Fig. 2.1 shows a pencil placed in a glass of water. When viewed from the side, the pencil appears bent at the water surface. (a) Name the process responsibl...
Question 1 Report
Fig. 2.1 shows a pencil placed in a glass of water. When viewed from the side, the pencil appears bent at the water surface.
(a) Name the process responsible for this effect. [1]
(b) Explain why the part of the pencil below the water surface appears to be in a different position. [3]
(c) The refractive index of the water is 1.33. The real depth of the bottom of the pencil below the surface is 12 cm. Calculate the apparent depth. [2]
Answer Details
(a) Name the process [1]
Refraction [1]
Refraction is the bending of light as it passes from one medium to another, caused by a change in speed.
(b) Why the submerged part appears in a different position [3]
Light from the submerged part of the pencil travels from water into air. [1] As it crosses the boundary, the light speeds up (air is less optically dense than water) and the rays bend away from the normal. [1] When the refracted rays reach the observer's eye, the brain traces them back in straight lines. These projected lines meet at a point that is above and closer to the surface than the real position of the pencil, so the submerged portion appears raised. [1]
(c) Calculate the apparent depth [2]
Using the formula: \(\text{apparent depth} = \dfrac{\text{real depth}}{n}\)
\(\text{apparent depth} = \dfrac{12}{1.33}\) [1]
\(\text{apparent depth} = \mathbf{9.0 \text{ cm}}\) [1]
The bottom of the pencil appears to be at 9.0 cm below the surface rather than 12 cm, a shift of 3.0 cm upward.
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