Light: Reflection, Refraction, Diffraction & Lenses – CSEC Physics | CSEC Hub
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Mastering Light: Reflection, Refraction, Diffraction & Lenses

CSEC Physics: The Behavior of Light

Essential Understanding: Light is an electromagnetic wave that exhibits wave-like behaviors including reflection, refraction, diffraction, and interference. Understanding these phenomena is crucial for optics, vision, and modern technology. Master ray diagrams and the laws governing light to excel in CSEC Physics.

🔑 Key Skill: Drawing Accurate Ray Diagrams
📈 Exam Focus: Snell’s Law Calculations
🎯 Problem Solving: Lens Equation & Magnification

The Four Key Behaviors of Light

↪️

Reflection

Definition: When light bounces off a surface.

Types:

  • Regular/Specular: Smooth surface, clear image
  • Diffuse: Rough surface, scattered light
📏 Laws of Reflection
  1. The incident ray, reflected ray, and normal all lie in the same plane
  2. Angle of incidence = Angle of reflection (\(i = r\))
↷

Refraction

Definition: Bending of light when it passes from one medium to another.

Caused by: Change in light’s speed in different media.

⚖️ Snell’s Law

\[ n_1 \sin\theta_1 = n_2 \sin\theta_2 \]

Where \(n\) = refractive index, \(\theta\) = angle to normal

🌊

Diffraction

Definition: Spreading out of waves when they pass through a gap or around an obstacle.

Most significant when: Gap width ≈ Wavelength of light

Applications: Diffraction gratings, CD rainbows

🔍

Lenses

Definition: Transparent material that refracts light to form images.

Types:

  • Convex (Converging): Thicker in middle
  • Concave (Diverging): Thinner in middle

Reflection: Plane Mirror Simulator

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Interactive Plane Mirror Ray Diagram

In a plane mirror, the image is formed where the reflected rays appear to intersect. Because the light does not actually pass through the mirror, we call this a Virtual Image.

Object
Image
Rays
Virtual: Cannot be projected on screen
Equidistant: \(u = v\)
Same Size: \(h_o = h_i\)
Lateral Inversion: Left <-> Right

*Drag the object arrow or use sliders.

Refraction & Snell’s Law

Refractive Index

The refractive index (\(n\)) measures how much light slows down in a medium:

\[ n = \frac{\text{speed of light in vacuum}}{\text{speed of light in medium}} = \frac{c}{v} \]

Also: \( n = \frac{\sin i}{\sin r} \) where \(i\) = angle in less dense medium

🎮

Snell’s Law Interactive Simulator

Adjust the incident angle and media to observe refraction. Watch for total internal reflection when passing from a dense to a less dense medium.

Incident
Refracted
Normal
Refracted Angle \(\theta_2\): —
Critical Angle \(\theta_c\): None
⚠️ TOTAL INTERNAL REFLECTION

Lenses & Ray Diagrams

Lens Type Ray Diagram Rules Image Characteristics
Convex (Converging) 1. Ray parallel to axis refracts through focus
2. Ray through center continues straight
3. Ray through focus refracts parallel 		Real or virtual, inverted or upright, depending on object position
Concave (Diverging) 1. Ray parallel to axis appears to come from focus
2. Ray through center continues straight
3. Ray toward focus refracts parallel 		Always virtual, upright, diminished
🔬

Lens Ray Diagram Simulator

Explore how images form using the thin lens equation: \(\frac{1}{f} = \frac{1}{u} + \frac{1}{v}\).

Object
Image
Rays
Image distance (\(v\)): —
Magnification (\(m\)): —
Nature: —

*Measurements are in pixels for visualization.

Diffraction & Its Applications

Diffraction Grating

A diffraction grating consists of many equally spaced slits. When light passes through, it produces a pattern of bright and dark fringes.

\[ d \sin\theta = n\lambda \]

Where:
\(d\) = slit separation
\(\theta\) = angle to the bright fringe
\(n\) = order number (0, 1, 2…)
\(\lambda\) = wavelength of light

