The problems are on the colour and lenses worksheet. Try them on your own first, then compare with 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 worked answers correct the 20 problems of the grade 8 science worksheet on colour and lenses, from spectrum order to image on the retina: each solution shows the formula, the substitution and the final answer with its unit.
Compare your working with these model answers. Each one gives the method first and the final result in bold. For colour problems, always list which colours are reflected, transmitted or absorbed before you decide what is seen.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS4-2
Problem 1 Spectrum order
1. The effect is dispersion.2. The order is red, orange, yellow, green, blue, violet.
3. Violet bends most: violet.
Problem 2 Mixing coloured lights
1. Red + green makes yellow.2. Green + blue makes cyan.
3. All three together make white.
Problem 3 Parts of the eye
1. A is the cornea, B the iris, C the lens, D the retina and E the optic nerve.2. The cornea (A) bends the light most: A, cornea.
3. The retina holds the light-sensitive cells: D, retina.
Problem 4 Blue torch filter
1. Only blue passes: blue.2. The filter absorbs the red and green light.
3. A red filter transmits only red, and there is no red in the beam, so no light emerges.
Problem 5 Lens vocabulary
1. 25 cm ÷ 100 = 0.25 m.2. The rays meet at the focal point, 25 cm from the lens.
3. It is thicker in the middle than at the edges.
4. A fatter lens bends light more, so the focal length is shorter.
Problem 6 Magnifying glass
1. M = hi ÷ ho = 7.5 ÷ 2.5 = 3.0.2. Image height = 3.0 × 4.0 cm = 12 cm.
Problem 7 Banana under coloured lamps
1. White light contains all colours, so it reflects red and green: yellow.2. Red light is reflected: red.
3. Blue light is absorbed and nothing else is present, so no light is reflected: black.
Problem 8 Focal length and lens power
1. The distance is 8.0 cm.2. 8.0 cm = 0.080 m.
3. P = 1 ÷ 0.080 = 12.5 D.
4. f = 0.050 m, so P = 1 ÷ 0.050 = 20 D. The second lens bends light more because its power is larger.
Problem 9 Diminished image
1. M = 2.4 ÷ 6.0 = 0.40.2. M is below 1, so the image is diminished.
3. Image height = 0.40 × 15 cm = 6.0 cm.
Problem 10 Pupil in bright light
1. Her pupils get smaller.2. The iris changes the size of the pupil.
3. A smaller pupil lets less light in, which protects the retina from too much light and keeps the image sharp. Less light enters the eye.
Problem 11 Image types from a table
1. 35 cm is beyond 2F (20 cm): real, inverted, smaller. 20 cm is at 2F: real, inverted, same size. 14 cm is between F and 2F: real, inverted, larger. 7 cm is closer than F: virtual, upright, larger.2. The first three are real: 35 cm, 20 cm and 14 cm.
3. The magnifying glass case is the object closer than F: 7 cm.
Problem 12 Spot the errors
1. A filter only removes colours. A red filter absorbs green and blue, so the beam is dimmer, not brighter.2. Violet is bent the most and red the least: violet bends most.
Problem 13 Camera and eye
1. The sensor matches the retina.2. The opening matches the pupil (controlled by the iris).
3. In both, the image is real and inverted.
Problem 14 Measuring the spread of a spectrum
1. Violet is turned more: violet.2. 40.0° − 38.0° = 2.0°.
3. Width = 2.0 × 3.5 cm = 7.0 cm.
Problem 15 Two filters in a row
1. Yellow transmits red and green.2. Cyan transmits green and blue, so only green passes both: green.
3. Magenta transmits red and blue only, so it absorbs the green: no light emerges.
Problem 16 Stage lights
1. Red + green makes yellow.2. The shirt reflects blue only, and yellow light (red + green) has none, so it absorbs everything: black.
3. Blue light is now present, so the shirt reflects it and looks blue. Red + green + blue make white on the backdrop.
Problem 17 Slide projector
1. 1.20 m = 120 cm.2. M = 120 ÷ 3.0 = 40.
3. Image distance = 600 cm, so object distance = 600 ÷ 40 = 15 cm.
4. The image is larger than the object (M above 1), so the slide is between F and 2F. Between F and 2F.
Problem 18 Short sight
1. Either the eyeball is too long, or the lens and cornea bend light too strongly: eyeball too long or lens too strong.2. A diverging (concave) lens.
3. It spreads the rays slightly before they reach the eye, which moves the focus back onto the retina.
Problem 19 Burning paper with sunlight
1. The Sun is far away, so its rays are parallel. The paper goes at the focal length: 12 cm.2. Area ratio = (8.0 ÷ 1.0)² = 64.
3. The same light power falls on an area 64 times smaller at the focus, so the light is 64 times more concentrated and the paper heats up.
Problem 20 Image on the retina
1. 2.0 cm = 0.020 m.2. M = 0.020 ÷ 5.1 = 0.0039 (0.00392).
3. Image height = 1.70 m × 0.00392 = 0.00667 m = 6.7 mm.
4. The image is real and inverted (and very small).
