass, and water to your eyes. You'll notice that the light bends at the interface between air and water, and again as it exits the water, due to the change in the speed of light in different media. This bending causes the misalignment of the image of the pencil, making the part in the water appear to be at a different angle compared to the part in the air.