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Physics Unit 9: Light and optics

Light is an electromagnetic wave that reflects, refracts, diffracts and interferes, and mirrors and lenses use these behaviors to form images.

Covers: The electromagnetic spectrum, reflection, refraction and Snell's law, index of refraction, total internal reflection, mirrors and lenses, diffraction and interference of light

Study guide

Light is an electromagnetic wave: a transverse wave that needs no medium and travels at c = 3.00 × 10⁸ m/s in a vacuum. The electromagnetic spectrum, from longest wavelength to shortest, is radio, microwaves, infrared, visible light (red to violet), ultraviolet, X-rays and gamma rays. All obey v = fλ, so shorter wavelengths have higher frequencies and more energy per photon. Visible light runs from about 700 nm (red) to 400 nm (violet).

Reflection follows the law of reflection: the angle of incidence equals the angle of reflection, both measured from the normal (a line perpendicular to the surface). A plane mirror forms a virtual, upright image the same size as the object, as far behind the mirror as the object is in front of it. Smooth surfaces give regular (specular) reflection; rough surfaces scatter light in diffuse reflection.

Refraction is the bending of light as it passes from one medium into another and changes speed. The absolute index of refraction is n = c/v; a higher n means light moves more slowly (glass about 1.5, water 1.33, air 1.00). Snell's law states n₁ sin θ₁ = n₂ sin θ₂. Light bends toward the normal when entering a medium with a higher index and away from the normal when entering a lower index. When light refracts, its frequency stays the same, but its speed and wavelength change. Total internal reflection happens when light moving into a lower-index medium hits the boundary at more than the critical angle, which makes fiber optics possible. Dispersion separates white light into colors because n depends on wavelength, as in prisms and rainbows. Converging (convex) lenses and concave mirrors focus light to a real focal point, while diverging lenses and convex mirrors spread it out; 1/f = 1/dₒ + 1/dᵢ relates focal length and distances. Light also shows wave behavior through diffraction (spreading around edges or through narrow openings) and interference, which produces bright and dark bands in a double-slit experiment. Polarization shows that light is a transverse wave.

Key ideas

• Light is a transverse electromagnetic wave traveling at 3.00 × 10⁸ m/s in a vacuum.
• Angle of incidence equals angle of reflection.
• n = c/v; light bends toward the normal entering a higher index.
• Frequency stays constant during refraction; speed and wavelength change.
• Diffraction, interference and polarization show light behaves as a wave.

Formulas and rules

Common mistakes

• Thinking light's frequency changes when it enters glass; only speed and wavelength change.
• Measuring angles from the surface instead of from the normal.

Words to know

Electromagnetic wave

A transverse wave of electric and magnetic fields that needs no medium.

Electromagnetic spectrum

The full range of EM waves from radio to gamma rays.

Normal

A line perpendicular to a surface at the point of incidence.

Law of reflection

The angle of incidence equals the angle of reflection.

Virtual image

An image that light only appears to come from.

Real image

An image formed where light rays actually meet.

Refraction

Bending of light as its speed changes between media.

Index of refraction

n = c/v, the ratio of light's speed in a vacuum to its speed in a medium.

Snell's law

n₁ sin θ₁ = n₂ sin θ₂.

Critical angle

The incident angle beyond which total internal reflection occurs.

Total internal reflection

Complete reflection of light back into a denser medium.

Dispersion

Separation of white light into colors.

Converging lens

A lens that brings light rays together at a focal point.

Diffraction

Spreading of waves around obstacles or through openings.

Interference

Waves combining to reinforce or cancel each other.

Polarization

Limiting a transverse wave to one plane of vibration.

Sample practice questions

20 more questions like these, from level 1 to 5, are in the practice set.