Learning outcomes
- Set up accurate ray-tracing experiments.
- Measure angles from the normal.
- Use pins to avoid parallax.
- Find a focal length using a sharp image.
- Evaluate optical alignment and image-judgment errors.
17.1 Reflection
Place the mirror on a traced line and mark its position. Draw a normal at the point of incidence. Use pins or a ray box to establish incident and reflected rays.
Angles of incidence and reflection are measured between each ray and the normal, not between the ray and mirror. Repeat for several incident angles and compare within experimental accuracy.
17.2 Pin alignment
Pins should be vertical and widely separated. When locating an image or emergent ray, move the eye until the relevant pins appear in one line and avoid parallax.
Mark pin positions with small precise dots after removing the apparatus, then draw thin straight ray lines through them.
17.3 Refraction through a block
Trace the block before placing pins. Draw the incident and emergent rays, remove the block and join the internal path. Construct normals at each boundary.
A rectangular block produces an emergent ray parallel to the incident ray but laterally displaced. A semicircular block can reduce refraction at the curved boundary when the ray travels through the centre.
17.4 Converging lens
Place object, lens and screen on a common principal axis and keep them vertical. Move the screen until a sharp real image is obtained. Measure distances from the optical centre of the lens.
For a distant object, the sharp image forms approximately one focal length from the lens. For finite object distance, measurements may be processed using a supplied relationship.
17.5 Optical evaluation
Main uncertainties are broad rays, thick pencil lines, pin parallax, tilted components and subjective sharpness. Improve by darkening the room, using narrow slits, thin lines, larger pin spacing and repeated focus approaches from both directions.
Do not look directly into an intense light source or laser. Use safe low-power school sources and screens.
Worked examples
Law of reflection
Measured pairs (i, r) of (20°, 21°), (35°, 34°) and (50°, 51°) are equal within about 1°, supporting i = r within experimental accuracy.
Lens measurement
A sharp distant image is 14.8 cm from the lens centre. Repeating gives 15.0 cm and 14.9 cm; mean focal length ≈ 14.9 cm.
Practical focus
Investigation or training activity
Trace refraction through a rectangular block for three incident angles, then determine the focal length of a converging lens using a distant object. Evaluate the dominant judgment uncertainty in each.
Examination guidance
- Measure angles from the normal.
- Keep pins vertical and well separated.
- Trace apparatus positions before removing them.
- Measure lens distances from the optical centre.
- Use a sharp, bright image and a darkened environment.
Check your understanding
- From which line is angle of incidence measured?
- Why should pins be widely separated?
- Where is lens distance measured from?
- Why approach focus from both directions?
Answers
- The normal.
- Alignment becomes more precise and angular uncertainty is reduced.
- The optical centre of the lens.
- To reduce subjective bias in choosing the sharpest position.
