Light and the electromagnetic spectrum
Reflection, refraction, total internal reflection, lenses, dispersion, and the uses and dangers of electromagnetic waves.
📌 Key points
- Law of reflection: angle of incidence = angle of reflection, both measured from the normal. The image in a plane mirror is virtual, upright, the same size as the object, and as far behind the mirror as the object is in front.
- Refraction: light changes speed at a boundary. Going into a denser medium (e.g. air to glass) it slows down and bends towards the normal.
- Refractive index n = sin isin r; it is also the ratio of the speed of light in air (vacuum) to its speed in the material.
- Total internal reflection happens when light in a denser medium hits the boundary at more than the critical angle c, where n = 1sin c. It is used in optical fibres.
- A converging lens brings rays parallel to the principal axis together at the principal focus. The distance from the lens to this point is the focal length.
- A converging lens used as a magnifying glass (object closer than the focal length) gives a virtual, upright, magnified image.
- A prism splits white light into a spectrum (dispersion): red, orange, yellow, green, blue, indigo, violet. Violet is refracted most.
- Electromagnetic spectrum in order of increasing frequency (decreasing wavelength): radio, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. All travel at 3.0 × 108 m/s in a vacuum.
- Uses: radio — broadcasting; microwaves — mobile phones, satellites, cooking; infrared — remote controls, thermal imaging; UV — security marking, sterilising water; X-rays — medical imaging, security; gamma — sterilising food and equipment, cancer treatment.
- Dangers: microwaves cause internal heating; infrared causes skin burns; ultraviolet damages skin and eyes and can cause skin cancer; X-rays and gamma rays can cause mutations and cancer.
📖 Key terms
- Normal
- A line at 90° to a surface at the point where a ray meets it.
- Refraction
- The change in direction of a wave as it changes speed at a boundary.
- Refractive index
- The ratio of the speed of light in air (vacuum) to its speed in a material.
- Critical angle
- The angle of incidence in the denser medium for which the angle of refraction is 90°.
- Total internal reflection
- Complete reflection of light inside a denser medium when the angle of incidence is greater than the critical angle.
- Principal focus
- The point where rays parallel to the principal axis meet after passing through a converging lens.
- Dispersion
- The splitting of white light into its colours.
- Virtual image
- An image that cannot be formed on a screen; the light only appears to come from it.
✏️ Worked example
Light travels from air into glass of refractive index 1.5. The angle of incidence is 40°. Find the angle of refraction.
- n = sin isin r, so sin r = sin in.
- sin r = sin 40°1.5 = 0.6431.5 = 0.429.
- r = sin−1(0.429).
- Answer: r = 25.4°, bent towards the normal.
Always measure angles of incidence, reflection and refraction from the normal, not from the surface.
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