Teacher sheet · Ages 15–16 · about 45–50 minutes
Light: rays, reflection, dispersion and the electromagnetic spectrum
Lesson page: echoesof.science/school/light/ages-15-16/ · Pupil worksheet: echoesof.science/school/light/ages-15-16/print/
Learning aims
- Use c = f·λ for electromagnetic waves and know the visible range.
- State and verify the law of reflection; distinguish specular and diffuse reflection.
- Explain dispersion through the wavelength dependence of the refractive index.
- Explain colour and atmospheric scattering.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: why can you see this page from anywhere, but your reflection only from one place? |
| 10 min | Read the text; work through one c = f·λ example. |
| 25 min | Law of reflection practical in pairs, five angles. |
| 5 min | Plot r against i and discuss errors. |
| 5 min | Quiz, or Visible or not? as homework. |
Materials for the experiment
- A plane mirror with a stand (or a block to hold it upright)
- A ray box with a single slit, or a torch with black card taped over it leaving a narrow slit
- A sheet of A4 paper, a ruler, a sharp pencil and a protractor
- A dim room
Safety: Ray boxes get hot: don't touch the bulb. If a laser pointer is used, only a low-power school laser under the teacher's supervision, and never point it at eyes or shiny surfaces facing people.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Angles are measured from the mirror surface. | Angles of incidence and reflection are measured from the normal. |
| A prism 'adds' colours to light. | It separates colours that are already in white light. |
| Red objects reflect red light under any light. | With no red in the light falling on it, a red object looks dark. |
Worksheet answers
- The angle of reflection equals the angle of incidence, both measured from the normal; the diagram shows both rays meeting at the mirror with equal angles to the normal.
- f = c/λ = 3.00×10⁸ / 7.00×10⁻⁷ ≈ 4.3×10¹⁴ Hz.
- The refractive index of glass is larger for shorter wavelengths, so violet is bent more than red at each surface and the colours separate.
- Results within a few degrees of r = i. Errors: ray width (use a narrow slit), mirror thickness (draw on the silvered back), protractor reading (measure carefully, repeat).
Quiz answers
- Green light has a wavelength of 500 nm. Its frequency is… 6.0 × 10¹⁴ Hz. f = c/λ = 3.00×10⁸ / 5.00×10⁻⁷ = 6.0×10¹⁴ Hz.[6]
- Which statement explains dispersion in a prism? The refractive index depends on wavelength. Each wavelength refracts by a different amount.[5]
- A ray hits a mirror at 25° to the surface (not the normal). The angle of reflection measured from the normal is… 65°. From the normal, i = 90° − 25° = 65°, and r = i.[4]
- Under red light only, a pure green leaf looks… dark (nearly black). It reflects green; there is no green light to reflect.[2]
- Why does the setting Sun look red? Short wavelengths are scattered out of the long path through the air. Blue is scattered away; long wavelengths remain.[1]
- Light takes about 500 s from the Sun. The distance is about… 1.5 × 10¹¹ m. d = c·t = 3.00×10⁸ × 500 = 1.5×10¹¹ m.[7][6]
Going further
Put the mirror at an angle of 90° to a second mirror (a corner reflector). Show with a ray diagram that light is sent back parallel to where it came from. Where is this used?
Sources used in this lesson
- [1] Why Is the Sky Blue? (NASA Space Place): reliability High
- [2] Wave Behaviors (NASA Science (Tour of the Electromagnetic Spectrum)): reliability High
- [3] College Physics 2e, 25.1 The Ray Aspect of Light (OpenStax, Rice University): reliability High
- [4] College Physics 2e AP, 25.2 The Law of Reflection (OpenStax, Rice University): reliability High
- [5] College Physics 2e, 25.5 Dispersion: The Rainbow and Prisms (OpenStax, Rice University): reliability High
- [6] College Physics 2e, 24.3 The Electromagnetic Spectrum (OpenStax, Rice University): reliability High
- [7] Origin of Light (NASA Scientific Visualization Studio): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/light/sources/
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