Quick answer: White light is a mixture of colours that a prism splits into a spectrum by dispersion, with violet bent most. This grade 8 science lab on colour and lenses gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How does the image made by a converging lens change as an object moves closer to it? In this lab you find the focal length of a lens from a distant object, then measure image distance and image height for several object distances and compare the magnification you get in two ways.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS4-2
Aim
To find the focal length of a converging lens and to see how the image changes as the object moves closer to the lens.
Equipment
- Converging lens (f about 10 cm) in a holder
- Bright LED torch or phone light with an arrow cut out of card
- White card screen in a holder
- Metre rule
- Ruler for measuring image height
Safety
Never look at the Sun through a lens, and never point a lens at the Sun, because the focused light can burn skin and damage eyes. Keep the lens away from paper and curtains near windows.
Method
- Hold the lens near a window and move the screen until a sharp picture of a distant building appears. Measure the lens-to-screen distance. Repeat three times.
- Put the lit arrow at one end of the metre rule (the arrow is 2.0 cm tall) and the lens 40.0 cm from it.
- Move the screen until the arrow image is sharp, then measure v and the image height.
- Repeat with u = 30.0, 20.0, 15.0 and 12.0 cm.
- Move the lens to u = 8.0 cm and try to find an image on the screen, then look through the lens at the arrow.
Results
Focal length from a distant object:
| Trial | Lens to screen (cm) |
|---|---|
| 1 | 10.1 |
| 2 | 10.3 |
| 3 | 10.0 |
Images of the 2.0 cm arrow:
| u (cm) | v (cm) | Image height (cm) | Image description |
|---|---|---|---|
| 40.0 | 13.5 | 0.7 | smaller, inverted |
| 30.0 | 15.0 | 1.0 | smaller, inverted |
| 20.0 | 20.0 | 2.0 | same size, inverted |
| 15.0 | 29.5 | 3.9 | larger, inverted |
| 12.0 | 58.0 | 9.6 | larger, inverted |
| 8.0 | no image on screen | – | looking through the lens: upright, larger |
Analysis questions
Question 1 Mean focal length
Calculate the mean lens-to-screen distance from the distant-object table and state it as the focal length.Question 2 Magnification of one image
Calculate the magnification for u = 40.0 cm in two ways: using image height ÷ object height, and using v ÷ u.Question 3 Larger images
Calculate the magnification for u = 15.0 cm and u = 12.0 cm, using image height ÷ object height.Question 4 Describing the trend
Describe how v and the image height change as u decreases from 40.0 cm to 12.0 cm.Question 5 Image at 2F
Use the row where the image is the same size as the object to estimate the focal length, and compare it with your answer to question 1.Question 6 Uncertainty
The sharpest position of the screen can be judged only to ±1.0 cm. Calculate the percentage uncertainty in v for u = 40.0 cm and for u = 12.0 cm, and state which is more reliable.Question 7 Object inside the focal length
Explain why no image appears on the screen at u = 8.0 cm, and describe what you see through the lens.Question 8 Sharper focus readings
Propose two changes to the procedure that would make the screen position easier to judge and the readings of v more trustworthy.
