Physics Unit 7 — Light and Optics.
This unit covers reflection, refraction, lenses and mirrors and electromagnetic spectrum — essential concepts for Physics. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers reflection, refraction, lenses and mirrors and electromagnetic spectrum — essential concepts for Physics. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Reflection
Reflection occurs when light bounces off a surface. Students must know the law of reflection: the angle of incidence equals the angle of reflection, both measured from the normal. They should also distinguish between specular (mirror-like) and diffuse reflection.
Key Points
- Angle of incidence = angle of reflection (measured from the normal, not the surface)
- The incident ray, reflected ray, and normal all lie in the same plane
- Specular reflection: smooth surface, parallel rays stay parallel after reflection
- Diffuse reflection: rough surface, parallel rays scatter in many directions
A light ray strikes a flat mirror at 35° to the surface. What is the angle of reflection?
The angle given (35°) is measured from the surface, not the normal. To find the angle of incidence from the normal, subtract from 90°: 90° − 35° = 55°. By the law of reflection, the angle of reflection is also 55° from the normal.
2 Refraction
Refraction is the bending of light as it passes from one medium to another due to a change in speed. Students must be able to apply Snell's Law (n₁ sin θ₁ = n₂ sin θ₂) and predict which direction light bends based on the indices of refraction. Total internal reflection occurs when light travels from a denser to a less dense medium and the angle of incidence exceeds the critical angle.
Key Points
- Light bends toward the normal when entering a denser medium (higher n), away when entering a less dense medium
- Snell's Law: n₁ sin θ₁ = n₂ sin θ₂, where n is the index of refraction
- Index of refraction: n = c/v; higher n means slower light speed in that medium
- Total internal reflection only occurs when going from high-n to low-n medium and angle > critical angle
A ray of light travels from water (n = 1.33) into air (n = 1.00) at an angle of incidence of 30°. Find the angle of refraction.
Apply Snell's Law: (1.33)(sin 30°) = (1.00)(sin θ₂). This gives (1.33)(0.50) = sin θ₂, so sin θ₂ = 0.665. Therefore θ₂ = sin⁻¹(0.665) ≈ 41.7°. Since light moves from a denser to a less dense medium, it bends away from the normal, consistent with θ₂ > θ₁.
3 Lenses and Mirrors
Students must apply the thin lens and mirror equation (1/f = 1/dₒ + 1/dᵢ) and the magnification equation (m = −dᵢ/dₒ). They need to know sign conventions, how to identify real vs. virtual images, and the behavior of converging vs. diverging lenses and concave vs. convex mirrors.
Key Points
- Thin lens/mirror equation: 1/f = 1/dₒ + 1/dᵢ; focal length f is positive for converging lenses and concave mirrors
- Real images: dᵢ positive, image on opposite side of lens (or same side as object for mirrors); can be projected
- Virtual images: dᵢ negative, image cannot be projected; diverging lenses and convex mirrors always produce virtual images
- Magnification m = −dᵢ/dₒ; |m| > 1 means enlarged, |m| < 1 means reduced; negative m means inverted image
An object is placed 20 cm in front of a converging lens with a focal length of 15 cm. Find the image distance and state whether the image is real or virtual.
Substitute into 1/f = 1/dₒ + 1/dᵢ: 1/15 = 1/20 + 1/dᵢ. Solving: 1/dᵢ = 1/15 − 1/20 = 4/60 − 3/60 = 1/60, so dᵢ = 60 cm. Because dᵢ is positive, the image is real and located 60 cm on the opposite side of the lens from the object.
4 Electromagnetic Spectrum
The electromagnetic spectrum organizes all electromagnetic waves by frequency and wavelength. Students must know the order of the spectrum, the relationship between frequency, wavelength, and energy (E = hf; c = fλ), and the properties that change across the spectrum versus those that stay constant.
Key Points
- Order from lowest to highest frequency (longest to shortest wavelength): Radio → Microwave → Infrared → Visible → Ultraviolet → X-ray → Gamma ray
- All EM waves travel at c = 3.0 × 10⁸ m/s in a vacuum regardless of frequency
- Energy increases with frequency: E = hf (h = 6.63 × 10⁻³⁴ J·s); gamma rays are most energetic, radio waves least
- Visible light spans roughly 400 nm (violet) to 700 nm (red)
A yellow light wave has a wavelength of 580 nm. Calculate its frequency.
Use c = fλ, rearranged to f = c/λ. Convert wavelength to meters: 580 nm = 580 × 10⁻⁹ m = 5.80 × 10⁻⁷ m. Then f = (3.0 × 10⁸ m/s) / (5.80 × 10⁻⁷ m) ≈ 5.17 × 10¹⁴ Hz. This falls in the visible light range, confirming the answer is physically reasonable.
Questions, answered.
What is Light and Optics?
Light and Optics is Unit 7 of Physics, covering reflection, refraction, lenses and mirrors and electromagnetic spectrum.
How to study for Physics Unit 7?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 28+ review questions across 5 different game modes.