3.2.3 Thin lenses (3)
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1.
Explain, using ray diagrams, how a converging lens forms an image. Include in your explanation the roles of the principal focus (F) and the focal point (P).
A converging lens forms an image by refracting (bending) light rays that pass through it. Here's an explanation using ray diagrams:
- Ray 1: A ray of light from the object travels parallel to the principal axis and refracts through the lens, passing through the focal point (P) on the other side of the lens.
- Ray 2: A ray of light from the object travels through the optical centre of the lens and continues straight through, without any refraction.
- Ray 3: A ray of light from the object travels parallel to the principal axis and refracts through the lens, appearing to come from the focal point (F) on the same side of the lens as the object.
When these three rays intersect, they form the image. The image formed is inverted because the rays cross. The image is also virtual because the rays do not actually meet; they appear to diverge from the focal point. The position of the image depends on the object's distance from the lens. If the object is beyond 2f, the image is inverted and diminished. If the object is between f and 2f, the image is inverted and magnified. If the object is between f and 2f, the image is virtual and magnified.
The principal focus (F) is the point where parallel rays of light converge after passing through the lens. The focal point (P) is the point where a ray of light passing through the optical centre of the lens continues straight.
2.
Explain, using a diagram, how a converging lens can be used to form a real image. Include in your explanation the roles of the object distance, image distance, and focal length.
To form a real image with a converging lens, the object must be placed beyond the focal point (2f). Here's how it works:
- A parallel beam of light from the object strikes the converging lens.
- The rays are refracted towards the principal axis.
- Rays passing through the center of the lens are not deviated.
- Rays passing closer to the principal axis are refracted more.
- The refracted rays converge at a point on the other side of the lens. This point is the real image.
The object distance is the distance between the object and the lens. The image distance is the distance between the image and the lens. The focal length (f) is the distance between the lens and the focal point. The relationship between these quantities is given by the lens formula: 1/f = 1/o + 1/i, where 'o' is the object distance and 'i' is the image distance. For a real image, the image distance 'i' is positive.
| Object Distance (o) | Image Distance (i) |
3.
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.