3.2.3 Thin lenses (3)
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1.
A student shines white light through a prism. The resulting spectrum displays the seven colours.
- Name the seven colours observed in the spectrum.
- State whether the frequency of the violet light is higher or lower than the frequency of the red light.
- Explain your answer.
1. The seven colours observed are: Red, Orange, Yellow, Green, Blue, Indigo, and Violet.
2. The frequency of the violet light is higher than the frequency of the red light.
3. The frequency of light is inversely proportional to its wavelength. Violet light has a shorter wavelength than red light. Since higher wavelength corresponds to lower frequency, violet light has a higher frequency than red light.
2.
Explain, using a ray diagram, how a converging lens can form a real, inverted image. Include a description of the position of the object and the location of the image.
Diagram: The ray diagram should show two rays from the object. One ray passes through the focal point. The other ray is parallel to the principal axis and refracts through the focal point. The intersection of these two refracted rays forms the real, inverted image. The image is on the same side of the lens as the object.
Explanation: For a real, inverted image to be formed, the object must be placed between the focal point and the principal axis (i.e., 0 < u < f). In this configuration, the refracted rays from the object do not diverge; they actually converge at a point on the other side of the lens. This point of convergence is the location of the real, inverted image. The image is real because the refracted rays actually meet at a point in space. The image is inverted because the refracted rays are diverging in a way that causes the image to be flipped upside down relative to the object.
3.
A parallel beam of light incident on a thin converging lens passes through the lens and focuses to a point. Describe the path of the light rays as they pass through the lens, including the terms focal length, focal point, and the direction of the rays.
When a parallel beam of light strikes a thin converging lens, the rays are refracted (bent) towards the principal axis. The light rays that are parallel to the principal axis are refracted so that they converge at a single point on the other side of the lens. This point is called the focal point. The focal length (f) is the distance between the principal axis and the focal point. Rays passing through the center of the lens are not deviated. Rays passing closer to the principal axis are refracted more than rays passing further away. The refracted rays emerge from the lens and appear to diverge from the focal point. The direction of the refracted rays is such that they converge at the focal point.