# Light & Optics — ACT Science
Light and optics questions on the ACT may involve the electromagnetic spectrum, reflection, refraction, or experiments with lenses and mirrors. Understanding basic concepts helps you interpret data efficiently.
Key Concepts
The Electromagnetic Spectrum
From lowest to highest frequency: Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma ray.
- All EM waves travel at the speed of light ().
- Higher frequency → shorter wavelength → more energy.
- Visible light: red (lowest frequency) to violet (highest frequency).
Reflection
- Angle of incidence = angle of reflection.
- Smooth surfaces → specular reflection (clear image).
- Rough surfaces → diffuse reflection (no clear image).
Refraction
- Light bends when passing between materials of different densities.
- Light bends toward the normal entering a denser medium.
- Light bends away from the normal entering a less dense medium.
Lenses
- Convex lens: converges light, can magnify images.
- Concave lens: diverges light, makes images smaller.
Color and Wavelength
- White light = all visible colors combined.
- A prism separates white light by bending each wavelength differently.
- Objects appear colored because they reflect/absorb specific wavelengths.
Worked Example
Passage summary: Students measure the angle of refraction as light passes from air into glass at different angles.
| Angle of incidence (°) | Angle of refraction (°) |
|---|---|
| 10 | 6.6 |
| 20 | 13.2 |
| 30 | 19.5 |
| 40 | 25.4 |
Q: What trend does the data show?
A: The refraction angle is always smaller than the incidence angle (light bends toward the normal entering a denser medium), and both increase together.
Practice Questions
1. Which type of EM radiation has the most energy per photon?
Gamma rays (highest frequency).
2. Why does a straw appear bent when placed in a glass of water?
Refraction — light bends as it passes from water to air, changing the apparent position of the straw.
3. A convex lens focuses sunlight to a point. What is that point called?
The focal point.
Want to check your answers and get step-by-step solutions?
Summary
- EM spectrum: higher frequency = more energy = shorter wavelength.
- Reflection: angle in = angle out.
- Refraction: light bends at boundaries between materials.
- Convex lenses converge; concave lenses diverge.