1
Example: A diffraction grating has 500 lines/mm. Find slit separation \(d\).
2
Solution: Lines/mm = 500, so \(d = \frac{1}{500} \, \text{mm} = \frac{1 \times 10^{-3}}{500} \, \text{m} = 2 \times 10^{-6} \, \text{m}\)
3
Calculation: For red light (\(\lambda = 650 \, \text{nm}\)) at first order (\(n=1\)):
\(\sin\theta = \frac{n\lambda}{d} = \frac{1 \times 650 \times 10^{-9}}{2 \times 10^{-6}} = 0.325\)
\(\theta = \sin^{-1}(0.325) = 19.0^\circ\)

CSEC Worked Examples

1
Example 1: Refraction Calculation
Light travels from air (\(n=1.00\)) into water (\(n=1.33\)) at an angle of 30° to the normal. Calculate the angle of refraction.
Solution: Using Snell’s Law: \(n_1 \sin\theta_1 = n_2 \sin\theta_2\)
\(1.00 \times \sin 30^\circ = 1.33 \times \sin\theta_2\)
\(0.5 = 1.33 \sin\theta_2\)
\(\sin\theta_2 = 0.5/1.33 = 0.376\)
\(\theta_2 = \sin^{-1}(0.376) = 22.1^\circ\)
2
Example 2: Critical Angle
Calculate the critical angle for light passing from glass (\(n=1.50\)) to air (\(n=1.00\)).
Solution: At critical angle, \(\theta_2 = 90^\circ\)
\(n_1 \sin\theta_c = n_2 \sin 90^\circ\)
\(1.50 \times \sin\theta_c = 1.00 \times 1\)
\(\sin\theta_c = 1/1.50 = 0.667\)
\(\theta_c = \sin^{-1}(0.667) = 41.8^\circ\)
3
Example 3: Lens Calculation
An object is placed 30 cm from a convex lens with focal length 20 cm. Find the image position and magnification.
Solution: Using lens formula: \(\frac{1}{f} = \frac{1}{u} + \frac{1}{v}\)
\(\frac{1}{20} = \frac{1}{30} + \frac{1}{v}\)
\(\frac{1}{v} = \frac{1}{20} – \frac{1}{30} = \frac{3-2}{60} = \frac{1}{60}\)
\(v = 60 \, \text{cm}\) (real image on opposite side)
\(m = \frac{v}{u} = \frac{60}{30} = 2\) (image is twice as large, inverted)
📝

CSEC Past Paper Question (2021)

Question: A ray of light enters a glass block at an angle of incidence of 40°. The refractive index of glass is 1.50.

(a) Calculate the angle of refraction. [3 marks]

(b) If the light ray instead travels from glass to air, calculate the critical angle. [2 marks]

(c) Explain why a concave lens always produces a virtual image. [2 marks]

CSEC Practice Arena

Test Your Light Knowledge

1
Which of these is NOT true about the image formed by a plane mirror?
It is virtual
It is magnified
It is laterally inverted
It is as far behind mirror as object is in front
Explanation: Plane mirrors produce images that are the same size as the object, not magnified. Magnification occurs with curved mirrors or lenses.
2
Light travels from water (n=1.33) to air (n=1.00). If the angle of incidence is 40°, what happens?
It refracts away from normal at about 60°
It refracts away from normal at about 58.7°
It refracts toward normal at about 29°
It continues straight without bending
Explanation: Critical angle for water-air is about 48.8° (sin⁻¹(1/1.33)). Since 40° < 48.8°, light will refract. Using Snell's Law: 1.33(sin40) = 1(sin r). 0.855 = sin r. r = 58.7°.
3
A convex lens has focal length 15 cm. An object is placed 30 cm from the lens. What is the magnification?
0.5
1.0
1.5
2.0
Solution: Using lens formula: 1/15 = 1/30 + 1/v → 1/v = 1/15 – 1/30 = 1/30 → v = 30 cm. Magnification m = v/u = 30/30 = 1.0.
🎯

CSEC Examination Mastery Tip

Ray Diagram Success Strategy:

  • Use a ruler! Straight lines are essential for accurate ray diagrams.
  • Label everything: Object (O), Image (I), Focus (F), Center of lens/mirror.
  • Arrows show direction: Always include arrows on rays showing light direction.
  • Virtual rays: Use dashed lines for virtual rays (behind mirror or on same side as object for lenses).
  • Check your work: All rays from object point should converge at image point (for real images) or appear to diverge from image point (for virtual images).

Common Pitfall: Forgetting that concave lenses and convex mirrors always produce virtual, diminished, upright images.

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