The method and the results are on the colour and lenses lab. Do the analysis yourself first, then check the answers below.
Quick answer: White light is a mixture of colours that a prism splits into a spectrum by dispersion, with violet bent most. These answers go with the grade 8 science lab on colour and lenses: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
Model answers use the sample measurements in the results tables. Your own numbers will differ slightly, but the pattern should be the same: the closer the object, the further away and larger the image, until the object reaches the focal length.
Mean = (10.1 + 10.3 + 10.0) ÷ 3 = 30.4 ÷ 3 = 10.13 cm, so the focal length is 10.1 cm.
Question 2 Magnification of one image
Using heights: M = 0.7 ÷ 2.0 = 0.35. Using distances: M = 13.5 ÷ 40.0 = 0.34. The values agree: M ≈ 0.35 (a diminished image).
Question 3 Larger images
u = 15.0 cm: M = 3.9 ÷ 2.0 = 1.95, and v ÷ u = 29.5 ÷ 15.0 = 1.97. u = 12.0 cm: M = 9.6 ÷ 2.0 = 4.8, and v ÷ u = 58.0 ÷ 12.0 = 4.83. The values are about 1.95 and 4.8.
Question 4 Describing the trend
As u gets smaller, v gets larger (13.5 cm to 58.0 cm) and the image gets larger (0.7 cm to 9.6 cm). The image is always inverted. The closer the object, the further away and larger the image.
Question 5 Image at 2F
At u = 20.0 cm the image is the same size, which happens at 2F. So f = 20.0 ÷ 2 = 10.0 cm, which agrees with 10.1 cm to within the uncertainty.
Question 6 Uncertainty
For v = 13.5 cm: 1.0 ÷ 13.5 × 100 = 7.4 %. For v = 58.0 cm: 1.0 ÷ 58.0 × 100 = 1.7 %. The reading at u = 12.0 cm is more reliable (1.7 % against 7.4 %).
Question 7 Object inside the focal length
At u = 8.0 cm the object is closer than F, so the rays leave the lens still spreading apart and never meet. The image is virtual. Looking through the lens you see an upright, larger image, like a magnifying glass.
Question 8 Sharper focus readings
Any two of: darken the room so the sharpest image is clear; move the screen back and forth and take the middle of the sharp range; repeat each reading and average; measure from the centre of the lens; keep the screen upright with a set square. Darken the room and average repeats are good examples.
